LTC3611
1
3611fd
ApplicAtions
typicAl ApplicAtion
Description
10A, 32V Monolithic
Synchronous Step-Down
DC/DC Converter
The LTC
®
3611 is a high efficiency, monolithic synchronous
step-down DC/DC converter that can deliver up to 10A
output current from a 4.5V to 32V (36V maximum) input
supply. It uses a constant on-time valley current mode
control architecture to deliver very low duty cycle opera-
tion at high frequency with excellent transient response.
The operating frequency is selected by an external resistor
and is compensated for variations in VIN and VOUT.
The LTC3611 can be configured for discontinuous or
forced continuous operation at light load. Forced continu-
ous operation reduces noise and RF interference while
discontinuous mode provides high efficiency by reducing
switching losses at light loads.
Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator and an optional
short-circuit shutdown timer. Soft-start capability for sup-
ply sequencing is accomplished using an external timing
capacitor. The regulator current limit is user programmable.
A power good output voltage monitor indicates when
the output is in regulation. The LTC3611 is available in a
compact 9mm × 9mm QFN package.
Efficiency and Power Loss
vs Load Current
FeAtures
n 10A Output Current
n Wide VIN Range = 4.5V to 32V (36V Maximum)
n Internal N-Channel MOSFETs
n True Current Mode Control
n Optimized for High Step-Down Ratios
n t0N(MIN) ≤ 100ns
n Extremely Fast Transient Response
n Stable with Ceramic COUT
n ±1% 0.6V Voltage Reference
n Power Good Output Voltage Monitor
n Adjustable On-Time/Switching Frequency (>1MHz)
n Adjustable Current Limit
n Programmable Soft-Start
n Output Overvoltage Protection
n Optional Short-Circuit Shutdown Timer
n Low Shutdown IQ: 15μA
n Available in a 9mm × 9mm 64-Pin QFN Package
n Point of Load Regulation
n Distributed Power Systems
High Efficiency Step-Down Converter
1µH
4.7µF
10µF
×3
VIN
4.5V TO 32V
VOUT
2.5V
10A
3611 TA01a
182k
0.1µF ION
VIN
SW
BOOST
RUN/SS
ITH
VON
SGND INTVCC
FCB
PGND
VFB
VRNG
0.22µF 100µF
×2
12.5k
39.2k
INTVCC
11k
LTC3611
680pF
EXTVCC
PGOOD
30.1k
9.5k
100pF
VOUT
LOAD CURRENT (A)
0.01
0
EFFICIENCY (%)
POWER LOSS (mW)
50
40
30
20
10
100
90
80
70
60
0.1 1 10
3611 TA01b
VOUT = 2.5V
VIN = 5V
VIN = 25V
POWER LOSS,
VIN = 25V
POWER LOSS,
VIN = 5V
1
10
100
1000
10000
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Patents including 5481178, 6100678, 6580258, 5847554, 6304066.
LTC3611
2
3611fd
pin conFigurAtionAbsolute MAxiMuM rAtings
(Note 1)
TOP VIEW
WP PACKAGE
64-LEAD (9mm × 9mm) QFN MULTIPAD
PGND 1
PGND 2
PGND 3
SW 4
SW 5
SW 6
SW 7
SW 8
SW 9
SW 10
SW 11
PVIN 12
PVIN 13
PVIN 14
PVIN 15
PVIN 16
48 SGND
47 SGND
46 SGND
45 SGND
44 EXTVCC
43 VFB
42 SGND
41 ION
40 SGND
39 FCB
38 ITH
37 VRNG
36 PGOOD
35 VON
34 SGND
33 SGND
PVIN 17
PVIN 18
PVIN 19
PVIN 20
PVIN 21
PVIN 22
PVIN 23
PVIN 24
PVIN 25
SW 26
NC 27
SGND 28
BOOST 29
RUN/SS 30
SGND 31
SGND 32
64 PGND
63 PGND
62 PGND
61 PGND
60 PGND
59 PGND
58 PGND
57 PGND
56 PGND
55 SW
54 INTVCC
53 INTVCC
52 SVIN
51 SVIN
50 SGND
49 SGND
68
SGND
67
PVIN
66
SW
65
PGND
TJMAX = 125°C, θJA = 28°C/W
orDer inForMAtion
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC3611EWP#PBF LTC3611EWP#TRPBF LTC3611WP 64-Lead (9mm × 9mm) Plastic QFN –40°C to 125°C
LTC3611IWP#PBF LTC3611IWP#TRPBF LTC3611WP 64-Lead (9mm × 9mm) Plastic QFN –40°C to 125°C
LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC3611EWP LTC3611EWP#TR LTC3611WP 64-Lead (9mm × 9mm) Plastic QFN –40°C to 125°C
LTC3611IWP LTC3611IWP#TR LTC3611WP 64-Lead (9mm × 9mm) Plastic QFN –40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
Input Supply Voltage (VIN, ION) .................. 36V to –0.3V
Boosted Topside Driver Supply Voltage
(BOOST) ................................................ 42V to –0.3V
SW Voltage ............................................ 36V to –0.3V
INTVCC, EXTVCC, (BOOST – SW), RUN/SS,
PGOOD Voltages .......................................... 7V to –0.3V
FCB, VON, VRNG Voltages ............ INTVCC + 0.3V to –0.3V
ITH, VFB Voltages ....................................... 2.7V to –0.3V
Operating Junction Temperature Range
(Notes 2, 4) ............................................ –40°C to 125°C
Storage Temperature Range ...................–55°C to 125°C
LTC3611
3
3611fd
electricAl chArActeristics
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Main Control Loop
VIN Operating Input Voltage Range 4.5 32 V
IQInput DC Supply Current
Normal
Shutdown Supply Current
900
15
2000
30
µA
µA
VFB Feedback Reference Voltage ITH = 1.2V (Note 3)
–40°C to 85°C
–40°C to 125°C
l
0.594
0.590
0.600
0.600
0.606
0.610
V
V
ΔVFB(LINEREG) Feedback Voltage Line Regulation VIN = 4V to 30V, ITH = 1.2V (Note 3) 0.002 %/V
ΔVFB(LOADREG) Feedback Voltage Load Regulation ITH = 0.5V to 1.9V (Note 3) –0.05 –0.3 %
IFB Feedback Input Current VFB = 0.6V –5 ±50 nA
gm(EA) Error Amplifier Transconductance ITH = 1.2V (Note 3) l1.4 1.7 2 mS
VFCB Forced Continuous Threshold l0.54 0.6 0.66 V
IFCB Forced Continuous Pin Current VFCB = 0.6V –1 –2 µA
tON On-Time ION = 60μA, VON = 1.5V
ION = 60μA, VON = 0V
190 250
120
310 ns
ns
tON(MIN) Minimum On-Time ION = 180μA, VON = 0V 60 100 ns
tOFF(MIN) Minimum Off-Time ION = 30μA, VON = 1.5V 290 500 ns
IVALLEY(MAX) Maximum Valley Current VRNG = 0V, VFB = 0.56V, FCB = 0V
VRNG = 1V, VFB = 0.56V, FCB = 0V
l
l
6
8
10
15
A
A
IVALLEY(MIN) Maximum Reverse Valley Current VRNG = 0V, VFB = 0.64V, FCB = 0V
VRNG = 1V, VFB = 0.64V, FCB = 0V
–6
–8
A
A
ΔVFB(OV) Output Overvoltage Fault Threshold 7 10 13 %
VRUN/SS(ON) RUN Pin Start Threshold l0.8 1.5 2 V
VRUN/SS(LE) RUN Pin Latchoff Enable Threshold RUN/SS Pin Rising 4 4.5 V
VRUN/SS(LT) RUN Pin Latchoff Threshold RUN/SS Pin Falling 3.5 4.2 V
IRUN/SS(C) Soft-Start Charge Current VRUN/SS = 0V –0.5 –1.2 –3 µA
IRUN/SS(D) Soft-Start Discharge Current VRUN/SS = 4.5V, VFB = 0V 0.8 1.8 3 µA
VIN(UVLO) Undervoltage Lockout VIN Falling l3.4 3.9 V
VIN(UVLOR) Undervoltage Lockout Release VIN Rising l3.5 4 V
RDS(ON) Top Switch On-Resistance
Bottom Switch On-Resistance
15
9
22
14
mΩ
mΩ
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted.
