ISENSE
ADJ
VIN
PGATE
FB
EN
LM3489 GND
PGND
1
2
3
4
5
6
7
8
+ +
Q1
VIN VOUT
CIN1 COUT
D1
L
RADJ
RIS
CADJ
R1
R2
Cff
CIN2
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An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM3489
,
LM3489-Q1
SNVS443C –MAY 2006–REVISED DECEMBER 2016
LM3489x Hysteretic PFET Buck Controller With Enable Pin
1
1 Features
1• Qualified for Automotive Parts
• AEC-Q100 Qualified With the Following Results:
– Device Temperature Grade 1: –40°C to 125°C
Ambient Operating Temperature Range
– Device HBM ESD Classification Level 2
– Device CDM ESD Classification Level C5
• Easy-to-Use Control Methodology
• No Control Loop Compensation Required
• Wide 4.5-V to 35-V Input Range
• 1.239 V to VIN Adjustable Output Range
• High Efficiency: 93%
• ±1.3% (±2% Over Temperature) Internal
Reference
• 100% Duty Cycle Operation
• Maximum Operation Frequency > 1 MHz
• Current Limit Protection
• Dedicated Enable Pin (on if Unconnected)
• Shutdown Mode Draws Only 7-µA Supply Current
• 8-Pin VSSOP Package
•
2 Applications
• Set-Top Boxes
• DSL or Cable Modems
• PC/IA
• Auto PCs
• TFT Monitors
• Battery-Powered Portable Applications
• Distributed Power Systems
• Always-On Power
• High-Power LED Drivers
• Automotive
3 Description
The LM3489 device is a high-efficiency PFET
switching regulator controller that can be used to
quickly and easily develop a small, cost-effective,
switching buck regulator for a wide range of
applications. The hysteretic control architecture
provides for simple design without any control loop
stability concerns using a wide variety of external
components. The PFET architecture also allows for
low component count as well as ultra-low dropout,
100% duty cycle operation. Another benefit is high
efficiency operation at light loads without an increase
in output ripple. A dedicated enable pin provides a
shutdown mode drawing only 7 µA. Leaving the
enable pin unconnected defaults to on.
Current limit protection can be implemented by
measuring the voltage across the PFET’s RDS(ON),
thus eliminating the need for a sense resistor. A
sense resistor may be used to improve current limit
accuracy if desired. The cycle-by-cycle current limit
can be adjusted with a single resistor, ensuring safe
operation over a range of output currents.
Device Information(1)
PART NUMBER PACKAGE BODY SIZE (NOM)
LM3489
LM3489-Q1 VSSOP (8) 3.00 mm × 3.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application Circuit
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Table of Contents
1 Features.................................................................. 1
2 Applications ........................................................... 1
3 Description............................................................. 1
4 Revision History..................................................... 2
5 Pin Configuration and Functions......................... 3
6 Specifications......................................................... 4
6.1 Absolute Maximum Ratings ...................................... 4
6.2 ESD Ratings: LM3489 .............................................. 4
6.3 ESD Ratings: LM3489-Q1 ........................................ 4
6.4 Recommended Operating Conditions....................... 4
6.5 Thermal Information.................................................. 5
6.6 Electrical Characteristics........................................... 5
6.7 Typical Characteristics.............................................. 6
7 Detailed Description.............................................. 9
7.1 Overview................................................................... 9
7.2 Functional Block Diagram......................................... 9
7.3 Feature Description................................................... 9
7.4 Device Functional Mode ......................................... 14
8 Application and Implementation ........................ 15
8.1 Application Information............................................ 15
8.2 Typical Application.................................................. 15
9 Power Supply Recommendations...................... 19
10 Layout................................................................... 19
10.1 Layout Guidelines ................................................. 19
10.2 Layout Examples................................................... 19
11 Device and Documentation Support................. 20
11.1 Related Links ........................................................ 20
11.2 Receiving Notification of Documentation Updates 20
11.3 Community Resources.......................................... 20
11.4 Trademarks........................................................... 20
11.5 Electrostatic Discharge Caution............................ 20
11.6 Glossary................................................................ 20
12 Mechanical, Packaging, and Orderable
Information........................................................... 20
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision B (February 2013) to Revision C Page
• Added ESD Ratings table, Feature Description section, Device Functional Modes,Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section.................................................................................................. 1
• Added AEC-Q100 Qualification bullets to Features............................................................................................................... 1
• Deleted Lead temperature (Vapor phase and Infrared maximums)....................................................................................... 4
• Added Thermal Information table........................................................................................................................................... 5
Changes from Revision A (February 2013) to Revision B Page
• Changed layout of National Semiconductor Data Sheet to TI format .................................................................................... 1
1ISENSE 8 VIN
2GND 7 PGATE
3EN 6 PGND
4FB 5 ADJ
Not to scale
3
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5 Pin Configuration and Functions
DGK Package
8-Pin VSSOP
Top View
Pin Functions
PIN I/O DESCRIPTION
NO. NAME
1 ISENSE I The current sense input pin. This pin must be connected to the PFET drain terminal directly or
through a series resistor up to 600 Ωfor 28 V > VIN > 35 V.
