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Switch Mode Secondary Side Post Regulator
UCC1583
UCC2583
UCC3583
FEATURES
• Precision Secondary Side Post
Regulation for Multiple Output Power
Supplies
• Useful for Both Single Ended and
Center Tapped Secondary Circuits
• Ideal Replacement for Complex
Magnetic Amplifier Regulated Circuits
• Leading Edge Modulation
• Does Not Require Gate Drive
Transformer
• High Frequency (>500kHz) Operation
• Applicable for Wide Range of Output
Voltages
• High Current Gate Driver (0.5A
Sink/1.5A Source)
• Average Current Limiting Loop
DESCRIPTION
The UCC3583 is a synchronizable secondary side post regulator for precision regulation of the auxiliary outputs of multiple output power supplies. It
contains a leading edge pulse width modulator, which generates the gate
drive signal for a FET power switch connected in series with the rectifying
diode. The turn-on of the power switch is delayed from the leading edge of
the secondary power pulse to regulate the output voltage. The UCC3583
contains a ramp generator slaved to the secondary power pulse, a voltage
error amplifier, a current error amplifier, a PWM comparator and associated logic, a gate driver, a precision reference, and protection circuitry.
The ramp discharge and termination of the gate drive signal are triggered
by the synchronization pulse, typically derived from the falling edge of the
transformer secondary voltage. The ramp starts charging again once its
low threshold is reached. The gate drive signal is turned on when the ramp
voltage exceeds the control voltage. This leading edge modulation technique prevents instability when the UCC3583 is used in peak current mode
primary controlled systems.
The controller operates from a floating power supply referenced to the output voltage being controlled. It features an undervoltage lockout (UVLO)
circuit, a soft start circuit, and an averaging current limit amplifier. The current limit can be programmed to be proportional to the output voltage, thus
achieving foldback operation to minimize the dissipation under short circuit
conditions.
TYPICAL APPLICATION AND BLOCK DIAGRAM
(continued)
Note: Pin connections shown for 14-pin packages.
11/98
UDG-96201-2
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ABSOLUTE MAXIMUM RATINGS
VDD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15V
I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15mA
VDD
RAMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to V
I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5mA
RAMP
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I
REF
PCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.2V to 0.2V
I
(twp < 1µS and Duty Cycle < 10%) . . . . . . –0.8A to 1.8A
GATE
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –5mA to 5mA
I
COMP
I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –5mA to 5mA
CAO
. . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.6V to V
V
SYNC
I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –05mA to 5mA
SYNC
INV, SS, ILIM, ISENSE. . . . . . . . . . . . . . –0.3V to VREF + 0.3V
Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
Junction Temperature. . . . . . . . . . . . . . . . . . . –55°C to +150°C
Lead Temperature (Soldering, 10 sec.). . . . . . . . . . . . . +300°C
All voltages are with respect to the COM terminal unless otherwise stated. Currents are positive into, negative out of the
specified terminal. Consult Packaging Section of Databook for
thermal limitations and considerations of packages.
REF
+ 1V
DD
–30mA
+0.3V
UCC1583
UCC2583
UCC3583
CONNECTION DIAGRAMS
DIL-14, SOIC-14 (Top View)
J, N, or D Packages
PLCC-20 (Top View)
Q Package
ELECTRICAL CHARACTERISTICS: Unless otherwise stated, these specifications apply for T
= –55°C to 125°C for
A
UCC1583, –40°C to 85°C for UCC2583, and 0°C to 70°C for UCC3583; VDD = 12V, RT= 60k, CT= 200pF, TA= TJ.
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
Ramp Generation and Synchronization
Maximum Input Operating Frequency For input with 5% to 90% duty cycle (Note 1) 500 kHz
Ramp Frequency, Free Running 95 100 105 kHz
Ramp Discharge Current V
= 0.5V 2.0 3.6 mA
RAMP
Low Threshold Voltage No min, no max, 0=TYP 0 V
High Threshold Voltage 3.75 4 4.25 V
Synchronizing Threshold Voltage (On) (Note 1) 1 V
Synchronizing Comparator Hysteresis 1 V
Output Duty Cycle
Minimum Duty Cycle Output D/C = Output PW / Input PW 0 %
Maximum Duty Cycle Output D/C = Output PW / Input PW 100 %
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UCC1583
UCC2583
UCC3583
ELECTRICAL CHARACTERISTICS:
Unless otherwise stated, these specifications apply for TA= –55°C to 125°C for
UCC1583, –40°C to 85°C for UCC2583, and 0°C to 70°C for UCC3583; VDD = 12V, RT= 60k, CT= 200pF, TA= TJ.