LTC3611
4
3611fd
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Internal VCC Regulator
VINTVCC Internal VCC Voltage 6V < VIN < 30V, VEXTVCC = 4V l4.7 5 5.6 V
ΔVLDO(LOADREG) Internal VCC Load Regulation ICC = 0mA to 20mA, VEXTVCC = 4V –0.1 ±2 %
VEXTVCC EXTVCC Switchover Voltage ICC = 20mA, VEXTVCC Rising l4.5 4.7 V
ΔVEXTVCC EXTVCC Switch Drop Voltage ICC = 20mA, VEXTVCC = 5V 150 300 m/V
ΔVEXTVCC(HYS) EXTVCC Switchover Hysteresis 500 m/V
PGOOD Output
ΔVFBH PGOOD Upper Threshold VFB Rising 7 10 13 %
ΔVFBL PGOOD Lower Threshold VFB Falling –7 –10 –13 %
ΔVFB(HYS) PGOOD Hysteresis VFB Returning 1 2.5 %
VPGL PGOOD Low Voltage IPGOOD = 5mA 0.15 0.4 V
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: TJ is calculated from the ambient temperature TA and power
dissipation PD as follows:
TJ = TA + (PD • 28°C/W) (θJA is simulated per JESD51-7 high
effective thermal conductivity test board)
θJC = 1°C/W (θJC is simulated when heatsink is applied at the
bottom of the package)
Note 3: The LTC3611 is tested in a feedback loop that adjusts VFB to
achieve a specified error amplifier output voltage (ITH). The specification at
85°C is not tested in production. This specification is assured by design,
characterization, and correlation to testing at 125°C.
Note 4: The LTC3611 is tested under pulsed load conditions such that
TJ ≈ TA. The LTC3611E is guaranteed to meet specifications from
0°C to 125°C junction temperature. Specifications over the –40°C to
125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3611I is guaranteed over the full –40°C to 125°C operating junction
temperature range. Note that the maximum ambient temperature
consistent with these specifications is determined by specific operating
conditions in conjunction with board layout, the rated package thermal
impedance and other environmental factors.
typicAl perForMAnce chArActeristics
Transient Response
Transient Response
(Discontinuous Mode) Start-Up
VIN = 25V
VOUT = 2.5V
RLOAD = 0.5Ω
FIGURE 6 CIRCUIT
3611 G03
40ms/DIV
RUN/SS
2V/DIV
VOUT
1V/DIV
IL
5A/DIV
LOAD = 1A TO 10A
VIN = 25V
VOUT = 2.5V
FCB = INTVCC
FIGURE 6 CIRCUIT
3611 G02
VOUT
200mV/DIV
IL
5A/DIV
ILOAD
5A/DIV
40µs/DIV
LOAD STEP 0A TO 8A
VIN = 25V
VOUT = 2.5V
FCB = 0
FIGURE 6 CIRCUIT
3611 G01
40µs/DIV
VOUT
200mV/DIV
IL
5A/DIV
ILOAD
5A/DIV
electricAl chArActeristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted.
LTC3611
5
3611fd
typicAl perForMAnce chArActeristics
Frequency vs Load Current
Load Current vs ITH Voltage
and VRNG On-Time vs ION Current On-Time vs VON Voltage
Efficiency vs Load Current Efficiency vs Input Voltage Frequency vs Input Voltage
ITH VOLTAGE (V)
0
LOAD CURRENT (A)
5
10
15
25
20
2
3611 G10
0
–5
–10
0.5 11.5 32.5
0.5V
0.7V
VRNG = 1V
VON VOLTAGE (V)
0
ON-TIME (ns)
400
600
3611 G12
200
01 2 3
1000 ION = 30µA
800
ION CURRENT (µA)
1
10
ON-TIME (ns)
100
1000
10000
10 100
3611 G11
VVON = 0V
LOAD CURRENT (A)
0
ITH VOLTAGE (V)
1.0
1.5
3611 G09
0.5
05 10 15
2.5
2.0
CONTINUOUS
MODE
DISCONTINUOUS
MODE
FIGURE 6 CIRCUIT
LOAD CURRENT (A)
0
∆VOUT (%)
8
3611 G08
–0.80
–0.60
–0.40
–0.20
0.60
0.40
0.20
0
2 4 6 10
0.80 FIGURE 6 CIRCUIT
LOAD CURRENT (A)
0
0
FREQUENCY (kHz)
50
100
150
200
250
300
350
400
450
500
550
600
650
2 4 6 8
3611 G07
10
CONTINUOUS MODE
DISCONTINUOUS MODE
INPUT VOLTAGE (V)
5
FREQUENCY (kHz)
480
520
25 30 35
3611 G06
440
400 10 15 20
640
600
560
ILOAD = 10A
FCB = 0V
FIGURE 6 CIRCUIT
ILOAD = 0A
INPUT VOLTAGE (V)
5
EFFICIENCY (%)
90
95
100
25 30
3611 G05
85
80
10 15 20 35
FCB = 5V
FIGURE 6 CIRCUIT
ILOAD = 10A
ILOAD = 1A
LOAD CURRENT (A)
0.01
50
EFFICIENCY (%)
70
60
100
90
80
0.1 1 10
3611 G04
DISCONTINUOUS
CONTINUOUS
VIN = 12V
VOUT = 2.5V
EXTVCC = 5V
FIGURE 6 CIRCUIT
Load Regulation ITH Voltage vs Load Current
LTC3611
6
3611fd
typicAl perForMAnce chArActeristics
Maximum Valley Current Limit
vs Temperature
Input Voltage vs Maximum
Valley Current
Maximum Valley Current Limit
in Foldback
Feedback Reference Voltage
vs Temperature Error Amplifier gm vs Temperature
On-Time vs Temperature
Maximum Valley Current Limit
vs VRNG Voltage
Maximum Valley Current Limit
vs RUN/SS Voltage
TEMPERATURE (°C)
–50
ON-TIME (ns)
200
250
300
25 75
3611 G13
150
100
–25 0 50 100 125
50
0
IION = 30µA
VVON = 0V
VRNG VOLTAGE (V)
0.5
MAXIMUM VALLEY CURRENT LIMIT (A)
5
10
20
15
0.9
3611 G15
0.6 0.7 0.8 1
RUN/SS VOLTAGE (V)
1.65
MAXIMUM VALLEY CURRENT LIMIT (A)
3
6
9
12
18
15
2.65
3611 G16
01.9 2.15 2.4 3.15 3.42.9
FIGURE 6 CIRCUIT
TEMPERATURE (°C)
–50
MAXIMUM VALLEY CURRENT LIMIT (A)
5
10
20
15
50
3611 G17
0–25 025 100 12575
VRNG = 1V
INPUT VOLTAGE (V)
4
MAXIMUM VALLEY CURRENT (A)
6
8
10
16
18
14
12
20
3611 G27
4812 16 28 32 3624
VFB (V)
0
MAXIMUM VALLEY CURRENT LIMIT (A)
5
10
20
15
0.4
3611 G14
00.1 0.2 0.3 0.60.5
VRNG = 1V
TEMPERATURE (°C)
–50
0.58
FEEDBACK REFERENCE VOLTAGE (V)
0.59
0.60
0.61
0.62
–25 0 25 50
3611 G18
75 100 125
TEMPERATURE (°C)
–50 –25
1.0
gm (mS)
1.4
2.0
050 75
3611 G19
1.2
1.8
1.6
25 100 125
LTC3611
7
3611fd
typicAl perForMAnce chArActeristics
EXTVCC Switch Resistance
vs Temperature FCB Pin Current vs Temperature
RUN/SS Pin Current
vs Temperature
RUN/SS Pin Current
vs Temperature
Undervoltage Lockout Threshold
vs Temperature
Input and Shutdown Currents
vs Input Voltage INTVCC Load Regulation IEXTVCC vs Frequency
INPUT VOLTAGE (V)
0
INPUT CURRENT (µA)
SHUTDOWN CURRENT (µA)
800
1000
1400
1200
15 25
3611 G20
600
400
5 10 20 30
200
0
30
25
15
5
40
35
20
10
0
EXTVCC OPEN
EXTVCC = 5V
SHUTDOWN
INTVCC LOAD CURRENT (mA)
0
∆INTVCC (%)
0.10
0.20
0.30
40
3611 G21
0
–0.20
–0.10
–0.40
–0.30
10 20 30 50
FREQUENCY (KHz)
400
IEXTVCC (mA)
5
10
15
30
25
20
800
3611 G28
0500 600 700 1000900
VIN = 24V
VOUT = 2.5V
TEMPERATURE (°C)
–50 –25
0
EXTVCC SWITCH RESISTANCE (Ω)
4
10
0 50 75
3611 G22
2
8
6
25 100 125
TEMPERATURE (°C)
–50
FCB PIN CURRENT (µA)
–0.50
–0.25
0
25 75