2 GND — Signal ground
3 EN I Enable pin. Connect EN pin to ground to shutdown the part or float to enable operation (Internally
pulled high). This pin can also be used to perform UVLO function.
4 FB I The feedback input. Connect the FB to a resistor voltage divider between the output and GND for an
adjustable output voltage.
5 ADJ I Current limit threshold adjustment. Connected to an internal 5.5-µA current source. A resistor is
connected between this pin and VIN. The voltage across this resistor is compared with the ISENSE
pin voltage to determine if an overcurrent condition has occurred.
6 PGND — Power ground
7 PGATE O Gate drive output for the external PFET. PGATE swings between VIN and VIN 5-V.
8 VIN I Power supply input pin
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(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
(2) This pin is internally pulled high and clamped at 8 V (typical). The absolute maximum and operating maximum rating specifies the input
level allowed for an external voltage source applied to this pin without triggering the internal clamp with margin.
(3) The maximum allowable power dissipation is a function of the maximum junction temperature, TJ_MAX, the junction-to-ambient thermal
resistance, RθJA and the ambient temperature, TA. The maximum allowable power dissipation at any ambient temperature is calculated
using: PD= (TJ– TA) / RθJA. Exceeding the maximum allowable power dissipation will lead to excessive die temperature.
6 Specifications
6.1 Absolute Maximum Ratings
See (1).MIN MAX UNIT
VIN voltage –0.3 36 V
PGATE voltage –0.3 36 V
FB voltage –0.3 5 V
ISENSE voltage –1 36 V
–1 (<100
ns)
ADJ voltage –0.3 36 V
EN voltage(2) –0.3 6 V
Power dissipation, TA= 25°C(3) 417 mW
Junction temperature, TJ–40 150 °C
Storage temperature, Tstg –65 150 °C
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
(2) JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
6.2 ESD Ratings: LM3489 VALUE UNIT
V(ESD) Electrostatic discharge Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001(1) ±2000 V
Charged-device model (CDM), per JEDEC specification JESD22-C101(2) ±750
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
6.3 ESD Ratings: LM3489-Q1 VALUE UNIT
V(ESD) Electrostatic discharge Human body model (HBM), per AEC Q100-002(1) ±2000 V
Charged device model (CDM), per AEC Q100-011 ±750
(1) This pin is internally pulled high and clamped at 8 V (typical). The absolute maximum and operating maximum rating specifies the input
level allowed for an external voltage source applied to this pin without triggering the internal clamp with margin.
(2) High junction temperatures degrade operating lifetimes. Operating lifetime is de-rated for junction temperatures greater than 125°C.
6.4 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted) MIN MAX UNIT
VIN Supply voltage 4.5 35 V
EN voltage(1) 5.5 V
TJOperating junction temperature(2) LM3489 –40 125 °C
LM3489-Q1 –40 150 °C
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(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
6.5 Thermal Information
THERMAL METRIC(1) LM3489
UNITDGK (VSSOP)
8 PINS
RθJA Junction-to-ambient thermal resistance 163.7 °C/W
RθJC(top) Junction-to-case (top) thermal resistance 56.6 °C/W
RθJB Junction-to-board thermal resistance 83.3 °C/W
ψJT Junction-to-top characterization parameter 5.4 °C/W
ψJB Junction-to-board characterization parameter 82 °C/W
RθJC(bot) Junction-to-case (bottom) thermal resistance — °C/W
(1) The VFB is the trip voltage at the FB pin when PGATE switches from high to low.
(2) Bias current flows out from the FB pin.
(3) A 1000-pF capacitor is connected between VIN and PGATE.
6.6 Electrical Characteristics
Typical values correspond to TJ= 25°C. Minimum and maximum limits apply over TJ= –40°C to 125°C for the LM3489 and
LM3489-Q1. VIN = 12 V, VISNS = VIN −1 V, and VADJ = VIN −1.1 V (unless otherwise noted).
