/ .
SLUS431A – MAY 2000 – REVISED DECEMBER 2000
Operates with Secondary–Side PWM
Multi-Mode Overcurrent Protection with
Control
Isolated PWM Command through a Pulse
Transformer
Initial Free-Running Soft-Startup with
Duty-Cycle Clamping
Up to 400-kHz Synchronizable Switching
Frequency
High-Current FET Drive (1.5-A Sink, 0.75-A
Source)
Restart, Latching, or Cycle-by-Cycle
Current Limiting
Programmable Volt-Second Clamp for
Transformer Reset
Undervoltage Lockout with 2-V Hysteresis
Low-Current Startup with Optional
Disconnect
Programmable Overvoltage and
Undervoltage Protection
8–pin Version Also Available (UCC3960)
typical application diagram
VIN
UCC3961
14
UVS
1
OVS
UV/OV
STOP
START
13
VDD
12
OPTIONAL
DEPLETION
MODE
SWITCH
+
–
REF
8
6
VS
VOLT
SEC
CLAMP
OUT
11
CS
9
4
FB
3
SS
5
RT
SD
2
7
AGND PGND
10
FEED
BACK
SOFT START
FREQ
SET
FAULT LATCH DELAY
CSD
UDG–99039
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.
Copyright 2000, Texas Instruments Incorporated
!$%'#)!%$ !( *''$) ( % &*"!)!%$ )
'%*)( %$%'# )% (&!!)!%$( &' ) )'#( % ,( $()'*#$)(
()$' +''$)- '%*)!%$ &'%((!$ %( $%) $(('!"- !$"*
)()!$ % "" &'#)'(
POST OFFICE BOX 655303
• DALLAS, TEXAS 75265
1
SLUS431A – MAY 2000 – REVISED DECEMBER 2000
description
The UCC3961 advanced primary-side startup controller is a unique solution that provides all the primary-side
functions required for a single-ended, isolated, switch-mode power converter incorporating secondary-side
PWM control. It is usable with a wide range of secondary control circuits and is especially well suited for systems
where sophisticated handling of overload conditions is required.
Secondary-side control assumes that output voltage and current measurements are interfaced directly to an
output ground-referenced PWM stage that develops the power switch command for the supply. This digital
PWM command can then be transmitted to the primary-side power switch through a simple and low-cost
isolating pulse transformer. With secondary-side control, it is much easier to monitor and control the system load
with tightly coupled analog control loops. Load-oriented features such as output current sharing and
synchronous rectification are implemented more easily.
The UCC3961 provides all the circuitry required on the primary side of a secondary-side controlled power
supply. It features a free running 60-kHz to 360-kHz oscillator which is synchronizable to the secondary-side
PWM signal and also has the ability to accept start/stop PWM commands from the isolating pulse-edge
transformer (PET). The use of an extremely small and low-cost pulse transformer allows for higher converter
bandwidth. This also eliminates the loop-gain variations due to initial accuracy and aging of an opto-coupler
feedback element or the size penalty of a gate transformer. It also includes an undervoltage lockout circuit with
2-V hysteresis, a low-current startup with active low during UVLO, a soft-start capability, a 5-V reference and
a high-current power output.
Advanced features of UCC3961 allows implementation of initial startup with optional high-voltage disconnect
after starting, and input voltage monitoring with turnoff for either undervoltage or overvoltage conditions. Other
features include power-switch current protection, pulse-by-pulse current limiting, shutdown after a
programmable delay and continuous input volt*second clamp. The UCC3961 also provides a multi purpose,
bidirectional shutdown pin (SD), that can be used in conjunction with the devices on the START pin, to modify
the converters behavior in overload conditions. The possible configurations include continuous-peak current
limiting, delayed or immediate shutdown with full cycle soft restart or fully latched overcurrent shutdown.
In a non-typical use, the UCC3961 can accommodate an analog feedback signal through an opto-isolator where
the UCC3961 then can operate in voltage-mode control mode with primary-side peak current limiting.