3611 G23
–0.75
–1.00
–25 0 50 100 125
–1.25
–1.50
TEMPERATURE (°C)
–50 –25
–2
RUN/SS PIN CURRENT (µA)
0
3
0 50 75
3611 G24
–1
2
1
25 100 125
PULL-UP CURRENT
PULL-DOWN CURRENT
TEMPERATURE (°C)
–50
3.0
RUN/SS PIN CURRENT (μA)
3.5
4.0
4.5
5.0
–25 0 25 50
3611 G25
75 100 125
LATCHOFF ENABLE
LATCHOFF THRESHOLD
TEMPERATURE (°C)
–50
2.0
UNDERVOLTAGE LOCKOUT THRESHOLD (V)
2.5
3.0
3.5
4.0
–25 0 25 50
3611 G26
75 100 125
LTC3611
8
3611fd
pin Functions
PGND (Pins 1, 2, 3, 56, 57, 58, 59, 60, 61, 62, 63, 64,
65): Power Ground. Connect this pin closely to the (–)
terminal of CVCC and the (–) terminal of CIN.
SW (Pins 4, 5, 6, 7, 8, 9, 10, 11, 26, 55, 66): Switch
Node Connection to the Inductor. The (–) terminal of the
bootstrap capacitor, CB, also connects here. This pin swings
from a diode voltage drop below ground up to VIN.
PVIN (Pins 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,
23, 24, 25, 67): Main Input Supply. Decouple this pin to
power PGND with the input capacitance, CIN.
NC (Pin 27): No Connection.
SGND (Pins 28, 31, 32, 33, 34, 40, 42, 45, 46, 47, 48,
49, 50, 68): Signal Ground. All small-signal components
and compensation components should connect to this
ground, which in turn connects to PGND at one point.
BOOST (Pin 29): Boosted Floating Driver Supply. The (+)
terminal of the bootstrap capacitor, CB, connects here.
This pin swings from a diode voltage drop below INTVCC
up to VIN + INTVCC.
RUN/SS (Pin 30): Run Control and Soft-Start Input. A
capacitor to ground at this pin sets the ramp time to full
output current (approximately 3s/μF) and the time delay
for overcurrent latchoff (see Applications Information).
Forcing this pin below 0.8V shuts down the device.
VON (Pin 35): On-Time Voltage Input. Voltage trip point for
the on-time comparator. Tying this pin to the output volt-
age or an external resistive divider from the output makes
the on-time proportional to VOUT. The comparator input
defaults to 0.7V when the pin is grounded and defaults to
2.4V when the pin is tied to INTVCC. Tie this pin to INTVCC
in high VOUT applications to use a lower RON value.
PGOOD (Pin 36): Power Good Output. Open-drain logic
output that is pulled to ground when the output voltage
is not within ±10% of the regulation point.
VRNG (Pin 37): Current Limit Range Input. The voltage
at this pin adjusts maximum valley current and can be
set from 0.7V to 1V by a resistive divider from INTVCC.
It defaults to 0.7V if the VRNG pin is tied to ground which
results in a typical 10A current limit.
ITH (Pin 38): Current Control Threshold and Error Amplifier
Compensation Point. The current comparator threshold
increases with this control voltage. The voltage ranges
from 0V to 2.4V with 0.8V corresponding to zero sense
voltage (zero current).
FCB (Pin 39): Forced Continuous Input. Tie this pin to
ground to force continuous synchronous operation at low
load, to INTVCC to enable discontinuous mode operation at
low load or to a resistive divider from a secondary output
when using a secondary winding.
ION (Pin 41): On-Time Current Input. Tie a resistor from VIN
to this pin to set the one-shot timer current and thereby
set the switching frequency.
VFB (Pin 43): Error Amplifier Feedback Input. This pin
connects the error amplifier input to an external resistive
divider from VOUT.
EXTVCC (Pin 44): External VCC Input. When EXTVCC ex-
ceeds 4.7V, an internal switch connects this pin to INTVCC
and shuts down the internal regulator so that controller and
gate drive power is drawn from EXTVCC. Do not exceed
7V at this pin and ensure that EXTVCC < VIN.
SVIN (Pins 51, 52): Supply Pin for Internal PWM
Controller.
INTVCC (Pins 53, 54): Internal 5V Regulator Output. The
driver and control circuits are powered from this voltage.
Decouple this pin to power ground with a minimum of
4.7μF low ESR tantalum or ceramic capacitor.
LTC3611
9
3611fd
FunctionAl DiAgrAM
0.7V
1.4V
VRNG
–
+
–+
–+
–+
–
+
ION
VON
ICMP
0.7V
FCB EXTVCC SVIN
1µA
RON
VVON
IION
tON = (10pF) R
S Q
20k
IREV
×
(0.5 TO 2)
1V
SHDN
SWITCH
LOGIC
ON
FCNT
0.6V
–
+
4.7V
OV
1
240k
0.4V
ITH
CSS
EA
SS
0.6V
+
–
+
–
×3.3
RUN/SS
3611 FD
SGND
R1
RUN
SHDN
PGND
PGOOD
VFB
SW
PVIN
CIN
BOOST
M1
M2
INTVCC
–
+
–
+
UV
0.54V
OV
0.66V
6V
0.6V
REF
5V
REG
R2
2.4V
37
35 41 39 44
29
NC
27
4, 5, 6, 7, 8, 9,
10, 11, 26, 55,
66
12, 13, 14, 15,
16, 17, 18, 19,
20, 21, 22, 23,
24, 25, 67
53, 54
51, 52
1, 2, 3, 56, 57,
58, 59, 60, 61,
62, 63, 64, 65
28, 31, 32, 33, 34,
40, 42, 45, 46, 47,
48, 49, 50, 68
36
43
3038
VOUT
L1
COUT
CVCC
+
Q1Q3
Q4Q2
0.8V
ITHB
Q6
CB
DB
F
1.2µA
LTC3611
10
3611fd
operAtion
Main Control Loop
The LTC3611 is a high efficiency monolithic synchronous,
step-down DC/DC converter utilizing a constant on-time,
current mode architecture. It operates from an input voltage
range of 4.5V to 32V and provides a regulated output voltage
at up to 10A of output current. The internal synchronous
power switch increases efficiency and eliminates the need
for an external Schottky diode. In normal operation, the
top MOSFET is turned on for a fixed interval determined
by a one-shot timer OST. When the top MOSFET is turned
off, the bottom MOSFET is turned on until the current
comparator ICMP trips, restarting the one-shot timer and
initiating the next cycle. Inductor current is determined
by sensing the voltage between the PGND and SW pins
using the bottom MOSFET on-resistance. The voltage on
the ITH pin sets the comparator threshold corresponding
to inductor valley current. The error amplifier, EA, adjusts
this voltage by comparing the feedback signal VFB from
the output voltage with an internal 0.6V reference. If the
load current increases, it causes a drop in the feedback
voltage relative to the reference. The ITH voltage then
rises until the average inductor current again matches
the load current.