PARAMETER TEST CONDITIONS MIN TYP MAX UNIT
ISHDN Shutdown input supply
current EN = 0 V 7 15 µA
VEN Enable threshold voltage Enable rising 1.15 1.5 1.85 V
VEN_HYST Enable threshold hysteresis 130 mV
IQQuiescent current at ground
pin FB = 1.5 V (not switching) 280 400 µA
VFB Feedback voltage(1) 1.214 1.239 1.264 V
VHYST Comparator hysteresis 10 20 mV
VCL_OFFSET Current limit comparator
offset VFB = 1 V –20 0 20 mV
ICL_ADJ Current limit ADJ current
source VFB = 1.5 V 3 5.5 7 µA
TCL Current limit one-shot off-
time VADJ = 11.5 V, VISNS = 11 V, VFB = 1 V 6 9 14 µs
RPGATE Driver resistance Source, ISOURCE = 100 mA 5.5 Ω
Sink, ISINK = 100 mA 8.5
IPGATE Driver output current Source, VIN = 7 V, PGATE = 3.5 V 0.44 A
Sink, VIN = 7 V, PGATE = 3.5 V 0.1
IFB FB pin bias current(2) VFB = 1 V 300 750 nA
TONMIN_NOR Minimum ON time in normal
operation VISNS = VADJ + 0.1 V, Cload on OUT = 1000 pF(3) 100 ns
TONMIN_CL Minimum ON time in current
limit VISNS = VADJ – 0.1 V, VFB = 1 V,
Cload on OUT = 1000 pF(3) 200 ns
%VFB/ΔVIN Feedback voltage line
regulation 4.5 V ≤VIN ≤35 V 0.01% V
-40 -20 0 20 40 60 80 100 140
JUNCTION TEMPERATURE (°C)
120
2
6
10
14
18
VHYST (mV)
IOUT = 0
VIN = 12 V
-40 -20 0 20 40 60 80 100 140
JUNCTION TEMPERATURE (°C)
120
4.5
5
5.5
6
6.5
ICL_ADJ (mA)
VFB = 1.5 V
4.5 V
18 V 35 V
1.214
1.224
1.234
1.244
1.254
1.264
-40 -20 0 20 40 60 80 100 140
JUNCTION TEMPERATURE (°C)
VFB (V)
120
IOUT = 200 mA
18 V
35 V
12 V
4.5 V
0 10 20 30 40
2
6
10
14
16
VHYST (mV)
VIN (V)
IOUT = 0
TJ = 25 °C
0 10 20 30 40
0
100
200
300
400
500
IIN (PA)
VIN (V)
VFB = 1.5 V, VEN = 5.5 V
-40 °C
25 °C
125 °C
0 10 20 30 40
0
3
6
9
12
15
IIN (PA)
VIN (V)
VFB = 1.5 V, VEN = 5.5 V
-40 °C
25 °C
125 °C
6
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6.7 Typical Characteristics
At TA= 25°C and applicable to both LM3489 and LM3489-Q1 at VIN = 12 V with configuration in Detailed Description (unless
otherwise noted).
Figure 1. Quiescent Current vs Input Voltage Figure 2. Shutdown Current vs Input Voltage
Figure 3. Feedback Voltage vs Temperature Figure 4. Feedback Voltage Hysteresis vs Input Voltage
Figure 5. Feedback Voltage Hysteresis vs Temperature Figure 6. Current Limit ADJ Current vs Temperature
0 0.2 0.4 0.6 0.8 1
0
2
4
6
8
10
OPERATING ON TIME (ms)
LOAD CURRENT (A)
VIN = 12 V
VIN = 6 V
VIN = 24 V
0 40
0
100
200
300
400
500
600
OPERATING FREQUENCY (kHz)
VIN (V)
10 20 30
L = 10 mH
L = 15 mH
L = 22 mH
VOUT = 3.3 V
IOUT = 500 mA
Cff = 100 pF
-40 -10 20 50 80 110 140
JUNCTION TEMPERATURE, TJ (oC)
100
150
200
250
300
TONMIN_CL (ns)
VIN = 12 V
VIN = 4.5 V
VIN = 24 V
-40 -10 20 50 80 110 140
JUNCTION TEMPERATURE, TJ (°C)
40
60
80
100
120
140
160
TONMIN_NOR (ns)
VIN = 4.5 V
VIN = 24 V
VIN = 12 V
-40 -10 20 50 80 110 140
JUNCTION TEMPERATURE, TJ (°C)
8
8.5
9
9.5
10
TCL (ms)
VIN = 12 V
VIN = 4.5 V
VIN = 24 V
VIN = 35 V
0 40
3
3.5
4
4.5
5
5.5
6
VIN -VPGATE (V)
VIN (V)
10 20 30
-40 °C
25 °C
125 °C
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Typical Characteristics (continued)
At TA= 25°C and applicable to both LM3489 and LM3489-Q1 at VIN = 12 V with configuration in Detailed Description (unless
otherwise noted).