The UCC3961 and the UCC2961 are available in the 14-pin SOIC (D) and PDIP (P) packages. For applications
where only startup and control are desired, a simplified version of this product is offered in an 8-pin package
as the UCC2960 and UCC3960.
D OR N PACKAGES
(TOP VIEW)
AVAILABLE OPTIONS
PACKAGED DEVICES
SOIC–14
SMALL OUTLINE
(D)
PDIP–14
PLASTIC DIP
(N)
–40C to 85C
UCC2961D
UCC2961N
0C to 70C
UCC3961D
UCC3961N
TA
† The SOIC (D) packages are available taped and reeled. Add an R suffix
to the device type (e.g., UCC2961DR) to order quantities of 2500
devices per reel.
2
POST OFFICE BOX 655303
• DALLAS, TEXAS 75265
OVS
1
14
UVS
SD
2
13
START
SS
3
12
VDD
FB
4
11
OUT
RT
5
10
PGND
REF
6
9
CS
AGND 7
8
VS
SLUS431A – MAY 2000 – REVISED DECEMBER 2000
absolute maximum ratings over operating free-air temperature range (unless otherwise noted)†‡
Input voltage, VI(VDD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 V
Input current, II(VDD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 V
Output current , IO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –1.5 A / 2 A
Output voltage: REF, START . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to VDD+0.3 V
Input voltage: OVS, SD, SS, RT, VS, CS, UVS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to VDD+0.3 V
FB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –7.0 V to VDD+0.3 V
Operating junction temperature range, TJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –55C to 150C
Storage temperature range, Tstg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –65C to 150C
Lead temperature soldering 1,6 mm (1/16 inch) from case for 10 seconds . . . . . . . . . . . . . . . . . . . . . . . 300C
† 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.
‡ Unless otherwise indicated, voltages are reference to ground and currents are positive into and negative out of the specified terminals. Pulsed is
defined as a less than 10% duty cycle with a maximum duration of 500 µs. All voltages are with respect to ground unless otherwise stated.
Currents are positive into, negative out of the specified terminal. Consult Packaging Section of the Power Supply Control Data Book (TI Literature
Number SLUD003) for thermal limitations and considerations of packages.
4nF load with 4-Ω series resistor.
POST OFFICE BOX 655303
• DALLAS, TEXAS 75265
3
SLUS431A – MAY 2000 – REVISED DECEMBER 2000
electrical characteristics VDD = 12 V, RT = 53.3 kΩ, CVDD = 1 µF, CREF = 0.1 µF, CSS = 0.01 µF,
ROUT = 4 Ω, COUT = 1 nF and TA = TJ (unless otherwise stated)
supply section (VDD)
PARAMETER
Clamp voltage
TEST CONDITIONS
MIN
TYP
MAX
UNITS
16
17.5
19
V
Operating current
IVDD = 10 mA
No load, COUT = 0
1.8
2.3
2.8
mA
Starting current
VDD = 9 V
100
150
200
µA
undervoltage lockout section
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Start threshold voltage
9.5
10
10.5
V
Hysteresis voltage
1.7
2
2.3
V
voltage reference section (REF)
PARAMETER
TEST CONDITIONS
Reference voltage
Load regulation voltage
Line regulation voltage
MIN
4.75
IREF = 0 mA to –2.5 mA
VDD = 10 V to 12 V
Short-circuit current
8
TYP
MAX
UNITS
5.0
5.25
V
3
5
mV
1
5
mV
10
16
mA
overvoltage sense section (OVS)
PARAMETER
TEST CONDITIONS
Threshold voltage
MIN
3.68
TYP
4
Input bias current
MAX
UNITS
4.32
V
0.2
µA
undervoltage sense section (UVS)
PARAMETER
TEST CONDITIONS
Threshold voltage
MIN
3.68
TYP
4
Input bias current
MAX
UNITS
4.32
V
0.2
µA
soft start section (SS)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Discharge current
SD = 4.5 V pulsed
3
5
7
µA
Charge current
SD = 4.5 V pulsed
–5
–7
–10
µA
Low–threshold voltage
0.9
1
1.1
V
Clamp threshold
4.5
5
5.5
V
600
800
1000
Ω
MAX
UNITS
On resistance
VDD = 7.5 V
shutdown section (SD)
PARAMETER
TEST CONDITIONS
MIN
TYP
Threshold voltage
3.68
4
4.32
V
Discharge current
0.4
0.6
0.8
µA
Charge current
–4
–6
–8
µA
VDD = 7.5 V
2.5
3.3
NOTES: 1. OUT Low, nominal of 0.7 V reflects the 3 Ω DMOS ON resistance plus 4 Ω RSERIES.
