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PRIMARY-SIDE PUSH-PULL OSCILLATOR
WITH DEAD-TIME CONTROL
1
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FEATURES
DPush-Pull Oscillator With Programmable
Deadtime
DHigh-Current Totem-Pole Dual Output Stage
Drives Push-Pull Configuration with 1-A Sink
and 0.5-A Source Capability
DCan be Used in Push-Pull, Half-Bridge, or
Full-Bridge Topologies
DOscillator Synchronization Output
DLow Start-Up Current of 130 µA and 1.4-mA
Typical Run Current
DOver-Current Shutdown
DDigitally Controlled Over-Current/Retry
Feature
DUndervoltage Lockout With Hysteresis
APPLICATIONS
DHigh Efficiency Cascaded Converters
DInverters
DElectronic Ballasts
DUninterruptable Power Supplies (UPS)
DAC or DC Links
DESCRIPTION
The UCC28089 is a versatile BiCMOS controller for
dc-to-dc or off-line fixed-frequency switching power
supplies. The UCC28089 has dual alternating output
stages in dual-alternating push-pull configuration. Both
outputs switch at half the oscillator frequency using a
toggle flip-flop and duty cycle is limited to less than 50%.
TYPICAL APPLICATION
2
1
4
3
7
8
5
6
UCC28089
SYNC
DIS
CT
CS
VDD
OUTA
OUTB
GND
+
−
2
4
18
UCC2540
REF
SYNCIN
G1
15
16
14
VDD
PGND
G2
−
BIAS
UDG-04112
RA
RB
CT
VIN = 48 V
VO = 1.2
V
+
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments
semiconductor products and disclaimers thereto appears at the end of this data sheet.
! "
#$ % $
%
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DESCRIPTION (CONTINUED)
The UCC28089 is optimized for use as the primary-side companion controller for a cascaded converter that has
secondary-side control. The device incorporates dead-time programming. The synchronization output also
provides dead-time information. The retry and soft-start duration scales with the oscillator clock frequency for
high performance fault recovery.
The UCC28089 also provides primary side under-voltage protection (UVLO), and over-current protection. Both
the soft start and retry after fault durations scale with oscillator frequency for high performance. The turn-on/off
UVLO thresholds are 10.5 V/8.0 V.
ORDERING INFORMATION
TEMPERATURE RANGE
PACKAGED DEVICES{
TEMPERATURE RANGE
TA = TJSOIC−8 (D) SON−8 (DRB)}
−40°C to 105°C UCC28089D UCC28089DRB
†D (SOIC−8) and DRB (SON−8) packages are available taped and reeled. Add R suffix to device type (e.g. UCC28089DR or UCC28089DRBR)
to order quantities of 2,500 devices per reel (for D), and 1,000 devices per reel (for DRB).
‡Contact factory through TI sales for the availability of this package. Target availability is October 2004.
CONNECTION DIAGRAM
1
2
3
4
8
7
6
5
SYNC
DIS
CT
CS
VDD
OUTA
OUTB
GND
D PACKAGE (SOIC−8)
(TOP VIEW)
8
7
6
5
SYNC
DIS
CT
CS
VDD
OUTA
OUTB
GND
1
2
3
4
DRB PACKAGE (SON−8)
(TOP VIEW)
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ABSOLUTE MAXIMUM RATINGS
over operating free-air temperature (unless otherwise noted)†}
PARAMETER SYMBOL RATING UNITS
Supply voltage (IDD < 10 mA) VDD 15 V
Supply current IDD 20 mA
OUTA/OUTB sink current (peak) IOUT(sink) 1.0
A
OUTA/OUTB source current (peak) IOUT(source) −0.5 A
SYNC sink current (peak) 50
mA
SYNC source current (peak) −50 mA
Analog inputs (DIS, CT, CS) −0.3 to VDD + 0.3, not to exceed 5 V
Power dissipation at TA = 25°C (D package) 650
mW
Power dissipation at TA = 25°C (DRB package) TBD mW
Junction operating temperature TJ−55 to 150
Storage temperature Tstg −65 to 150 oC
Lead temperature (soldering, 10 sec.) Tsol +300
oC
†Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under “recommended operating conditions” is not
implied. Exposure to absolute−maximum−rated conditions for extended periods may affect device reliability.