Figure 7. Current Limit One Shot OFF Time vs Temperature Figure 8. VIN – VPGATE vs VIN
Figure 9. Minimum ON Time
vs Temperature (Normal Operation) Figure 10. Minimum ON Time
vs Temperature (Current Limit)
Figure 11. Operating ON Time vs Load Current Figure 12. Operating Frequency vs Input Voltage
VOUT RIPPLE (50 mVac/Div)
Switch Node Voltage, VD1 (10V/Div)
IL (1A/Div)
TIME (2 ms/DIV)
VOUT RIPPLE (20 mVac/Div)
Switch Node Voltage, VD1 (10V/Div)
IL (500 mA/Div)
TIME (4 ms/DIV)
0.0 0.2 0.4 0.6 0.8 1.0 1.2
OUTPUT CURRENT (A)
40
50
60
70
80
90
100
EFFICIENCY (%)
L = 22 mH
R1 = 60.7 k
R2 = 20 k
VIN = 24 V
VIN = 12 V
0.0 0.2 0.4 0.6 0.8 1.0 1.2
OUTPUT CURRENT (A)
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
DVOUT (%)
VIN = 24 V
VIN = 12 V
L = 22 mH
R1 = 60.7 k
R2 = 20 k
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Typical Characteristics (continued)
At TA= 25°C and applicable to both LM3489 and LM3489-Q1 at VIN = 12 V with configuration in Detailed Description (unless
otherwise noted).
VOUT = 5 V, L = 22 µH
Figure 13. Efficiency vs Load Current VOUT = 5 V, L = 22 µH
Figure 14. VOUT Regulation vs Load Current
VIN = 12 V, VOUT = 3.3 V, IOUT = 500 mA
Figure 15. Continuous Mode Operation VIN = 12 V, VOUT =3.3 V, IOUT = 50 mA
Figure 16. Discontinuous Mode Operation
VOUT = 3.3 V, 500 mA loaded
Figure 17. Enable Transient VOUT = 3.3 V, 500 mA loaded
Figure 18. Shutdown Transient
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7 Detailed Description
7.1 Overview
The LM3489 is a buck (step-down) DC-DC controller that uses a hysteretic control scheme. The control
comparator is designed with approximately 10 mV of hysteresis. In response to the voltage at the FB pin, the
gate drive (PGATE pin) turns the external PFET on or off. When the inductor current is too high, the current limit
protection circuit engages and turns the PFET off for approximately 9 µs.
Hysteretic control does not require an internal oscillator. Switching frequency depends on the external
components and operating conditions. The operating frequency reduces at light loads resulting in excellent
efficiency compared to other architectures.
The output voltage can be programmed by two external resistors. The output can be set in a wide range from
1.239 V (typical) to VIN.
7.2 Functional Block Diagram
7.3 Feature Description
7.3.1 Hysteretic Control Circuit
When the FB input to the control comparator falls below the reference voltage (1.239 V), the output of the
comparator switches to a low state. This results in the driver output, PGATE, pulling the gate of the PFET low
and turning on the PFET. With the PFET on, the input supply charges COUT and supplies current to the load
through the series path through the PFET and the inductor. Current through the Inductor ramps up linearly and
the output voltage increases. As the FB voltage reaches the upper threshold, which is the internal reference
voltage plus 10 mV, the output of the comparator changes from low to high, and the PGATE responds by turning
the PFET off. As the PFET turns off, the inductor voltage reverses, the catch diode turns on, and the current
through the inductor ramps down. Then, as the output voltage reaches the internal reference voltage again, the
next cycle starts.
IN OUT
OUT
IN HYST IN
V V ESR
V
FV V L V delay ESR
u
u u D u u u
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Feature Description (continued)
The LM3489 operates in discontinuous conduction mode at light-load current or continuous conduction mode at
heavy-load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up
to the peak then ramps down to zero. The next cycle starts when the FB voltage reaches the reference voltage.
Until then, the inductor current remains zero and the output capacitor supplies the load. The operating frequency
is lower and switching losses reduced. In continuous conduction mode, current always flows through the inductor
and never ramps down to zero.
The output voltage (VOUT) can be programmed by 2 external resistors. It can be calculated with Equation 1.
VOUT = 1.239 × (R1 + R2) / R2 (1)
Figure 19. Hysteretic Window
The minimum output voltage ripple (VOUT_PP) can be calculated in the same way with Equation 2.
VOUT_PP = VHYST (R1 + R2) / R2 (2)
For example, with VOUT set to 3.3 V, VOUT_PP is 26.6 mV in Equation 3.
VOUT_PP = 0.01 × (33k + 20k) / 20k = 0.0266 V (3)
Operating frequency (F) is determined by knowing the input voltage, output voltage, inductor, VHYST, ESR
(Equivalent Series Resistance) of output capacitor, and the delay. It can be approximately calculated using
Equation 4.
where
•α: (R1 + R2) / R2 (4)
7.3.1.1 Delay
It includes the LM3489 propagation delay time and the PFET delay time. The propagation delay is 90 ns typically
(see Figure 20).