2. OUT High (VDD–OUT), nominal of 0.56 V reflects the 10W HVPMOS ON resistance plus 4W RSERIES.
5
kΩ
On resistance
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• DALLAS, TEXAS 75265
SLUS431A – MAY 2000 – REVISED DECEMBER 2000
electrical characteristics VDD = 12 V, RT = 53.3 kΩ, CVDD = 1 µF, CREF = 0.1 µF, CSS = 0.01 µF,
ROUT = 4 Ω, COUT = 1 nF and TA = TJ (unless otherwise stated) (continued)
current sense section (CS)
PARAMETER
Threshold
Input bias
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Pulse-by-pulse
0.9
1
1.1
V
Immediate
1.3
1.4
1.5
V
0.2
µA
60
100
140
ns
600
800
1000
Ω
MAX
UNITS
CS = 1.1 V pulsed
Delay time CS to OUT
On resistance
oscillator section
PARAMETER
TEST CONDITIONS
Frequency
MIN
135
Frequency change with voltage
VDD = 10 V to 12 V
Minimum duty cycle
TYP
150
165
kHz
0.02
0.2
%/V
0%
Maximum duty cycle
69%
72%
75%
volt-second section (VS)
PARAMETER
TEST CONDITIONS
Threshold
MIN
3.68
TYP
MAX
4
4.32
V
0.2
µA
1000
Ω
MAX
UNITS
Input bias
On resistance
600
800
UNITS
output section (OUT)
PARAMETER
Low-level output voltage
TEST CONDITIONS
MIN
TYP
See Note 1
0.7
1.0
V
High-level output voltage
IOUT = 100 mA (dc)
IOUT = –40 mA (dc)
See Note 2
0.56
1.0
V
Low-level output voltage during UVLO
IOUT = 20 mA (dc),
VDD = 7.5 V
1.5
V
Rise time
30
60
ns
Fall time
15
30
ns
bias regulator section (START)
PARAMETER
Bias regulator
TEST CONDITIONS
VDD – START = 0.5 V
VDD – START = 1.0 V,
Override voltage
MIN
11.7
See Note 3
TYP
MAX
12.1
12.5
12.2
UNITS
V
V
feedback section (FB)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
0.4
µA
–7.2
–7.6
V
40
70
100
ns
50
85
120
ns
input bias current
FB = 4.5 V,
SS = 0 V
Negative compliance voltage
IFB = –100 mA,
SS = 0 V
–6.8
Delay time, FB–SS to OUT, rising edge
FB–SS Pulsed = 2 V,
FB = SS
Delay time, FB–SS to OUT, falling edge
FB–SS Pulsed = 2 V,
FB = SS
NOTES: 1. OUT Low, nominal of 0.7 V reflects the 3 Ω DMOS ON resistance plus 4 Ω RSERIES.
2. OUT High (VDD–OUT), nominal of 0.56 V reflects the 10 Ω HVPMOS ON resistance plus 4 Ω RSERIES.
3. The override VDD voltage for shutting off the bias regulation is 100 mV higher than the bias regulator voltage.
POST OFFICE BOX 655303
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SLUS431A – MAY 2000 – REVISED DECEMBER 2000
Terminal Functions
TERMINAL
NAME
NO.