At light load, the inductor current can drop to zero and
become negative. This is detected by current reversal
comparator IREV which then shuts off M2 (see Func-
tional Diagram), resulting in discontinuous operation. Both
switches will remain off with the output capacitor supplying
the load current until the ITH voltage rises above the zero
current level (0.8V) to initiate another cycle. Discontinu-
ous mode operation is disabled by comparator F when
the FCB pin is brought below 0.6V, forcing continuous
synchronous operation.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to main-
tain regulation. The one-shot timer generates an on-time
that is proportional to the ideal duty cycle, thus holding
frequency approximately constant with changes in VIN.
The nominal frequency can be adjusted with an external
resistor, RON.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback volt-
age exits a ±10% window around the regulation point.
Furthermore, in an overvoltage condition, M1 is turned
off and M2 is turned on and held on until the overvoltage
condition clears.
Foldback current limiting is provided if the output is
shorted to ground. As VFB drops, the buffered current
threshold voltage ITHB is pulled down by clamp Q3 to
a 1V level set by Q4 and Q6. This reduces the inductor
valley current level to one sixth of its maximum value as
VFB approaches 0V.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2μA current source to charge
up an external soft-start capacitor, CSS. When this voltage
reaches 1.5V, the controller turns on and begins switching,
but with the ITH voltage clamped at approximately 0.6V
below the RUN/SS voltage. As CSS continues to charge,
the soft-start current limit is removed.
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most of
the internal controller circuitry is derived from the INTVCC
pin. The top MOSFET driver is powered from a floating
bootstrap capacitor, CB. This capacitor is recharged from
INTVCC through an external Schottky diode, DB, when
the top MOSFET is turned off. When the EXTVCC pin is
grounded, an internal 5V low dropout regulator supplies
the INTVCC power from VIN. If EXTVCC rises above 4.7V,
the internal regulator is turned off, and an internal switch
connects EXTVCC to INTVCC. This allows a high efficiency
source connected to EXTVCC, such as an external 5V sup-
ply or a secondary output from the converter, to provide
the INTVCC power. Voltages up to 7V can be applied to
EXTVCC for additional gate drive. If the input voltage is
low and INTVCC drops below 3.5V, undervoltage lockout
circuitry prevents the power switches from turning on.
LTC3611
11
3611fd
The basic LTC3611 application circuit is shown on the front
page of this data sheet. External component selection is
primarily determined by the maximum load current. The
LTC3611 uses the on-resistance of the synchronous power
MOSFET for determining the inductor current. The desired
amount of ripple current and operating frequency also
determines the inductor value. Finally, CIN is selected for its
ability to handle the large RMS current into the converter
and COUT is chosen with low enough ESR to meet the
output voltage ripple and transient specification.
VON and PGOOD
The LTC3611 has an open-drain PGOOD output that
indicates when the output voltage is within ±10% of the
regulation point. The LTC3611 also has a VON pin that
allows the on-time to be adjusted. Tying the VON pin high
results in lower values for RON which is useful in high VOUT
applications. The VON pin also provides a means to adjust
the on-time to maintain constant frequency operation in
applications where VOUT changes and to correct minor
frequency shifts with changes in load current.
VRNG Pin and ILIMIT Adjust
The VRNG pin is used to adjust the maximum inductor
valley current, which in turn determines the maximum
average output current that the LTC3611 can deliver. The
maximum output current is given by:
IOUT(MAX) = IVALLEY(MAX) + 1/2 ΔIL
The IVALLEY(MAX) is shown in the figure “Maximum Valley
Current Limit vs VRNG Voltage” in the Typical Performance
Characteristics.
An external resistor divider from INTVCC can be used to
set the voltage on the VRNG pin from 1V to 1.4V, or it can
be simply tied to ground force a default value equivalent
to 0.7V. Do not float the VRNG pin.
ApplicAtions inForMAtion
Operating Frequency
The choice of operating frequency is a trade-off between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3611 applications is de-
termined implicitly by the one-shot timer that controls the
on-time, tON, of the top MOSFET switch. The on-time is
set by the current into the ION pin and the voltage at the
VON pin according to:
tON =VVON
IION
(10pF)
Tying a resistor RON from VIN to the ION pin yields an
on-time inversely proportional to VIN. The current out of
the ION pin is:
IION =VIN
RON
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
f=VOUT
VVON RON(10pF) [Hz]
To hold frequency constant during output voltage changes,
tie the VON pin to VOUT or to a resistive divider from VOUT
when VOUT > 2.4V. The VON pin has internal clamps that
limit its input to the one-shot timer. If the pin is tied below
0.7V, the input to the one-shot is clamped at 0.7V. Similarly,
if the pin is tied above 2.4V, the input is clamped at 2.4V.
In high VOUT applications, tying VON to INTVCC so that
the comparator input is 2.4V results in a lower value for
LTC3611
12
3611fd
RON. Figures 1a and 1b show how RON relates to switching
frequency for several common output voltages.
Because the voltage at the ION pin is about 0.7V, the cur-
rent into this pin is not exactly inversely proportional to
VIN, especially in applications with lower input voltages.
To correct for this error, an additional resistor, RON2,
connected from the ION pin to the 5V INTVCC supply will
further stabilize the frequency.
RON2 =5V
0.7V RON
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
ApplicAtions inForMAtion
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the ITH pin to the VON pin and VOUT. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the ITH pin to the VON pin
as shown in Figure 2a. Place capacitance on the VON pin
to filter out the ITH variations at the switching frequency.
The resistor load on ITH reduces the DC gain of the error
amp and degrades load regulation, which can be avoided
by using the PNP emitter follower of Figure 2b.
RON (kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3611 F01a
VOUT = 3.3V
VOUT = 1.5V VOUT = 2.5V
RON (kΩ)
100
100
SWITCHING FREQUENCY (kHz)
1000
1000 10000
3611 F01b
VOUT = 3.3V
VOUT = 12V
VOUT = 5V
Figure 1a. Switching Frequency vs RON (VON = 0V)
Figure 1b. Switching Frequency vs RON (VON = INTVCC)
LTC3611
13
3611fd
Minimum Off-time and Dropout Operation
The minimum off-time, tOFF(MIN), is the smallest amount
of time that the LTC3611 is capable of turning on the bot-
tom MOSFET, tripping the current comparator and turning
the MOSFET back off. This time is generally about 250ns.
The minimum off-time limit imposes a maximum duty
cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
VIN(MIN) =VOUT
tON +tOFF(MIN)
tON
A plot of maximum duty cycle vs frequency is shown in
Figure 3.