INPUT VOLTAGE - OUTPUT VOLTAGE (V)
PROPOGATION DELAY (ns)
0
20
40
60
80
100
120
140
0510 15 20 25 30 35
L=10 µH
L=4.7 µH
L=22 µH
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Feature Description (continued)
Figure 20. Propagation Delay
The operating frequency and output ripple voltage can also be significantly influenced by the speed up capacitor
(Cff). Cff is connected in parallel with the high side feedback resistor, R1. The location of this capacitor is similar
to where a phase lead capacitor would be located in a PWM control scheme. However it's effect on hysteretic
operation is much different. Cff effectively shorts out R1 at the switching frequency and applies the full output
ripple to the FB pin without dividing by the R2/R1 ratio. The end result is a reduction in output ripple and an
increase in operating frequency. When adding Cff, calculate the formula above with α= 1. The value of Cff
depend on the desired operating frequency and the value of R2. A good starting point is 470-pF ceramic at 100-
kHz decreasing linearly with increased operating frequency. Also note that as the output voltage is programmed
below 2.5 V, the effect of Cff will decrease significantly.
7.3.2 Current Limit Operation
The LM3489 has a cycle-by-cycle current limit. Current limit is sensed across the VDS of the PFET or across an
additional sense resistor. When current limit is activated, the LM3489 turns off the external PFET for a period of
9 µs (typical). The current limit is adjusted by an external resistor, RADJ.
The current limit circuit is composed of the ISENSE comparator and the one-shot pulse generator. The positive
input of the ISENSE comparator is the ADJ pin. An internal 5.5-µA current sink creates a voltage across the
external RADJ resistor. This voltage is compared to the voltage across the PFET or sense resistor. The ADJ
voltage can be calculated with Equation 5.
VADJ = VIN −(RADJ × 3 µA)
where
• 3 µA is the minimum ICL-ADJ value (5)
The negative input of the ISENSE comparator is the ISENSE pin that must be connected to the drain of the
external PFET. The inductor current is determined by sensing the VDS. It can be calculated with Equation 6.
VISENSE = VIN −(RDSON × IIND_PEAK)=VIN −VDS (6)
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Feature Description (continued)
Figure 21. Current Sensing by VDS
The current limit is activated when the voltage at the ADJ pin exceeds the voltage at the ISENSE pin. The ISENSE
comparator triggers the 9-µs one-shot pulse generator forcing the driver to turn the PFET off. The driver turns the
PFET back on after 9 µs. If the current has not reduced below the set threshold, the cycle will repeat
continuously.
A filter capacitor, CADJ, must be placed as shown in Figure 21. CADJ filters unwanted noise so that the ISENSE
comparator will not be accidentally triggered. A value of 100 pF to 1 nF is recommended in most applications.
Higher values can be used to create a soft-start function (see Start Up).
The current limit comparator has approximately 100 ns of blanking time. This ensures that the PFET is fully on
when the current is sensed. However, under extreme conditions such as cold temperature, some PFETs may not
fully turn on within the blanking time. In this case, the current limit threshold must be increased. If the current limit
function is used, the on time must be greater than 100 ns. Under low duty cycle operation, the maximum
operating frequency is limited by this minimum on-time.
During current limit operation, the output voltage drops significantly as does operating frequency. As the load
current is reduced, the output returns to the programmed voltage. However, there is a current limit foldback
phenomenon inherent in this current limit architecture (see Figure 22).
Figure 22. Current Limit Foldback Phenomenon
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Feature Description (continued)
At high input voltages (> 28 V) increased undershoot at the switch node can cause an increase in the current
limit threshold. To avoid this problem, a low Vf Schottky catch diode must be used (see Catch Diode Selection
(D1)). Additionally, a resistor can be placed between the ISENSE pin and the switch node. Any value in the
range of 220 Ωto 600 Ωis recommended.
7.3.3 Start Up
The current limit circuit is active during start-up. During start-up, the PFET stays on until either the current limit or
the feedback comparator is tripped
If the current limit comparator is tripped first, then take the the foldback characteristic into account. Start-up into
full load may require a higher current limit set point or the load must be applied after start-up.
One problem with selecting a higher current limit is inrush current during start-up. Increasing the capacitance
(CADJ) in parallel with RADJ results in a soft-start characteristic. CADJ and RADJ create an RC time constant forcing
current limit to activate at a lower current. The output voltage will ramp more slowly when using this technique.
There is example start-up plot for CADJ equal to 1 nF in Typical Characteristics. Lower values for CADJ will have
little to no effect on soft-start.