‡All voltages are with respect to GND. Currents are positive into, negative out of the specified terminal. Consult Packaging Section of the Databook
for thermal limitations and considerations of packages.
RECOMMENDED OPERATION CONDITIONS
Parameter Symbol MIN TYP MAX UNITS
Supply voltage (IDD < 10 mA) VDD 8.5 14 V
SYNC sink current (peak) 0 10 25
mA
SYNC source current (peak) −25 −10 0 mA
Analog inputs (DIS, CT, CS) 0 4 V
Timing capacitor range CT 100 100,000 pF
Timing charge resistor range RA 32 750
kΩ
Discharge resistor range RB 0 250 kΩ
Timing charge current ICHG(RA+RB) 10 300 mA
Switching Frequency fSW 1000 kHz
Junction temperature TJ−40 105 °C
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ELECTRICAL CHARACTERISTICS:
TA = −40°C to 105°C for UCC28089, VDD = 9 V (see Note 1), 1 µF capacitor from VDD to GND, RA = 110 kΩ, RB = 182 Ω, CT = 220 pF, TA = TJ,
(unless otherwise noted).
PARAMETER TEST CONDITION MIN TYP MAX UNITS
Overall Section
Startup current VDD < UVLO start threshold (see Note 2) 130 260 µA
Operating supply current CS = 0 V, (see Note 1, Note 2) 1.4 2.0 mA
Undervoltage Lockout
Start threshold See Note 1 9.5 10.5 11.5
Minimum operating voltage after start 7.4 8.0 8.4 V
Hysteresis 2.1 2.5 2.9
V
Oscillator
Oscillator frequency 2 x OUTx frequency, Measured at output(s) 180 200 220 kHz
Current Sense
Current Shutdown threshold Resetting current limit 0.650 0.725 0.800 V
CS to output delay CS from 0 mV to 900 mV 45 100 ns
Output
Dead Time
Measured at OUTA or OUTB 90 100 110
ns
Dead Time Over temperature 80 125 ns
Minimum duty cycle CS = 0.9 V 0 %
VOL (OUTA or OUTB) IOUT = 75 mA 0.5 1
V
VOH (OUTA or OUTB) IOUT = −35 mA, (VDD – VOUT) 1.0 1.3 V
Output resistance high TA = 25°C IOUT = −1 mA (see Note 4) 70 80 90
TA = full range IOUT = −1 mA (see Note 4) 40 80 135
Ω
Output resistance low TA = 25°C IOUT = 1 mA (see Note 4) 6.5 7.5 8.5 Ω
TA = full range IOUT = 1 mA (see Note 4) 4 7.5 14
tr, Rise Time CLOAD = 1 nF 28 50
ns
tf, Fall Time CLOAD = 1 nF 13 30 ns
SYNC
SYNC duration Measured at SYNC pin 75 95 115
tr, delay Rising SYNC until falling OUTA or OUTB 0 8.5 30 ns
tf, delay Falling SYNC until rising OUTA or OUTB 0 14 50
ns
SYNC VOH ISYNC = −5 mA (VDD – VSYNC) 0.3 1
V
SYNC VOL ISYNC = 5 mA 0.3 1 V
tr, Rise Time CLOAD = 100 pF 15 30
ns
tf, Fall Time CLOAD = 100 pF 15 30 ns
Soft Start & Fault
OUTA/OUTB start delay time Cycles as measured at CT pin 57 59 62
OUTA/OUTB soft start duration First output stage cycle to first full output stage cycle,
CS ≤ 0.6 V 4 5 7 cycles
NOTES: 1. Set VDD above the start threshold before setting at 9V.