Setting the Output Voltage
The LTC3611 develops a 0.6V reference voltage between
the feedback pin, VFB, and the signal ground as shown in
Figure 6. The output voltage is set by a resistive divider
according to the following formula:
VOUT =0.6V 1+R2
R1
⎛
⎝
⎜⎞
⎠
⎟
ApplicAtions inForMAtion
To improve the frequency response, a feedforward capaci-
tor C1 may also be used. Great care should be taken to
route the VFB line away from noise sources, such as the
inductor or the SW line.
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
ΔIL=VOUT
f L
⎛
⎝
⎜⎞
⎠
⎟1−VOUT
VIN
⎛
⎝
⎜⎞
⎠
⎟
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a trade-off between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of IOUT(MAX). The largest ripple current
occurs at the highest VIN. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
L=VOUT
fΔIL(MAX)
⎛
⎝
⎜⎞
⎠
⎟1−VOUT
VIN(MAX)
⎛
⎝
⎜⎞
⎠
⎟
CVON
0.01µF
RVON2
100k
RVON1
30k
CC
VOUT
RC
(2a)
(2b)
VON
ITH
LTC3611
CVON
0.01µF
RVON2
10k
Q1
2N5087
RVON1
3k
10k
CC3611 F02
VOUT
INTVCC RC
VON
ITH
LTC3611
2.0
1.5
1.0
0.5
0
0 0.25 0.50 0.75
3611 F03
1.0
DROPOUT
REGION
DUTY CYCLE (VOUT/VIN)
SWITCHING FREQUENCY (MHz)
Figure 2. Correcting Frequency Shift with Load Current Changes
Figure 3. Maximum Switching Frequency vs Duty Cycle
LTC3611
14
3611fd
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores.
A variety of inductors designed for high current, low volt-
age applications are available from manufacturers such as
Sumida, Panasonic, Coiltronics, Coilcraft and Toko.
CIN and COUT Selection
The input capacitance, CIN, is required to filter the square
wave current at the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
IRMS ≅IOUT(MAX)
VOUT
VIN
VIN
VOUT
– 1
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT(MAX)/2. This simple worst-case condition is
commonly used for design because even significant de-
viations do not offer much relief. Note that ripple current
ratings from capacitor manufacturers are often based on
only 2000 hours of life which makes it advisable to derate
the capacitor.
The selection of COUT is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The output ripple ΔVOUT is approximately bounded by:
ΔVOUT ≤ ΔILESR +1
8fCOUT
⎛
⎝
⎜⎞
⎠
⎟
Since ΔIL increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfied, the capacitance is adequate
for filtering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount pack-
ages. Special polymer capacitors offer very low ESR but
have lower capacitance density than other types. Tantalum
ApplicAtions inForMAtion
capacitors have the highest capacitance density but it is
important to only use types that have been surge tested
for use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can be used
in cost-sensitive applications providing that consideration
is given to ripple current ratings and long-term reliability.
Ceramic capacitors have excellent low ESR characteris-
tics but can have a high voltage coefficient and audible
piezoelectric effects. The high Q of ceramic capacitors with
trace inductance can also lead to significant ringing. When
used as input capacitors, care must be taken to ensure that
ringing from inrush currents and switching does not pose
an overvoltage hazard to the power switches and control-
ler. To dampen input voltage transients, add a small 5μF
to 50μF aluminum electrolytic capacitor with an ESR in
the range of 0.5Ω to 2Ω. High performance through-hole
capacitors may also be used, but an additional ceramic
capacitor in parallel is recommended to reduce the effect
of their lead inductance.
Top MOSFET Driver Supply (CB, DB)
An external bootstrap capacitor, CB, connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. This capacitor is charged through diode DB from
INTVCC when the switch node is low. When the top MOSFET
turns on, the switch node rises to VIN and the BOOST pin
rises to approximately VIN + INTVCC. The boost capacitor
needs to store about 100 times the gate charge required by
the top MOSFET. In most applications an 0.1μF to 0.47μF,
X5R or X7R dielectric capacitor is adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when in-
ductor current reverses. The load current at which current
reverses and discontinuous operation begins depends on
the amplitude of the inductor ripple current and will vary
LTC3611
15
3611fd
LTC3611
SGND 48
SGND 47
SGND 46
SGND 45
EXTVCC
44
VFB
43
SGND 42
ION
41 R4
SGND 40
FCB 39
ITH
38
VRNG
37
PGOOD 36
VON
35
SGND 34
SGND
SGND
PGND
33
PVIN
17
PVIN
18
PVIN
19
PVIN
20
PVIN
21
PVIN
22
PVIN
23
PVIN
24
PVIN
25
SW
SW
SW
26
NC
27
SGND
28
BOOST
29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
55
INTVCC
54
INTVCC
53
SVIN
52
SVIN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PVIN
VIN 12
PVIN
13
PVIN
14
PVIN
15
PVIN
16
3611 F04
CSEC
1µF
VOUT2
VOUT1
COUT
CIN
IN4148
OPTIONAL EXTVCC
CONNECTION
5V < VOUT2 < 7V
T1
1:N
R3
+
+
•
•
+
SGND
with changes in VIN. Tying the FCB pin below the 0.6V
threshold forces continuous synchronous operation, al-
lowing current to reverse at light loads and maintaining
high frequency operation.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to maintain a
flyback winding output when the primary is operating
in discontinuous mode. The secondary output VOUT2 is
normally set as shown in Figure 4 by the turns ratio N
of the transformer. However, if the controller goes into
discontinuous mode and halts switching due to a light
primary load current, then VOUT2 will droop. An external
resistor divider from VOUT2 to the FCB pin sets a minimum
voltage VOUT2(MIN) below which continuous operation is
forced until VOUT2 has risen above its minimum:
VOUT2(MIN) =0.6V 1+R4
R3
⎛
⎝
⎜⎞
⎠
⎟
ApplicAtions inForMAtion
Fault Conditions: Current Limit and Foldback
The LTC3611 has a current mode controller which inher-
ently limits the cycle-by-cycle inductor current not only
in steady state operation but also in transient. To further
limit current in the event of a short circuit to ground, the
LTC3611 includes foldback current limiting. If the output
falls by more than 25%, then the maximum sense voltage is
progressively lowered to about one sixth of its full value.
INTVCC Regulator and EXTVCC Connection
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3611. The INTVCC pin can supply up to 50mA
RMS and must be bypassed to ground with a minimum of
4.7μF tantalum or ceramic capacitor. Good bypassing is
necessary to supply the high transient currents required
by the MOSFET gate drivers.
Figure 4. Secondary Output Loop and EXTVCC Connection
LTC3611
16
3611fd
The EXTVCC pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTVCC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTVCC
pin to INTVCC. INTVCC power is supplied from EXTVCC
until this pin drops below 4.5V. Do not apply more than
7V to the EXTVCC pin and ensure that EXTVCC ≤ VIN. The
following list summarizes the possible connections for
EXTVCC:
1. EXTVCC grounded. INTVCC is always powered from the
internal 5V regulator.
2. EXTVCC connected to an external supply. A high efficiency
supply compatible with the MOSFET gate drive require-
ments (typically 5V) can improve overall efficiency.
3. EXTVCC connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
will start-up using the internal linear regulator until the
boosted output supply is available.
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the LTC3611
as well as a timer for soft-start and overcurrent latchoff.
Pulling the RUN/SS pin below 0.8V puts the LTC3611 into
a low quiescent current shutdown (IQ < 30μA). Releasing
the pin allows an internal 1.2μA current source to charge
up the external timing capacitor, CSS. If RUN/SS has
been pulled all the way to ground, there is a delay before
starting of about:
tDELAY =1.5V
1.2µACSS =1.3s/µF
( )
CSS
When the voltage on RUN/SS reaches 1.5V, the LTC3611
begins operating with a clamp on ITH of approximately
0.9V. As the RUN/SS voltage rises to 3V, the clamp on ITH
is raised until its full 2.4V range is available. This takes an
ApplicAtions inForMAtion
additional 1.3s/μF, during which the load current is folded
back until the output reaches 75% of its final value.