7.3.4 External Sense Resistor
The VDS of a PFET tends to vary significantly over temperature. This will result an equivalent variation in current
limit. To improve current limit accuracy, an external sense resistor can be connected from VIN to the source of
the PFET, as shown in Figure 23. The current sense resistor, RCS must have value comparable with RDSON of the
PFET used, typically in the range of 50 mΩto 200 mΩ.Equation 6 in Current Limit Operation can be used by
replacing the RDSON with RCS.
Figure 23. Current Sensing by External Resistor
7.3.5 PGATE
When switching, the PGATE pin swings from VIN (off) to some voltage below VIN (on). How far the PGATE will
swing depends on several factors including the capacitance, on-time, and input voltage.
PGATE voltage swing will increase with decreasing gate capacitance. Although PGATE voltage will typically be
around VIN-5V, with very small gate capacitances, this value can increase to a typical maximum of VIN-8.3 V.
Additionally, PGATE swing voltage will increase as on-time increases. During long on-times, such as when
operating at 100% duty cycle, the PGATE voltage will eventually fall to its maximum voltage of VIN-8.3 V (typical)
regardless of the PFET gate capacitance.
The PGATE voltage will not fall below 0.4 V (typical). Therefore, when the input voltage falls below approximately
9 V, the PGATE swing voltage range is reduced. At an input voltage of 7 V, for instance, PGATE will swing from
7 V to a minimum of 0.4 V.
ISENSE
ADJ
VIN
PGATE
FB
EN
LM3489 GND
PGND
1
2
3
4
5
6
7
8
VIN
VEN
R3
R4
Copyright © 2016, Texas Instruments Incorporated
IN(UVLO_HYST) EN_HYST R4
V V 1 R3
§ ·
u
¨ ¸
© ¹
IN(UVLO) EN R4
V = V 1 R3
§ ·
¨ ¸
© ¹
14
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Feature Description (continued)
7.3.6 Adjustable UVLO
The undervoltage lockout (UVLO) function can be implemented as shown in Figure 24. By incorporating the
feature of the internal enable threshold, the lockout level can be programmed through an external potential
divider formed with R3 and R4. The input voltage information is detected and compared with the enable
threshold and the device operation is inhibited when VIN drops below the preset UVLO level. The UVLO and
hysteresis voltage can be calculated with Equation 7 and Equation 8.
(7)
where
• VEN is the enable rising threshold voltage
• VEN_HYST is the enable threshold hysteresis (8)
Figure 24. Adjustable UVLO
7.4 Device Functional Mode
7.4.1 Device Enable and Shutdown
The LM3489 can be remotely shutdown by forcing the enable pin to ground. With EN pin grounded, the internal
blocks other than the enable logic are deactivated and the shutdown current of the device is lowered to only 7 µA
(typical). Releasing the EN pin allows for normal operation to resume. The EN pin is internally pulled high with
the voltage clamped at 8 V typical. For normal operation, this pin must be left open. In case an external voltage
source is applied to this pin for enable control, the applied voltage must not exceed the maximum operating
voltage level specified in this datasheet (that is 5.5 V).
IN DS OUT
V V V D
Lif
u
'
ISENSE
ADJ
VIN
PGATE
FB
EN
LM3489 GND
PGND
7
2
3
4
5
6
1
8
+ +
Q 1 FDC5614P
VIN VOUT
CIN1 COUT
D1
L 22 PH
RADJ
CADJ
R1
R2
Cff
CIN2
7V ±35V
22 PF
50V
0.1 PF
50V
3.3V/0.5A
1 nF 24k
33k
20k
100 pF
100 PF
6.3V
MBRS140
SD*
*
Short to shutdown
the device
RIS
270
Copyright © 2016, Texas Instruments Incorporated
15
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
Hysteretic control is a simple control scheme. However the operating frequency and other performance
characteristics highly depend on external conditions and components. If either the inductance, output
capacitance, ESR, VIN, or Cff is changed, there is a change in the operating frequency and output ripple. The
best approach is to determine what operating frequency is desirable in the application and then begin with the
selection of the inductor and COUT ESR.
8.2 Typical Application
Figure 25. Typical Application Schematic for VOUT = 3.3 V, 500 mA
8.2.1 Design Requirements
The important parameters for the inductor are the inductance and the current rating. The LM3489 operates over
a wide frequency range and can use a wide range of inductance values. A rule of thumb is to use the equations
used for Simple Switchers®. The equations for inductor ripple (Δi) as a function of output current (IOUT) depend on
Iout:
For Iout < 2 A, Δi≤Iout × Iout−0.366726.
For Iout > 2 A, Δi≤Iout × 0.3.
8.2.2 Detailed Design Procedure
8.2.2.1 Inductor Selection (L)
The inductance can be calculated with Equation 9 and Equation 10 based upon the desired operating frequency.