AGND
7
CS
9
I/O
DESCRIPTION
This pin is the reference point for grounding all analog functions and must be kept as clean as possible from all
switching noise. It should be connected to PGND in only one location as close to the IC as practical.
I
Pulse-by-pulse and shutdown overcurrent sense input pin
FB
4
I
Control input for the signal from a secondary–side PWM controller
OUT
11
O
Drive pin for the MOSFET power switch
OVS
1
I
This pin is used with an external resistor divider of VIN to terminate operation if a voltage is sensed above the
internal 4 V threshold. Activation sets the shutdown latch that requires recycling the voltage on VDD to restart.
PGND
10
REF
6
RT
This power ground for the PWM output stage conducts any current transients from the power switch gate drive. It
should be closely bypassed to VDD and connected to AGND in only one location as close to the IC as possible.
O
This is a 5-V output usable with external loads of up to 10 mA. This voltage is also the source for all internal
analog threshold settings and should be bypassed with a minimum of 0.1-µF capacitance to AGND.
5
I
Sets the free-running startup oscillator frequency
SD
2
I
Input to the shutdown circuit
SS
3
I
The pin implements the primary-side soft-start function. This is the connection point for an external capacitor that
determines the rate of increase in commanded pulse width for the power switch at startup. It also serves as the
ac ground return for the feedback pulse transformer to provide a tracking bias for the FB input.
O
Used to develop a regulated 12 V at VDD and thereby minimize or eliminate continuous current drain
I
This pin is used with an external resistor divider of Vin to terminate operation if a voltage is sensed below the
internal 4-V threshold. Activation maintains the circuit in shutdown with the soft-start capacitor clamped low.
START
13
UVS
14
VDD
12
I
Power input connection for all control circuitry
VS
8
I
Volt-second clamp
detailed descriptions
current sense (CS)
This is the pulse-by-pulse and shutdown overcurrent sense input pin. This current-sense pin triggers a
pulse-by-pulse termination anytime a 1.0-V threshold is exceeded while a signal in excess of 1.375 V on this
pin initiates a complete shutdown. Each activation of pulse-by-pulse termination also sends a current pulse to
the SD pin where an external capacitor can be used to provide a delayed shutdown. Since the CS pin can be
noise sensitive, it is good practice to insert a small low-pass RC filter between this pin and the current sensor.
feedback (FB)
This is the control input for the signal from a secondary-side PWM controller whose pulse-width command has
been differentiated by the feedback pulse-edge transformer into positive start and negative stop pulses. These
signals are used to turn on and turn off the primary power switch and must have an amplitude of at least
VSS ±2.0V (4 V peak-to-peak) for at least 25 ns per pulse and no more than 200 ns per pulse. The maximum
amplitude allowed on this pin is VSS ± 7.0 V.
output drive (OUT)
This is the drive pin for the MOSFET power switch and sinks (1.5 A) and sources (0.75 A) fast, high-current
gate-drive pulses. During shutdown, this pin is self–biased to an active low state. A minimum of 4 W should
be added in series with the output to ensure that the on-chip driver safe operating area is not exceeded. (Data
from the IRF820/830/840 family of MOSFETs, commonly available in the TO–220 package, is used to derive
this value. The gate charge needed to provide full enhancement was used to establish an equivalent
capacitance of up to 4000 pF.)