After the controller has been started and given adequate
time to charge up the output capacitor, CSS is used as a
short-circuit timer. After the RUN/SS pin charges above 4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8μA current then
begins discharging CSS. If the fault condition persists until
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
The overcurrent protection timer requires that the soft-
start timing capacitor, CSS, be made large enough to
guarantee that the output is in regulation by the time CSS
has reached the 4V threshold. In general, this will depend
upon the size of the output capacitance, output voltage
and load current characteristic. A minimum soft-start
capacitor can be estimated from:
CSS > COUT VOUT RSENSE (10–4 [F/V s])
Generally 0.1μF is more than sufficient.
Overcurrent latchoff operation is not always needed or
desired. Load current is already limited during a short
circuit by the current foldback circuitry and latchoff op-
eration can prove annoying during troubleshooting. The
feature can be overridden by adding a pull-up current
greater than 5μA to the RUN/SS pin. The additional cur-
rent prevents the discharge of CSS during a fault and also
shortens the soft-start period. Using a resistor to VIN as
shown in Figure 5a is simple, but slightly increases shut-
down current. Connecting a resistor to INTVCC as shown
in Figure 5b eliminates the additional shutdown current,
but requires a diode to isolate CSS. Any pull-up network
must be able to pull RUN/SS above the 4.2V maximum
threshold of the latchoff circuit and overcome the 4μA
maximum discharge current.
LTC3611
17
3611fd
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3611 circuits:
1. DC I2R losses. These arise from the resistance of the
internal resistance of the MOSFETs, inductor and PC board
traces and cause the efficiency to drop at high output
currents. In continuous mode the average output current
flows through L, but is chopped between the top and bot-
tom MOSFETs. If the two MOSFETs have approximately
the same RDS(ON), then the DC I2R loss for one MOSFET
can simply be determined by [RDS(ON) + RL] • IO.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the
input voltage, load current, driver strength and MOSFET
capacitance, among other factors. The loss is significant
at input voltages above 20V and can be estimated from:
Transition Loss ≅ (1.7A–1) VIN2 IOUT CRSS f
ApplicAtions inForMAtion
3. INTVCC current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by sup-
plying INTVCC current through the EXTVCC pin from a
high efficiency source, such as an output derived boost
network or alternate supply if available.
4. CIN loss. The input capacitor has the difficult job of filtering
the large RMS input current to the regulator. It must have
a very low ESR to minimize the AC I2R loss and sufficient
capacitance to prevent the RMS current from causing ad-
ditional upstream losses in fuses or batteries.
Other losses, including COUT ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If
you make a change and the input current decreases, then
the efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ΔILOAD (ESR), where ESR is the effective series
resistance of COUT. ΔILOAD also begins to charge or dis-
charge COUT generating a feedback error signal used by the
regulator to return VOUT to its steady-state value. During
this recovery time, VOUT can be monitored for overshoot
or ringing that would indicate a stability problem. The ITH
pin external components shown in Figure 6 will provide
adequate compensation for most applications. For a
detailed explanation of switching control loop theory see
Application Note 76.
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
3.3V OR 5V RUN/SS
VIN
INTVCC
RUN/SS
D1
(5a) (5b)
D2*
CSS
RSS*
CSS
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
RSS*
3611 F05
2N7002
LTC3611
18
3611fd
Design Example
As a design example, take a supply with the following
specifications: VIN = 5V to 36V (12V nominal), VOUT =
2.5V ±5%, IOUT(MAX) = 10A, f = 550kHz. First, calculate
the timing resistor with VON = VOUT:
RON =2.5V
(2.4) 550kHz
( )
10pF
( )
=187k
and choose the inductor for about 40% ripple current at
the maximum VIN:
L=2.5V
550kHz
( )
0.4
( )
10A
( )
1−2.5V
36V
⎛
⎝
⎜⎞
⎠
⎟=1µH
Selecting a standard value of 1μH results in a maximum
ripple current of:
ΔIL=2.5V
550kHz
( )
1µH
( )
1– 2.5V
12V
⎛
⎝
⎜⎞
⎠
⎟=3.6A
ApplicAtions inForMAtion
Next, set up VRNG voltage and check the ILIMIT. Tying
VRNG to 1V will set the typical current limit to 15A, and
tying VRNG to GND will result in a typical current around
10A. CIN is chosen for an RMS current rating of about 5A
at 85°C. The output capacitors are chosen for a low ESR
of 0.013Ω to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
ΔVOUT(RIPPLE) = ΔIL(MAX) (ESR)
= (3.6A) (0.013Ω) = 47mV
However, a 0A to 10A load step will cause an output
change of up to:
ΔVOUT(STEP) = ΔILOAD (ESR) = (10A) (0.013Ω) =130mV
An optional 22μF ceramic output capacitor is included
to minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 6.
Figure 6. Design Example: 5V to 32V Input to 2.5V/10A at 550kHz
VOUT
2.5V AT
10A
GND
GND
VIN
VIN
5V TO 32V
COUT1
100µF
×2
C5
22µF
6.3V
L1
1µH
CIN
4.7µF
50V
×2
C6
100µF
50V
+
(OPTIONAL)
(OPTIONAL)
CIN = MURATA GRM32ER71H475K
COUT = MURATA GRM43SR60J107M
L1 = COOPER HCP0703-IRO
C5: MURATA GRM31CR60J226KE19
KEEP POWER AND SIGNAL GROUNDS SEPARATE.
CONNECT TO ONE POINT.
LTC3611
SGND 48
SGND 47
SGND 46
SGND 45
EXTVCC
EXTVCC
44
VFB
43
SGND 42
ION
41
SGND 40
FCB 39
ITH
38
VRNG
37
PGOOD 36
VON
35
SGND 34
SGND
VOUT
VOUT
33
PVIN
17
PVIN
18
PVIN
19
PVIN
20
PVIN
21
PVIN
22
PVIN
23
PVIN
24
PVIN
25
SW
SW
SW
SW
26
NC
27
SGND
28
BOOST
29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
55
INTVCC
INTVCC
INTVCC
54
INTVCC
INTVCC
53
SVIN
VIN
52
SVIN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PVIN
VIN
2Ω
12
PVIN
13
PVIN
14
PVIN
15
PVIN
16
3611 F06
CF
0.1µF
50V
CVCC
4.7µF
6.3V
CON
0.01µF
C4
0.01µF
C2C1
0.01µF
CB1
0.22µF
DB
CMDSH-3
CSS
0.1µF
VIN
RSS1
510k
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
R1
9.5k
1%
R2
30.1k
1%
RVON
0Ω
RPG1
100k
39.2k
RON
182k
1%
R5
12.5k
R3
11k
CC1
680pF
CC2
100pF
(OPTIONAL)
SGNDPGND
LTC3611
19
3611fd
ApplicAtions inForMAtion
How to Reduce SW Ringing
As with any switching regulator, there will be voltage ring-
ing on the SW node, especially for high input voltages.
The ringing amplitude and duration is dependent on the
switching speed (gate drive), layout (parasitic inductance)
and MOSFET output capacitance. This ringing contributes
to the overall EMI, noise and high frequency ripple. One
way to reduce ringing is to optimize layout. A good layout
minimizes parasitic inductance. Adding RC snubbers from
SW to GND is also an effective way to reduce ringing.
Finally, adding a resistor in series with the BOOST pin
will slow down the MOSFET turn-on slew rate to dampen
ringing, but at the cost of reduced efficiency. Note that
since the IC is buffered from the high frequency transients
by PCB and bondwire inductances, the ringing by itself is
normally not a concern for controller reliability.