2. Does not include current of the external oscillator network.
3. Ensured by design. Not 100% tested in production.
4. The pullup / pulldown circuits of the driver are bipolar and MOSFET transistors in parallel. The output resisstance is the RDS(ON)
of the MOSFET transistor when the voltage of the driver output is less than the saturation voltage of the bipolar transistor.
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FUNCTIONAL BLOCK DIAGRAM
OUTPUT LOGIC
UCC28089
6OUTB
7OUTA
8VDD
4
CS
2
DIS
1
SYNC
3
CT
5GND
VDD
UVLO
VDD = 10.5/8.0 V
VDD
OVER
CURRENT
COMP
SS LATCH
0.725 V
SS
COMP
Q
Q
S
R
+
0.2V
56 STEP
START
DELAY
R
GO 7 STEP SOFT−
START RAMP
REFGO
VDD/5
SQ
QR
CK
VDD/5
VDD/19.6
QT_
Q
SOFT−START & FAULT
OSCILLATOR
UDG-04101
PIN # NAME I/O FUNCTION
1 SYNC O Active when OUTA and OUTB are active, logic LO at all other times such as during under-voltage
lock-out and over-current shutdown. When active, SYNC is logic HI (VDD) during the discharge time
of the oscillator and logic LO (GND) at all other times. The pulse occur during the dead time.
2 DIS I Separate oscillator timing capacitor discharge pin that allows the dead time to be externally
programmed.
3 CT I Oscillator timing capacitor connection.
4 CS I Current sense pin. An over current shutdown event is triggered when the voltage of this pin rises
above 0.75 V.
5 GND − Ground pin. Analog and digital signals reference this pin and output drivers return current through
this pin
6 OUTB O Driver output, capable of sinking 1 A and sourcing 0.5 A. OUTB signal alternates with OUTA.
7 OUTA O Driver output, capable of sinking 1 A and sourcing 0.5 A. OUTA signal alternates with OUTB.
8 VDD I Power input connection for this device.
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APPLICATION INFORMATION
UCC28089 is an alternating dual-driver output oscillator with over-current and under-voltage fault protection.
This feature set is ideal as a start-up controller for isolated power systems where the majority of control functions
are performed on the secondary side. This device is especially useful for dc link for topologies such as the
cascaded buck converter [1], ac link inverter topologies [2], and inexpensive modified square wave inverters.
The UCC28089 has a brief 5 to 7 cycle leading-edge modulated soft-start cycle so that it will not interfere with
secondary-side controlled soft start. Both systems with off-line self bias and auxiliary bias supplies are more
fault tolerant with the UCC28089 because it consistently responds to a fault with a delay of at least 56 oscillator
cycles before retry.
Detailed Functional Description
VDD: Power input connection for this device. Although quiescent VDD current is very low, total supply current
is higher, depending on OUTA and OUTB current and the programmed oscillator frequency. During fault
response, the current drops to a lower level because the oscillator is disabled.
In order to avoid noise problems, position a 1-µF ceramic bypass capacitor, connected from VDD to GND, as
close to the chip as possible. The ceramic bypass capacitor is in addition to any energy storage capacitance
that would be used to hold up the VDD voltage during start-up transients.
GND: Ground pin. Analog signals reference this pin and output drivers return current through this pin. For best
results, use this pin as a local ground point in a star ground configuration.
OUTA and OUTB: Output drivers capable of sinking 1 A and sourcing 0.5 A. The output pulse alternates
between O UTA and OUTB. In addition, a T latch forces the output pulses to alternate in order to reduce flux build
up in a transformer during low duty ratio operation. Each output is capable of driving the gate of a power
MOSFET.
CT and DIS: Oscillator timing capacitor pin and timing capacitor discharge pin. The UCC28089 oscillator tracks
VDD and GND internally in order to minimize oscillator frequency changes due to variations in the voltage of
VDD. Figure 1 shows the oscillator block diagram.