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SLUS431A – MAY 2000 – REVISED DECEMBER 2000
timing resistor (RT)
A resistor from this pin to AGND establishes a current,
I
SET
2V
RT
that is mirrored internally for several functions. It establishes the free-running startup switching frequency with
an internal capacitor according to the relationship,
fs 8.0 10
RT
9
The startup oscillator has a rise and fall time set to limit the duty-cycle of the power switch to a maximum of 72%,
a limit that is maintained even after the feedback signal takes command. The range of RT is 22.2 kΩ to 133 kΩ,
giving a free-run frequency range range of 60 kHz to 360 kHz, respectively. Variations in the free-running
oscillator frequency overtemperature are very small. The typical temperature coefficient is –40 Hz per degree
Celsius, measured at 150 kHz.
shutdown (SD)
This pin is the input to the shutdown circuit. Like OVS, this pin also sets the shutdown latch when a threshold
above 4 V is exceeded. The primary intent of this input is to allow the use of an external capacitor to program
a delay between the onset of current limiting and the issuance of a shutdown command by integrating current
pulses that appear on this pin with each activation of the CS input. This pin is pulled low with a current sink of
0.33/RT when there is no CS signal. The shutdown function can be disabled by connecting SD pin to AGND.
soft-start (SS)
The pin implements the primary-side soft-start function. This is the connection point for an external capacitor
that determines the rate of increase in commanded pulse width for the power switch at startup. It also serves
as the ac ground return for the feedback pulse transformer to provide a tracking bias for the FB input.
start bias regulator (START)
In conjunction with an external depletion-mode N-channel FET, such as the Supertex DN2530, this pin can be
used to develop a regulated voltage of 12 V at VDD and thereby minimize or eliminate continuous current drain
when starting from a variable high voltage source. If this function is unused, this pin can be left open.
power (VDD)
This is the power input connection for all the control circuitry and, in addition, conducts all the gate charge current
for the power FET. It should be closely bypassed with at least 1.0-µF to PGND and 1.0-µF to AGND. This pin
is internally shunt regulated to clamp at 17.5V to protect the internal components so if a voltage source above
this value is possible, external current limiting must be provided.
volt-second clamp (VS)
This pin provides a volt–second clamp for the operation of the transformer–driving power switch with the aid
of an external capacitor to ground and a high value resistor to the transformer’s voltage source (Vin). With the
initiation of each power pulse, the circuit releases an internal grounding clamp across the capacitor allowing
it to charge with a current from the resistor proportional to the input voltage. If this pin reaches 4 V prior to output
termination from other control functions, then this ends the power pulse.
T
VS
R
VS
1.61 R
VS
C
VS
100 POST OFFICE BOX 655303
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SLUS431A – MAY 2000 – REVISED DECEMBER 2000
pin descriptions (continued)
UNDER–VOLTAGE
UVS
VDD REG
14
12 V
+
13 START
4V
SOFT
START
OVER–VOLTAGE
OVS
1
VS
8
4V
VOLT–SEC
LMT
1V
UVLO
S/D
LATCH
Q
R
Q
S
10 / 8 V
12
VDD
6
VREF
11
OUT
REF 5.0 V
GEN
17 V
1V
PWM
LATCH
STOP
+
FB
4
1V
5V
ISS
SS
START
Q
S
Q
SD
S/S
PWM
LO
HI
1.0V
9
CS
2
SD
ISD
5
HI
SYNC
LO
MAX D/C
OSCILL
CT
10 PGND
5V
2 ISS
AGND
1.375 V
CURRENT
LIMIT
3
5V
RT
+
R
PWM DRIVE
RAMP
CLK
SHUT DOWN
4V
ISD
10
7
UDG–00160
Figure 1. Block Diagram
DETAILED DESCRIPTION
start-up oscillator
The RT pin is connected to an internal 2.0 V nominal, unity-gain closed-loop amplifier that is referenced to a
voltage divider off the 5.0-V reference. When a 22.2-kΩ resistor is connected from the RT pin to GND an
approximate IRT = 90µA internal current is realized that charges the internal oscillator capacitor that is
approximately 58 pF. IRT = 90µA produces a maximum free running oscillator frequency of 360 KHz at a 72%
duty cycle. When a 133.3-kΩ resistor is connected from the RT pin to GND an approximate IRT = 15 mA internal
current is realized that produces a minimum free running oscillator frequency of 60 KHz at approximately 72%
duty cycle. This maximum duty cycle is setup by on-chip MOSFET current mirrors that are not programmable.