PC Board Layout Checklist
When laying out a PC board follow one of the two sug-
gested approaches. The simple PC board layout requires
a dedicated ground plane layer. Also, for higher currents,
a multilayer board is recommended to help with heat
sinking of power components.
• The ground plane layer should not have any traces and
it should be as close as possible to the layer with the
LTC3611.
• Place CIN and COUT all in one compact area, close to
the LTC3611. It may help to have some components
on the bottom side of the board.
• Keep small-signal components close to the LTC3611.
• Ground connections (including LTC3611 SGND and
PGND) should be made through immediate vias to
the ground plane. Use several larger vias for power
components.
• Use a compact plane for the switch node (SW) to improve
cooling of the MOSFETs and to keep EMI down.
• Use planes for VIN and VOUT to maintain good voltage
filtering and to keep power losses low.
• Flood all unused areas on all layers with copper. Flood-
ing with copper reduces the temperature rise of power
components. Connect these copper areas to any DC
net (VIN, VOUT, GND or to any other DC rail in your
system).
When laying out a printed circuit board without a ground
plane, use the following checklist to ensure proper opera-
tion of the controller. These items are also illustrated in
Figure 7.
• Segregate the signal and power grounds. All small-
signal components should return to the SGND pin at
one point, which is then tied to the PGND pin.
• Connect the input capacitor(s), CIN, close to the IC. This
capacitor carries the MOSFET AC current.
• Keep the high dV/dT SW, BOOST and TG nodes away
from sensitive small-signal nodes.
• Connect the INTVCC decoupling capacitor, CVCC, closely
to the INTVCC and PGND pins.
• Connect the top driver boost capacitor, CB, closely to
the BOOST and SW pins.
• Connect the VIN pin decoupling capacitor, CF
, closely
to the VIN and PGND pins.
LTC3611
20
3611fd
ApplicAtions inForMAtion
Figure 7. LTC3611 Layout Diagram
LTC3611
SGND 48
SGND 47
SGND 46
SGND 45
EXTVCC
44
VFB
43
SGND 42
ION
41
SGND 40
FCB 39
ITH
38
VRNG
37
PGOOD 36
VON
35
SGND 34
33
PVIN
17
PVIN
18
PVIN
19
PVIN
20
PVIN
21
PVIN
22
PVIN
23
PVIN
24
PVIN
25
SW
26
NC
27
SGND
28
BOOST
29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
SW
55
INTVCC
54
INTVCC
53
SVIN
52
SVIN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PVIN
12
PVIN
13
PVIN
14
PVIN
15
PVIN
16
3611 F07
CIN
COUT
VOUT
CVCC
CB
DBCSS
R1
RF
RON
R2
RC
CC1
CC2
LTC3611
21
3611fd
3.3V Input to 1.5V/10A at 750kHz
typicAl ApplicAtions
LTC3611
SGND 48
SGND 47
SGND 46
SGND 45
EXTVCC
44
VFB
43
SGND 42
ION
41
SGND 40
FCB 39
ITH
38
VRNG
37
PGOOD 36
VON
35
SGND 34
SGND
PGND SGND
VOUT
1.5V AT
10A
VOUT
VOUT
GND
GND
33
PVIN
17
PVIN
18
PVIN
19
PVIN
20
PVIN
21
PVIN
22
PVIN
23
PVIN
24
PVIN
25
SW
SW
26
NC
27
SGND
28
BOOST
29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
55
INTVCC
INTVCC
INTVCC
54
INTVCC
INTVCC
53
SVIN
VIN2 = 5V
52
SVIN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PVIN
VIN
VIN 12
PVIN
13
PVIN
14
PVIN
15
PVIN
16
3611 TA02
COUT1
100µF
×2
C5
22µF
6.3V
L1
0.47µH
CF
0.1µF
50V
CIN
4.7µF
50V
×2
CVCC
4.7µF
6.3V
C6
100µF
50V
+
(OPTIONAL)
(OPTIONAL)
C5: TAIYO YUDEN JMK316BJ226ML-T
CIN: MURATA GRM31CR71H475K
COUT1: MURATA GRM435R60J107M
L1: TOKO FDV0630-R47M
KEEP POWER AND SIGNAL GROUNDS SEPARATE.
CONNECT TO ONE POINT.
CVON
CON
0.01µF
C4
0.01µF
C2C1
CB1
0.22µF 2Ω
CSS
0.1µF
VIN
RSS1
510k
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
R1
20.43k
1%
R2
30.1k
1%
RPG1
100k
11k
39.2k
RON
113k
1%
R5
12.5k
CC1
1500pF
CC2
100pF
VIN
3.3V
LTC3611
22
3611fd
5V to 24V Input to 1.2V/10A at 550kHz
typicAl ApplicAtions
VOUT
1.2V AT
10A
GND
GND
VIN
COUT1
100µF
×2
C5
22µF
6.3V
L1
0.47µH
CIN
4.7µF
50V
×2
C6
100µF
50V
+
(OPTIONAL)
(OPTIONAL)
C5: TAIYO YUDEN JMK316BJ226ML-T
CIN: MURATA GRM32ER71H475K
COUT1: MURATA GRM435R60J167M
L1: TOKO HCPO703-OR47
KEEP POWER AND SIGNAL GROUNDS SEPARATE.
CONNECT TO ONE POINT.
LTC3611
SGND 48
SGND 47
SGND 46
SGND 45
EXTVCC
EXTVCC
44
VFB
43
SGND 42
ION
41
SGND 40
FCB 39
ITH
38
VRNG
37
PGOOD 36
VON
35
SGND 34
SGND
SGND
PGND SGND
VOUT
VOUT
33
PVIN
17
PVIN
18
PVIN
19
PVIN
20
PVIN
21
PVIN
22
PVIN
23
PVIN
24
PVIN
25
SW
26
NC
27
SGND
28 29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
55
INTVCC
INTVCC
54
INTVCC
INTVCC
53
SVIN
VIN
52
SVIN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PVIN
VIN
12
PVIN
13
PVIN
14
PVIN
15
PVIN
16
3611 TA03
CF
0.1µF
50V
CVCC
4.7µF
6.3V
CON
0.01µF
C4
0.01µF
C2C1
DB
CMDSH-3
CSS
0.1µF
2Ω
VIN
RSS1
510k
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
R1
30k
1%
R2
30.1k
1%
RPG1
100k
39.2k
11k
RON
182k
1%
R5
4.75k
CC1
1500pF
CC2
100pF
CVON
(OPTIONAL)
VIN
5V TO 24V
SW
BOOST
INTVCC
CB1
0.22µF
LTC3611
23
3611fd
typicAl ApplicAtions
5V to 28V Input to 1.8V/10A All Ceramic 1MHz
VOUT
1.8V AT
10A
GND
VIN
VIN
5V TO 28V
COUT
100µF
×2
C5
22µF
6.3V
L1
0.68µH
CIN
4.7µF
50V
×2
(OPTIONAL)
COUT: MURATA GRM32ER60J107ME20L
CIN: MURATA GRM32ER71H475K
L1: VISHAY IHLP2525CZERR68M01
KEEP POWER AND SIGNAL GROUNDS SEPARATE.
CONNECT TO ONE POINT.