2DIS
3CT
RA
RB
CT
VDD
OSCILLATOR
SQ
QR
D
VROK
CK
VDD
15.7Rx
2.90Rx
Rx
UCC28089
VDD/19.6
VDD/5
V(CT)
CK
Figure 1. Block Diagram for Oscillator
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APPLICATION INFORMATION
The recommended oscillator frequency range is up to 1 MHz. In order to avoid noise issues, RA and RB should
be small enough for the oscillator to have at least 10 µA of current. There are two sets of oscillator programming
equations that model the oscillator over its wide programming range. Measure the charge and the discharge
times at the SYNC pin in order to avoid affecting the oscillator with probe impedances or output driver delays.
The approximate first order equations in the table are adequate for switching frequencies below 50 kHz and/or
discharge times that are greater than 1 µs. The specific requirements for using the first order equations versus
the second order equations are related to the timing capacitor size and the discharge resistor. Keep in mind that
the 1st order equations and 2nd order equations are merely approximations that are typically within +/−20% of
the actual operating point. The frequency, charge and discharge times are relatively insensitive to temperature
but larger values of CT and RB exhibit the least sensitivity to temperature. Incidentally, the second order
equations apply for the operating conditions that are in the Electrical Characteristics table. The oscillator
frequency is set according to the following equations:
1ST ORDER EQUATIONS 2ND ORDER EQUATIONS
Condition RA > 300 Ω AND CT > 300pF 100 Ω < RA < 300 Ω OR 100pF < CT < 300pF
TCHARGE 0.169ǒRA)RBǓCT0.175ǒRA)RBǓǒCT)40 pFǓ)20 ns
TDISCHARGE 1.36 RBCT(1.37)ǒRB)44ǓǒCT)14 pFǓ)20 ns
fOSC 5.9
ǒRA)8.0 RBǓCT1
TCHARGE )TDISCHARGE
Where RA and RB are in Ohms; CT is in Farads; fOSC is in Hz; tCHARGE and tDISCHARGE are in seconds.
The oscillator is optimized for a CT timing capacitor range from 100 pF to 1000 nF and RB more than 100 Ω.
If the shortest discharge time possible is desired, it is permissible to short DIS to CT for all recommended CT
values (100 pF to 0.100 µF).
SYNC: This SYNC pin produces an output pulse from 0 to VDD that can be used to synchronize a secondary
side-buck controller to the free running isolating power stage. The proper timing of this signal enables zero
voltage switching on the primary side MOSFETs. The clean signal also solves a problem of getting a
synchronization signal from the secondary side of the transformer, which can have leakage inductance voltage
spikes that may cause false triggering. The SYNC pulse width is the oscillator discharge time, which is
approximately equal to the dead time. Pulse frequency is the oscillator frequency. During fault conditions, the
SYNC pulses are terminated and the SYNC output is held low for at least 56 oscillator cycles. During soft start,
SYNC precedes the first output pulse by at least one oscillator cycle.
CS: Connect the current sense device to this pin. A voltage threshold of 0.725 V triggers a shutdown sequence.
An over-current fault triggers an immediate shutdown. After the fault clears, a total of 64 oscillator cycles are
required for an entire soft start sequence to occur. First, the outputs and SYNC are kept OFF for at least 56
oscillator cycles. Next, after one or two SYNC pulses, the soft start progressively increases the output duty ratio
over the next five to seven oscillator cycles.
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APPLICATION INFORMATION
Using the UCC28089 as the Primary-Side start-up Controller in a Cascaded Push-Pull Buck
Two-Stage Converter
The cascaded push-pull topology is ideal for converting from moderate bus voltages, such as 48-V telecom
buses, to sub 2-V output voltages. The general topology is shown in Figure 2 using the UCC28089 as the
primary-side start-up controller and the UCC2540 as the secondary-side regulator [3].