(9)
OUT IN OUT
RSM_CIN OUT IN
V (V V )
I I V
u
pk OUT i
I I 1.1
2
'
§ ·
u
¨ ¸
© ¹
pk OUT i
I I 1.1
2
'
§ ·
u
¨ ¸
© ¹
16
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Typical Application (continued)
where
• D is the duty cycle
• VDis the diode forward voltage
• VDS is the voltage drop across the PFET (10)
The inductor must be rated with Equation 11.
(11)
The inductance value and the resulting ripple is one of the key parameters controlling operating frequency. The
second is the inductor ESR that contribute to the steady-state power loss due to current flowing through the
inductor.
8.2.2.2 Output Capacitor Selection (COUT)
The ESR of the output capacitor times the inductor ripple current is equal to the output ripple of the regulator.
However, the VHYST sets the first-order value of this ripple. As ESR is increased with a given inductance,
operating frequency increases as well. If ESR is reduced then the operating frequency reduces.
The use of ceramic capacitors has become a common desire of many power supply designers. However,
ceramic capacitors have a very low ESR resulting in a 90° phase shift of the output voltage ripple. This results in
low operating frequency and increased output ripple. To fix this problem a low-value resistor must be added in
series with the ceramic output capacitor. Although counter intuitive, this combination of a ceramic capacitor and
external series resistance provides highly accurate control over the output voltage ripple. Other types capacitor,
such as Sanyo POS CAP and OS-CON, Panasonic SP CAP, and Nichicon NA series, are also recommended
and may be used without additional series resistance.
For all practical purposes, any type of output capacitor may be used with proper circuit verification.
8.2.2.3 Input Capacitor Selection (CIN)
A bypass capacitor is required between the input source and ground. It must be located near the source pin of
the external PFET. The input capacitor prevents large voltage transients at the input and provides the
instantaneous current when the PFET turns on.
The important parameters for the input capacitor are the voltage rating and the RMS current rating. Follow the
manufacturer's recommended voltage derating. For high-input voltage applications, low-ESR electrolytic,
Nichicon UD series or the Panasonic FK series are available. The RMS current in the input capacitor can be
calculated with Equation 12.
(12)
The input capacitor power dissipation can be calculated with Equation 13.
PD(CIN) = IRMS_CIN2× ESRCIN (13)
The input capacitor must be able to handle the RMS current and the dissipation. Several input capacitors may be
connected in parallel to handle large RMS currents. In some cases it may be much cheaper to use multiple
electrolytic capacitors than a single low-ESR, high-performance capacitor such as OS-CON or Tantalum. The
capacitance value must be selected such that the ripple voltage created by the switch current pulses is less than
10% of the total DC voltage across the capacitor.
For high VIN conditions (> 28 V), the fast switching, high swing of the internal gate drive introduces unwanted
disturbance to the VIN rail and the current limit function can be affected. To eliminate this potential problem, a
high-quality ceramic capacitor of 0.1 µF is recommended to filter out the internal disturbance at the VIN pin. This
capacitor must be placed right next to the VIN pin for best performance.
DSON
ADJ IND_PEAK CL_ADJ
R
R I I
u
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Typical Application (continued)
8.2.2.4 Programming the Current Limit (RADJ)
The current limit is determined with Equation 14 by connecting a resistor (RADJ) between input voltage and the
ADJ pin, pin 5.
where
• RDSON is Drain-Source ON resistance of the external PFET
• ICL_ADJ is 3 µA minimum
• IIND_PEAK = ILOAD + IRIPPLE / 2 (14)
Using the minimum value for ICL_ADJ (3 µA) ensures that the current limit threshold is set higher than the peak
inductor current.
The RADJ value must be selected to ensure that the voltage at the ADJ pin does not fall below 3.5 V. With this in
mind, RADJ_MAX = (VIN – 3.5) / 7 µA. If a larger RADJ value is needed to set the desired current limit, either use a
PFET with a lower RDSON or use a current sense resistor as shown in Figure 23.
The current limit function can be disabled by connecting the ADJ pin to ground and ISENSE to VIN.
8.2.2.5 Catch Diode Selection (D1)
The important parameters for the catch diode are the peak current, the peak reverse voltage, and the average
power dissipation. The average current through the diode can be calculated with Equation 15.
ID_AVE = IOUT × (1 – D) (15)
The off-state voltage across the catch diode is approximately equal to the input voltage. The peak reverse
voltage rating must be greater than input voltage. In nearly all cases a Schottky diode is recommended. In low-
output voltage applications, a low forward voltage provides improved efficiency. For high-temperature
applications, diode leakage current may become significant and require a higher reverse voltage rating to
achieve acceptable performance.
8.2.2.6 P-Channel MOSFET Selection (Q1)
The important parameters for the PFET are the maximum Drain-Source voltage (VDS), the ON resistance
(RDSON), Current rating, and the input capacitance.