LTC3611
SGND 48
SGND 47
SGND 46
SGND 45
EXTVCC
EXTVCC
44
VFB
43
SGND 42
ION
41
SGND 40
FCB 39
ITH
38
VRNG
37
PGOOD 36
VON
35
SGND 34
SGND
PGND SGND
VOUT
VOUT
33
PVIN
17
PVIN
18
PVIN
19
PVIN
20
PVIN
21
PVIN
22
PVIN
23
PVIN
24
PVIN
25
SW
26
NC
27
SGND
28 29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
55
INTVCC
INTVCC
54
INTVCC
INTVCC
53
SVIN
VIN
52
SVIN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PVIN
VIN
12
PVIN
13
PVIN
14
PVIN
15
PVIN
16
3611 TA04
CF
0.1µF
50V
CVCC
4.7µF
6.3V
CON
0.01µF
C4
0.01µF
C2
C1
47pF
DB
CMDSH-3
CSS
0.1µF
VIN
RSS1
510k
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
R1
10k
1%
R2
20k
1%
RPG1
100k
9.31k 39.2k
RON
102k
1%
R5
12.7k
CC1
680pF
CC2
100pF
CVON
(OPTIONAL)
SW
BOOST
INTVCC
CB1
0.22µF
2Ω
LTC3611
24
3611fd
pAckAge Description
WP Package
64-Lead QFN Multipad (9mm × 9mm)
(Reference LTC DWG # 05-08-1812 Rev A)
9.00
BSC
9.00
BSC
17
16
32
33
BOTTOM VIEW
(BOTTOM METALLIZATION DETAILS)
TOP VIEW 0.90 ± 0.10
// ccc C
0.20 REF
0.00 – 0.05
0.30 – 0.50
WP64 QFN REV A 0707
0.20 – 0.30
NX b
SEATING PLANE
6
5
0.08 C
aaa C
aaa C
MAC Bbbb
NX
A
B
2x
2x
3.30
1.19 49
48
50 51 52 53 54 64
1
1.39
1.17
0.53
(2x)
1.92
2.01
3.06
4.10
3.30
0.30
(2x)
0.95
3.50 0.87
3.60
0.50
1.81
2.04
2.98
3.99
4.53
NOTE:
1. DIMENSIONING AND TOLERANCING CONFORM TO ASME Y14.5M-1994
2. ALL DIMENSIONS ARE IN MILLIMETERS, ANGLES ARE IN DEGREES (°)
3. N IS THE TOTAL NUMBER OF TERMINALS
4. THE LOCATION OF THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING
CONVENTION CONFORMS TO JEDEC PUBLICATION 95 SPP-002
6COPLANARITY APPLIES TO THE TERMINALS AND ALL OTHER SURFACE
METALLIZATION
5DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED
BETWEEN 0.15mm AND 0.30mm FROM THE TERMINAL TIP.
SYMBOL
aaa
bbb
ccc
TOLERANCE
0.15
0.10
0.10
3.85
1.42
PAD 1
CORNER
RECOMMENDED SOLDER PAD LAYOUT
TOP VIEW
0.30 – 0.50
3.30
1.19
1.17
1.92
2.01
3.06
4.10
0.53
(2x)
1.39
3.30
0.30
(2x)
2.30
3.50
0.87
3.60
PIN 1
0.50
1.81
2.04
2.98
3.99
4.53
0.20 – 0.30
3.85 1.42
0.95
1.30
1.30
LTC3611
25
3611fd
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
revision history
REV DATE DESCRIPTION PAGE NUMBER
D 06/10 Updated SW voltage range in Absolute Maximum Ratings.
Note 4 updated.
2
4
(Revision history begins at Rev D)
LTC3611
26
3611fd
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
LINEAR TECHNOLOGY CORPORATION 2008
LT 0610 REV D • PRINTED IN USA
relAteD pArts
typicAl ApplicAtion
PART NUMBER DESCRIPTION COMMENTS
LTC1778 No RSENSE Current Mode Synchronous Step-Down Controller Up to 97% Efficiency, VIN: 4V to 36V, 0.8V ≤ VOUT ≤ (0.9)(VIN),
IOUT Up to 20A
LTC3411 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT: 0.8V, IQ: 60μA, ISD: <1μA,
MS Package
LTC3412 2.5A (IOUT) 4MHz Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN): 0.8V, IQ: 60mA,
ISD: <1mA, TSSOP16E
LTC3414 4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64μA,
ISD: <1μA, TSSOP20E Package
LTC3418 8A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, Thermally
Enhanced 38-Lead QFN Package
LTC3610 12A Current Mode Monolithic Synchronous Step-Down Converter Up to 24V Input (28V Maximum), Current Mode Extremely Fast
Transient Response
LTC3770 Fast, No RSENSE Step-Down Synchronous Controller with
Margining, Tracking, PLL
±0.67% 0.6V Reference Voltage; Programmable Margining;
True Current Mode; 4V ≤ VIN ≤ 32V
LTC3778 Low VOUT, No RSENSE Synchronous Step-Down Controller 0.6V ≤ VOUT ≤ (0.9) VIN, 4V ≤ VIN ≤ 36V, IOUT Up to 20A
LT3800 60V Synchronous Step-Down Controller Current Mode, Output Slew Rate Control
LTM4600HV 10A Complete Switch Mode Power Supply 92% Efficiency, VIN: 4.5V to 28V, VOUT: 0.6V, True Current Mode
Control, Ultrafast Transient Response
LTM4601HV 12A Complete Switch Mode Power Supply 92% Efficiency, VIN: 4.5V to 28V, VOUT: 0.6V, True Current Mode
Control, Ultrafast Transient Response
LTM4602HV 6A Complete Switch Mode Power Supply 92% Efficiency, VIN: 4.5V to 28V, VOUT: 0.6V, True Current Mode
Control, Ultrafast Transient Response
LTM4603HV 6A Complete Switch Mode Power Supply 93% Efficiency, VIN: 4.5V to 28V, with PLL, Output Tracking and Margining
14V to 32V Input to 12V/5A at 500kHz
VOUT
12V AT
5A
GND
GND
VIN
VIN
14V TO 32V
COUT
180µF
16V
C5
22µF
25V
L1
4.7µH
CIN
4.7µF
50V
×2
+
C6
100µF
50V
+
(OPTIONAL)
(OPTIONAL)
CIN: GRM31CR71H475K
COUT: SANYO 16SVP180MX
L1: HCP0703-4R7-R
KEEP POWER AND SIGNAL GROUNDS SEPARATE.
CONNECT TO ONE POINT.
LTC3611
SGND 48
SGND 47
SGND 46
SGND 45
EXTVCC
EXTVCC
44
VFB
43
SGND 42
ION
41
SGND 40
FCB 39
ITH
38
VRNG
37
PGOOD 36
VON
35
SGND 34
SGND
PGND SGND
INTVCC
VOUT
33
PVIN
17
PVIN
18
PVIN
19
PVIN
20
PVIN
21
PVIN
22
PVIN
23
PVIN
24
PVIN
25
SW
26 27
SGND
28 29
RUN/SS
30
SGND
31
SGND
32
PGND
64
PGND
63
PGND
62
PGND
61
PGND
60
PGND
59
PGND
58
PGND
57
PGND
56
SW
55
INTVCC
INTVCC
54
INTVCC
INTVCC
53
SVIN
VIN
52
SVIN
51
SGND
50
SGND
49
PGND
1
PGND
2
PGND
3
SW
4
SW
5
SW
6
SW
7
SW
8
SW
9
SW
10
SW
11
PVIN
VIN
12
PVIN
13
PVIN
14
PVIN
15
PVIN
16
3611 TA05
CF
0.1µF
50V
CVCC
4.7µF
6.3V
CON
0.01µF
C4
0.01µF
C2C1
RUN/SS
CSS
0.1µF
VIN
RSS1
510k
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
R1
1.58k
1%
R2
30.1k
1%
RPG1
100k
RON
1M
1%
R5
20k
CC1
560pF
CC2
100pF
(OPTIONAL)
CVON
(OPTIONAL)
NC
DB
CMDSH-3
SW
BOOST
INTVCC
CB1
0.22µF