UDG-04100
CT
RB
RA 10 V
CR1 CR2
VIN = 48 V VO
1.2 V
2
1
4
3
7
8
5
6
UCC28089
SYNC
DIS
CT
CS
VDD
OUTA
OUTB
GND
+
−
IN OUT
COM
L.REG
2
1
4
3
19
20
17
18
UCC2540
ISET/SD
REF
G2C
SYNCIN
SWS
BST
G1
SW
6
5
8
7
RAMP
GND
VEA−
CEA−
10
9 COMP
TR
15
16
13
14
VDD
PGND
G2
VDRV
11
12G2S
SS
+
−
Figure 2. Cascaded Push-Pull Buck Two-Stage Converter
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APPLICATION INFORMATION
Program the oscillator frequency of the UCC28089 to equal the desired switching frequency of the output post
regulator. The secondary-side controller may also need corresponding switching frequency programming, such
as RAMP and G2C capacitor values for the UCC2540. Program the dead time to be approximately 1/4 of the
resonant period of the equivalent parasitic L−C circuit that is established by the primary leakage inductance of
the transformer and the total drain-source capacitance of the primary-side power MOSFET transistors (COSS
+ stray capacitances). Remember that COSS predictably varies over input line voltage. If the variation is too great
and/or 1/4 the resonant period is less than 100 ns, connect additional capacitance (CR1 and CR2 in Figure 2)
between the drain and source of the primary transistors, which stabilizes the capacitance and raise the total
capacitance value.
If the secondary-side controller is compatible with pulse edges, the pulse edge transformer circuit in Figure 3
can provide an isolated pulse edge signal on the secondary side using a transformer core that is 6-mm diameter
or less. The recommended transformer (COEV #MGBBT−0001101) is compatible with all switching frequencies
and it is smaller than many opto-isolators.
UCC28089
R1
634
C1
680 pF
1
5GND
GND1
Primary Ground Secondary Ground
UCC2540
SYNCIN
REFSYNC
4
2
QCL
2N3904
T1
1:1
RCB
422
RBE
115
L1
15uH
Figure 3. Isolation and clamping the SYNC signal for Cascaded Buck Converters
Notice that the peak-pulse voltage is proportional to the UCC28089 bias voltage. The circuit in Figure 3 is well
suited to the full VDD bias voltage range of the UCC28089 bias voltage because it has a clamp circuit. The clamp
circuit in Figure 3 (RCB, RBE and QCL) is a VBE clamp rather than a Zener diode. A VBE clamp is used here
because it has much lower capacitance than typical Zener diodes so that the clamp does not affect the narrow
50-ns pulse width. The clamp may be replaced by a single resistor in applications, as in Figure 2, where the VDD
bias voltage of the UCC28089 is regulated within a +/−5% window.
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APPLICATION INFORMATION
Synchronization of Multiple UCC28089 Controllers to an External Signal
In systems where multiple UCC28089 parts need to be synchronized to a common clock, a 3.3-V logic-level
signal can be directly applied to the CT pin (the SYNC pin on UCC28089 only provides output sync signals).
As shown in Figure 4, the externally supplied sync pulse width determines the frequency and the dead time
between OUT A and OUT B. In this configuration, the discharge pin DIS should be grounded since it is not used.
The external sync signal should exceed the oscillator trip level of VDD/5 when high, and pull CT below VDD/20
when low.
UDG-04113
3.3V
OUT A
OUT B
Ext. Sync
DIS
SYNC
CT
CS GND
OUTB
OUTA
VDD
UCC28089
1
2
3
4 5
6
7
8
Ext. Sync
Applied to CT
U1
External Sync pulse width defines
output dead time
NC
1 µF
VDD
Figure 4. Synchronizing the UCC28089 to an External Signal
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APPLICATION INFORMATION
Using the UCC28089 as a Modified Square Wave Inverter
Remote or dc-only power systems often require a limited amount of 60-Hz ac line power to supply small
appliances. Compatible loads include universal motors, incandescent lamps, and other electronic devices with
switched mode power supplies to convert the 110-VAC to lower dc voltages. Many of these devices do not require
a perfect sinusoidal line voltage, and acceptable performance can be obtained with a modified square wave
voltage. Using the circuit in Figure 5, the UCC28089 can provide the appropriate waveform along with primary
side over-current protection. Components RA, RB, and CT are selected to program the desired modified square
waveform with the appropriate dead time.