Any frequency between these two limits (6:1 maximum to minimum frequency) is obtainable by linearly scaling
the RT resistance between the minimum 22.2-kΩ and maximum 133.3-kΩ values. The secondary-side PWM
frequency should be fixed at (1 / 0.9) or 1.11 times the user programmed primary-side free running oscillator
frequency for proper primary-side synchronization. Therefore, in all cases the recommended secondary-side
synchronization frequency shall be 1.11 times higher than the selected primary-side free running startup
frequency. Taking into consideration the two extreme limits for primary-side free running startup frequency, the
secondary-side operating frequency should be set between 400 kHz and 67 kHz.
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SLUS431A – MAY 2000 – REVISED DECEMBER 2000
DETAILED DESCRIPTION
soft start
The soft-start section contains all the circuitry required to produce a user programmable slowly increasing PWM
duty cycle, starting from 0% to a maximum of 72%. The soft-start cycle is triggered either by the initial
primary-side startup procedure or after any one of three user-programmable fault conditions and one fixed-fault
condition. The PWM duty cycle increases according to the charge rate of an user selectable external soft-start
capacitor, connected from the SS pin to GND. The SS capacitor is charged by a nominal 7-µA internal current
source.
Voltage comparators referenced to a 4.0-V threshold monitor the OVS pin, SD pin and the UVS pin and trigger
a soft-start cycle when a fault condition is detected on any of these pins. Should the CS pin rise in voltage above
1.375 V a soft-star cycle is also triggered. The soft-start cycle disables the output driver OUT and holds it in the
low state until the capacitor connected from the SS pin to GND is discharged below 1.0 V by an internal 5-µA
current sink. After this discharge period the PMW output OUT is enabled and the duty cycle is allowed to slowly
increase as before.
synchronization
The SS pin and the FB pin accept the secondary side of a small-signal synchronization transformer. A
series-blocking capacitor inserted in the primary-side of the synchronization transformer is intended to
differentiate the square-wave gate drive output of the secondary-side PWM controller while preventing the
transformer from saturation. The SS capacitor also provides an ac GND at the SS pin or the synchronization
transformer secondary. The small signal synchronization transformer provides galvanic isolation between
primary and secondary side and must have adequate voltage breakdown rating between the primary and
secondary windings.
Two comparators, with an approximate 1.0-V offset each, are connected to the FB pin to provide plus and minus
differential voltage comparison with a 2.0-V deadband between the FB and SS pins. The 2.0-V deadband
prevents inductive backswing of the small signal transformer from giving false secondary-side pulse-edge
detection.
Enough energy must be coupled into the comparator differential inputs to ensure reliable comparator switching.
This requires sufficient voltage overdrive above the 1.0-V comparator threshold and a specified transformer
circuit time constant to provide a minimum synchronization pulse width.
On receiving the first recognizable negative going voltage pulse (turnoff command) generated from the falling
edge of the differentiated square-wave gate drive signal on the secondary-side, the PWM latch is reset and a
synchronization latch is set. After this event all primary-side PMW driver output is slaved to the secondary-side
driver output in both frequency and duty cycle. The triggering of a soft-start cycle by a fault condition resets the
synchronization latch to again allow the internal startup oscillator to control the PWM latch.
pulse-width modulation
The PWM section consists of a reset dominant SR latch with necessary logical gating on the SET input to allow
control from the free running startup oscillator until feedback from the secondary-side PWM gate drive output
is detected. After the occurrence of detectable feedback from the secondary-side gate driver, the control of the
primary-side PWM latch is handed off to the secondary-side PWM controller. A nine-input OR gate on the PWM
latch reset dominant input allows the numerous fault conditions to reset the PWM latch and control from either
the startup oscillator or feedback from the secondary-side PWM output driver.