The voltage across the PFET when it is turned off is equal to the sum of the input voltage and the diode forward
voltage. The VDS must be selected to provide some margin beyond the input voltage.
PFET drain current, Id, must be rated higher than the peak inductor current, IIND-PEAK.
Depending on operating conditions, the PGATE voltage may fall as low as VIN – 8.3 V. Therefore, a PFET must
be selected with a VGS maximum rating greater than the maximum PGATE swing voltage.
As input voltage decreases below 9 V, PGATE swing voltage may also decrease. At 5-V input the PGATE will
swing from VIN to VIN – 4.6 V. To ensure that the PFET turns on quickly and completely, a low threshold PFET
must be used when the input voltage is less than 7 V.
Total power loss in the FET can be approximated using Equation 16.
PDswitch = RDSON × IOUT2× D + F × IOUT × VIN × (ton + toff) / 2
where
• ton is the FET turn on time
• toff is the FET turn off time (16)
A value of 10 ns to 20 ns is typical for ton and toff.
A PFET must be selected with a turnon rise time of less than 100 ns. Slower rise times will degrade efficiency,
can cause false current limiting, and in extreme cases may cause abnormal spiking at the PGATE pin.
0.0 0.2 0.4 0.6 0.8 1.0 1.2
OUTPUT CURRENT (A)
40
50
60
70
80
90
100
EFFICIENCY (%)
VIN = 4.5 V
VIN = 24 V
VIN = 12 V
0.0 0.2 0.4 0.6 0.8 1.0 1.2
OUTPUT CURRENT (A)
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
'VOUT (%)
VIN = 4.5 V
VIN = 24 V
VIN = 12 V
18
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Typical Application (continued)
The RDSON is used in determining the current limit resistor value, RADJ. Note that the RDSON has a positive
temperature coefficient. At 100°C, the RDSON may be as much as 150% higher than the 25°C value. This
increase in RDSON must be considered when determining RADJ in wide temperature range applications. If the
current limit is set based upon 25°C ratings, then false current limiting can occur at high temperature.
Keeping the gate capacitance below 2000 pF is recommended to keep switching losses and transition times low.
This will also help keep the PFET drive current low, which will improve efficiency and lower the power dissipation
within the controller.
As gate capacitance increases, operating frequency must be reduced and as gate capacitance decreases
operating frequency can be increased.
8.2.2.7 Interfacing With the Enable Pin
The enable pin is internally pulled high with clamping at 8 V typical. For normal operation this pin must be left
open. To disable the device, the enable pin must be connected to ground externally. If an external voltage source
is applied to this pin for enable control, the applied voltage must not exceed the maximum operating voltage level
specified in this datasheet, that is 5.5 V. For most applications, an open-drain or open-collector transistor can be
used to short this pin to ground to shutdown the device .
8.2.3 Application Curves
VOUT = 3.3 V, L = 22 µH
Figure 26. Efficiency vs Load Current VOUT = 3.3 V, L = 22 µH
Figure 27. VOUT Regulation vs Load Current
No load, CADJ = 1 nF Figure 28. Power Up VOUT = 3.3 V, 50 mA to 500 mA load
Figure 29. Load Transient
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9 Power Supply Recommendations
This device is designed to operate over a recommended input voltage supply range of 4.5 V to 35 V. The input
supply must be well regulated. If the input supply is located far from the LM3485 EVM and needs a long power
supply cable to connect, an additional bulk capacitor may be required. An electrolytic capacitor with a value of
47 µF can be used typically.
As mentioned in Current Limit Operation, at higher input voltages (> 28 V) an increased negative SW transient
spike at the switch node can lead to an increase in the current limit threshold due to the formation of the parasitic
NPN connection between the ISENSE pin, the internal substrate and the ADJ pin . To avoid this issue, a
Schottky catch diode with lower forward voltage drop must be used. In addition to that, a resistor must be placed
between the ISENSE pin and the external switch node. A resistor value in the range of 220 Ωto 600 Ωis
recommended.
10 Layout
10.1 Layout Guidelines
The PCB layout is very important in all switching regulator designs. Poor layout can cause switching noise into
the feedback signal and generate EMI problems. For minimal inductance, the wires indicated by heavy lines in
schematic diagram must be as wide and short as possible. Keep the ground pin of the input capacitor as close
as possible to the anode of the catch diode. This path carries a large AC current. The switching node, the node
with the diode cathode, inductor and FET drain must be kept short. This node is one of the main sources for
radiated EMI since it sees a large AC voltage at the switching frequency. It is always a good practice to use a
ground plane in the design, particularly for high-current applications.
The two ground pins, PGND and GND, must be connected by as short a trace as possible. They can be
connected underneath the device. These pins are