UDG-04105
CT
0.1 µF
RB
27 kΩ
RA
221 kΩ
NC 200 Ω
47 µF
12 V Bias
VIN = 145 VDC
F1
RS
4.7 µFMPSA42 MPSA42
4.7 µF
−145 V
145 V
0 V
VO
110 VAC(rms) 16.7 ms
NOTE: CS signal should be selected to limit peak inrush current to acceptable levels.
1N4003
2
1
4
3
7
8
5
6
UCC28089
SYNC
DIS
CT
CS
VDD
OUTA
OUTB
GND
+
−
6800 pF
6800 pF
50 Ω
200 Ω
200 Ω
50 Ω
200 Ω
20 Ω
Figure 5. Modified Square Wave Inverter
The high-side gate drives of the inverter in Figure 5 are suitable for low frequency applications with relatively
constant duty ratio. The NPN transistors and the charge pump diodes on the high-side gate drives must be rated
for high voltage (at least 145 V + VDD). The gates are protected from excessive negative voltage by the diodes
shown from gate to source.
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APPLICATION INFORMATION
If desired, the 60-Hz modified square wave inverter frequency could be programmed using an external sync
signal that might originate from a separate oscillator or digital controller. The following diagram in Figure 6 shows
a 50% duty cycle square wave fed into the CT pin, with a frequency of 120 Hz, and the resulting OUTA/OUTB
wave shapes.
3.3 V
OUT A
OUT B
Ext. Sync
DIS
SYNC
CT
CS GND
OUTB
OUTA
VDD
UCC28089
1
2
3
4 5
6
7
8
Ext. Sync
U1
External Sync example with 50% duty cycle
NC
8.33 ms
16.7 ms VDD
1 µF
Figure 6. External Synchronization Example with 50% Duty Cycle Square Wave
RELATED PRODUCTS
DEVICE DESCRIPTION
UCC2540 High-Efficiency Secondary-Side Synchronous-Buck PWM Controller
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TYPICAL CHARACTERISTICS
Figure 7
OSCILLATOR FREQUENCY
vs
TEMPERATURE
fs − Oscillator Frequency− kHz
Tj − Temperature − °C
−50 50
190
180
25 100750−25
185
195
210
200
205
215
220
RA = 110 kΩ
RB = 182 Ω
CT = 220 pF
Figure 8
Tj − Temperature − °C
−50 50 125
98
94
25 100750−25
96
100
106
102
104
108
110
90
92
OSCILLATOR FREQUENCY
vs
TEMPERATURE
fs − Oscillator Frequency− kHz
RA = 221 kΩ
RB = 3.32 Ω
CT = 220 pF
Figure 9
Tj − Temperature − °C
−50 50 125
−1.0%
−2.0%
25 100750−25
−1.5%
−0.5%
1.0%
0.0%
0.5%
1.5%
2.0%
OSCILLATOR FREQUENCY SHIFT
vs
TEMPERATURE
fs − Normalized to 25°C− kHz
RA = 110 kΩ
RB = 182 Ω
CT = 220 pF
RA = 221 kΩ
RB = 3.32 Ω
CT = 220 pF
VDD = 9 V
Figure 10
OSCILLATOR FREQUENCY SHIFT
vs
SUPPLY VOLTAGE
f − Frequency, Normalized − %
VDD − Supply Voltage − V
81215
0.0%
−0.5%
11 1413109
0.5%
1.5%
1.0%
2.0% RA = 110 kΩ
RB = 182 Ω
CT = 220 pF
RA = 221 kΩ
RB = 3.32 Ω
CT = 220 pF
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TYPICAL CHARACTERISTICS
Figure 11
OSCILATOR FREQUENCY
vs
RA x CT
fOSC − Oscilator Frequensy − Hz