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SLUS431A – MAY 2000 – REVISED DECEMBER 2000
DETAILED DESCRIPTION
undervoltage lockout
The undervoltage lockout (UVLO) circuit enables normal operation after VDD exceeds the 10.0-V turnon
threshold and permits operation until VDD falls below the 8-V turnoff threshold. While activated, the UVLO circuit
holds the PWM gate driver output (OUT) and the internal-reference buffer amplifier output REF low. To ensure
proper soft-start and to prevent false SD detection, internal N-channel MOSFET switches discharge external
capacitors connected to the SS and SD pins during undervoltage conditions.
voltage reference
The 5-V internal reference is connected to the REF pin and must be bypassed using a good quality,
high-frequency capacitor. This 5-V reference is not available externally while the chip is disabled by the
undervoltage lockout circuit.
current sensing
The current sense (CS) circuit monitors the voltage across a ground referenced current sense resistor,
connected between the source of the external power MOSFET and GND. The signal amplitude at the CS pin
is compared to two thresholds (1.0 V, and 1.375 V respectively) by two independent voltage comparators.
A voltage level greater than 1.0 V, but less than 1.375 V, sets the reset dominant shutdown latch and resets
the PWM latch. The SD latch is reset by the startup oscillator arriving at its 4.0 V compare threshold. When the
SD latch is set, a scaled current, ISD = –(1/6) × (IRT), charges an user-selected external capacitor connected
between the SD pin and GND. When the SD latch is reset, a scaled current ISD = (1/10)*(1/6) × (IRT), discharges
the user-selected capacitor connected between the SD pin and GND.
A current-sense voltage greater than 1.375 V immediately triggers a SD event and also reset the PWM latch.
During the off period of the PWM latch, any capacitance connected to the CS pin is discharged to GND potential
by an internal 800-Ω device.
volt-second clamp
The volt-second (VS) clamp circuit monitors the voltage at the VS pin produced by an external-series RC circuit.
The resistor is connected from the HV primary-side power input, that is derived from the rectified line voltage,
to the VS pin. The capacitor is from the VS pin to GND and being charged by the resistor during the on-time
of the OUT driver. The resulting exponential voltage at the VS pin is monitored by a voltage comparator with
a 4.0-V threshold. Should the voltage at the VS pin exceed 4.0-V, the PWM latch is reset, and as a result the
output drive signal is terminated. This RC circuit can be tailored to prevent the power transformer from saturation
by effectively limiting the applied maximum volt-second product across the primary winding. During the off
period of the PWM output driver the VS capacitor is discharged to GND potential by an internal switch with 800Ω
on resistance.
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DETAILED DESCRIPTION
start regulator and VDD clamp
To facilitate the primary-side startup, a VDD = 12V voltage regulator may be implemented by using an external
depletion-mode FET. The gate of this device is then connected to the START pin, the source terminal is attached
to the VDD pin, and the drain is tied to the HV primary-side power input that is derived from the rectified line
voltage. An auxiliary bootstrap winding off the main power transformer can be used to generate a bias voltage
greater than 12 V, that effectively shuts down the 12-V regulator and increase the efficiency of the biasing
solution during normal operation.
To ensure that the absolute-maximum voltage ratings of internal devices are not violated, an internal-shunt
voltage regulator is provided to clamp the VDD pin at a nominal 17.5-V maximum voltage. Similarly to other
shunt or Zener-like voltage regulator circuits, the current through the internal VDD clamp must be limited below
the maximum current level indicated in the datasheet. In addition to limiting the current through the clamp circuit,
the maximum power dissipation capability of the particular package used in the application must be considered.
OUT driver
An internal output driver (OUT) is provided to drive the gate of an external N–channel power MOSFET. The
output driver consists of a nominal 4.0-W ON-resistance P-channel MOSFET for turn-on, and a nominal 2.0-Ω
ON resistance DMOS FET utilized during the turn–off of the external MOSFET transistor. An external series gate
resistance is specified to maintain an acceptable safe operating area (SOA) for the DMOS device of the internal
output driver. As discussed in the UVLO section of this datasheet, the undervoltage lockout (UVLO) circuit holds
the PWM gate driver output low while UVLO conditions exists.
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SLUS431A – MAY 2000 – REVISED DECEMBER 2000
APPLICATION INFORMATION
The evaluation circuit of UCC3961 as the primary-side startup circuit and UCC38C45 as the secondary-side
controller is shown in Figure 2.
UP4–101
MUR610
VIN
470µF
470µF
MUR110
10 Ω
IRF530
1 kΩ
4.7 kΩ
22 kΩ
27 kΩ
0.5Ω
36 kΩ
UCC38C45
12 V
8 VREF
1 nF
3.9 kΩ
0.1µF
36 kΩ
COMP 1
VDD
FB 2
6 OUT
CS 3
5 GND
RT 4
7
6.2 kΩ
0.1µF
OUT
10 kΩ
4.7µF
6.8 kΩ
UCC3961
14 UVS
12 V
1000 pF
OVS
1
13 START
SD
2
12 VDD
SS
3
11
FB
4
10 PGND
ISET
5
9
REF
6
OUT
0.1µF
1000 pF
2.7 nF
200Ω
LM= 5.4µH
1 µF
CS
1 µF
0.1µF
100 pF
8
VS
AGND
300Ω
RT
80 kΩ
7
UDG–99041
Figure 2. Evaluation Circuit of UCC3961
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APPLICATION INFORMATION
pulse edge transmission circuit
The UCC3961 uses a pulse-edge-transmission (PET) circuit to transmit isolated gate-pulse information from
the secondary-side controller. It is important for the PET circuit to have proper frequency response and
adequately high damping (low Q-factor) in order to prevent excessive overshoot. The circuit is shown in
Figure 3.
C1
FB
4
VFB
+
UCC3961
SS
25 ns < T < 200 ns
R1
V2P
R2
0
3
CSS
LM
VSS+1.0
VSS
VSS–1.0
t
SECONDARY
GATE PULSE
25 ns < T < 200 ns
Figure 3. Pulse Edge Transmission Circuit
The pulse width measured at the FB pin must be between 25 ns when measured at 1 V above the soft-start
voltage and and 200 ns when measured at 1 V below the soft-start voltage. The FB voltage must not be
overdriven by more than 5 V above or 5 V below the soft-start voltage. In order to prevent false triggering, the
FB voltage must not ring below the soft-start voltage by more than ±0.9 V. This can be met if the PET circuit has
a resonant frequency of 880 kHz and a Q of 0.25. The following values meet the specifications for a 12-V
secondary-gate pulse signal, over the full range of UCC3961 operating frequencies.
T1
1:1 turns ratio, LM = 5.4-µH, Ferronics 11–622J, N1 = N2 = 4 turns
R1
300 Ω
C1
2700 pF
R2
200 Ω
Pulse-edge-transmission (PET) circuits in standard surface-mount packages are available from Pulse
Engineering (Part #PA0128, Part #PA0115) and from Cooper Electronic Technologies (Coiltronix),
(Part #CTX01–15157).
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SLUS431A – MAY 2000 – REVISED DECEMBER 2000
PARAMETER MEASUREMENT INFORMATION
OPERATING WAVEFORMS
DURING OVERLOAD
NORMAL OPERATING WAVEFORMS
Figure 5
Figure 4
OPERATING WAVEFORMS
DURING NO LOAD
Figure 6
ADDITIONAL REFERENCES
1. Dennis, Mark and Michael Madigan, 50-W Forward Converter with Synchronous Rectification and
Secondary-Side Control, SEM–1300, Topic 4, Texas Instruments Literature Number SLUP002.
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