RA x CT − s
10 M
1 M
100 K
10 K
1 K
100
1 M
RA = 76.8 kΩ
RB = 0 Ω
VDD = 10 V
10 µ100 µ1 µ
Figure 12
DISCHARGE TIME
vs
CT
Tdischarge − Discharge Time− s
CT− Farad
100 P 10 n
100 n
10 n
1 n 100 n
RA = 76.8 kΩ
RB = 0 Ω
VDD = 10 V
1 µ
1 µ
10 µ
Figure 13
SYNC PULSE WIDTH
vs
TEMPERATURE
SYNC − Pulse Width− ns
Tj − Temperature − °C
−50 50 125
85
75
25 100750−25
80
90
105
95
100
110
115 RA = 110 kΩ
RB = 182 Ω
CT = 220 pF
Figure 14
OUTPUT DEAD TIME
vs
TEMPERATURE
TO − Output Dead Time − ns
Tj − Temperature − °C
−50 50 125
95
90
25 100750−25
100
115
105
110
120
125
80
85
RA = 110 kΩ
RB = 182 Ω
CT = 220 pF
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Figure 15
PROPAGATION DELAY (SYNC RISE TO OUTPUT FALL)
vs
TEMPERATURE
Tprop(f) − Propagation Delay− ns
Tj − Temperature − °C
−50 50 125
7
5
25 100750−25
6
8
11
9
10
12
Figure 16
Tj − Temperature − °C
10
5
20
15
25
−50 50 12525 100750−25
PROPAGATION DELAY (SYNC FALL TO OUTPUT RISE)
vs
TEMPERATURE
Tprop(f) − Propagation Delay− ns
Figure 17
OSCILLATOR DISCHARGE ON-RESISTANCE
vs
TEMPERATURE
RDS(on) − Oscillator Discharge FET On Resistance− Ω
Tj − Temperature − °C
50
45
35
30
25
−50 50 12525 100750−25
40
vDIS = 1.5 V
VCT = 3.0 V
Figure 18
CURRENT SENSE THRESHOLD
vs
TEMPERATURE
CS − Current Sense Threshold− mV
Tj − Temperature − °C
−50 50 12
5
25 100750−25
810
770
710
690
650
750
670
730
790
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TYPICAL CHARACTERISTICS
Figure 19
SUPPLY CURRENT
vs
OSCILLATOR FREQUENCY (NO LOAD)
IDD − Supply Current − mA
fOSC − Oscillator Frequency − kHz
0 600 K 1 M
1.0
800 K400 K200 K
1.5
2.0
3.0
2.5
3.5
4.0
VDD = 14 V
VDD = 12 V
VDD = 9 V
Figure 20
0 600 K 1 M
6
4
400 K 800 K200 K
8
14
10
12
16
18
0
2
SUPPLY CURRENT
vs
OSCILLATOR FREQUENCY (1 nF LOADS)
IDD − Supply Current − mA
fOSC − Oscillator Frequency − kHz
VDD = 14 V
VDD = 12 V
VDD = 9 V
Figure 21
t − Time − 1 ms/div.
OUTA
OUTB
SYNC
CT
TYPICAL SOFT START WAVEFORMS
Figure 22
t − Time − 1 ms/div.
OUTA
OUTB
SYNC
CT
TYPICAL OVERALL START−UP WAVEFORMS
SLUS623 − SEPTEMBER 2004
17
www.ti.com
REFERENCES
1. Power Supply Seminar SEM−1300 Topic 1: Unique Cascaded Power Converter Topology for High Current
Low Output Voltage Applications, by L. Balogh, C. Bridge and B. Andreycak, Texas Instruments Literature
No. SLUP133
2. Low Cost Inverter Suitable for Medium-Power Fuel Cell Sources, by P.T. Krein and R Balog, IEEE Power
Electronics Specialists Conference Proceedings, 2002, vol. 1, pp. 321−326.
3. Datasheet, UCC2540 High-Efficiency Secondary-Side Synchronous-Buck PWM Controller, Texas
Instruments Literature No. SLUS539
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