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
Voltage Error Amplifier
V
V
INV
I
INV
Low V
V
COMP
V
High V
COMP
= VINV, 0°C to 70°C (UCC3583) 2.462 2.5 2.538 V
COMP
V
= VINV, All Other Temperature Ranges 2.45 2.5 2.55 V
COMP
V
= VINV 300 500 nA
COMP
= 2.6V, ICOMP = 100µA 450 700 mV
INV
= 2.4V, ICOMP = –100µA 5.0 5.5 6.0 V
INV
AVOL No Load 70 90 dB
GBW Product At f = 100kHz, T
A = 25°C (Note 1) 3 5 MHz
Current Error Amplifier
Input Offset Voltage 10 mV
Input CM Low Common Mode for CS and ILIM (Note 1) 0 V
Input CM High Common Mode for CS and ILIM (Note 1) 2 V
Low V
V
CAO
V
High V
CAO
+IN
+IN
= 0V, V
= 0V, V
= 0.1V, ICAO = 100µA 250 500 mV
–IN
= 0.1V, ICAO = –100µA 5.0 5.5 6.0 V
–IN
Input Current (ILIM and CS Pins) –50 0 50 nA
AVOL No Load 70 90 dB
GBW Product At f = 100kHz, T
= 25°C 2 4 MHz
A
Soft Start Current 10 25 µA
UVLO
VDD On Threshold Voltage 8.5 9.0 9.5 V
VDD Off Threshold Voltage 7.9 8.4 8.9 V
UVLO Hysteresis 0.3 0.6 0.9 V
Bias Supply
Supply Clamp Voltage 13 14 15 V
Supply Current (VDD) f = 100kHz With No Gate Output Load 3 5 mA
Output Driver
SAT High I
V
= –150mA 0.6 1.0 V
GATE
VSAT Low IGATE = 50mA 0.4 0.75 V
Rise Time C
GATE = 1nF 50 75 ns
Fall Time CGATE = 330pF 20 40 ns
Reference
VREF IREF = 0, 0°C to 70°C (UCC3583) 4.925 5 5.075 V
REF = 0, All Other Temperature Ranges 4.900 5 5.100 V
I
Line Regulation VDD = 10V to 14V 2 30 mV
Load Regulation IREF = 0mA to 2mA 1 20 mV
Note 1: Guaranteed by design. Not 100% tested in production.
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PIN DESCRIPTIONS
CAO: Output of the current error amplifier. Averaging of
the sensed current signal is provided by connecting an
integrating capacitor between ILIM and CAO. CAO feeds
into the PWM comparator input and controls the loop
when its voltage is higher than the voltage at COMP
(output of the voltage error amplifier).
COM: Signal ground for the chip. It is connected to the
positive terminal of the output voltage being regulated by
the IC.
COMP: Output of the voltage error amplifier fed into the
PWM comparator. Loop compensation components are
connected between COMP and INV.
CS: Non-inverting input of the current error amplifier. The
sensed current signal from the current sense resistor is
connected to this pin. By making the signal at CS
proportional to the output voltage, effective current
foldback limiting can be provided.
GATE: Gate drive output for the power switch FET. The
drive pin has a 0.5A sink/1.5A source capability and very
low output off-state impedance.
ILIM: Inverting input of the current error amplifier. It sets
the DC limit for the output current.
INV: Inverting input of the voltage error amplifier. The
feedback signal is connected to this pin using a resistive
divider between REF and –V
PCOM: Power ground for the chip. It is connected to the
source terminal of the MOSFET being regulated by the
IC.
RAMP: This pin is the input to the PWM comparator and
provides a ramp signal for generation of the PWM signal.
A capacitor to COM and a resistor to REF set the
charging rate for the ramp. An internal current source of
O.
UCC1583
UCC2583
UCC3583
1mA discharges RAMP when synchronization signal
appears or when RAMP crosses a 4V threshold. In the
intended mode of operation, the switching frequency is
determined by the secondary power pulse. The RC
components at RAMP should be selected to give an
appropriately sized ramp signal. In the absence of a
synchronizing pulse, these RC components determine
the free running frequency of the controller.
REF: Precision 5V reference pin. REF stays off until
VDD exceeds 9V and turns off again when VDD drops
below 8.4V. Bypass REF to COM.
SS: This pin provides a soft start function. A capacitor to
REF programs the soft start time. During soft start, the
PWM comparator is controlled by the soft start voltage
resulting in a slow increase in output duty cycle. Once
the soft start capacitor is discharged, output control is
dictated by the larger of the output at CAO or COMP.
SYNC: Synchronization input pin. It is connected to a
signal representative of the secondary power pulse. One
possible implementation is to use a resistive divider
between terminal S2 of the secondary winding shown in
Figure 1 and REF for generating the input to the SYNC
pin. The synchronizing comparator is referenced to 0.5V
and has ±500mV of hysteresis. The trip levels are
approximate 1.0V and 0.0V. The designer should prevent
the SYNC pin from exceeding 0.3V below ground as this
will turn on the ESD diode.
VD: Power supply for the output driver. VD should be
tied to VDD in the application.
VDD: Power supply for the chip. VDD should be
bypassed to COM. VDD has to be 9V for the IC to start
and 8.4V for it to remain operational. A shunt clamp from
VDD to COM limits the supply voltage to 14V.
APPLICATION INFORMATION
Power Stage Circuit Configuration
The UCC3583 is designed for use in a post regulator application for tightly regulating auxiliary outputs in a multiple output converter. The post regulation is applied to the
secondary side power pulse of a power transformer
where the power pulse is controlled by the feedback signal from the main output. In order to simplify the application of the UCC3583, it is required that the IC be
referenced to the positive output terminal and the output
filter inductor be placed in the return path. The placement of the inductor in the return path facilitates better
EMI performance, in addition to making magnetic de-
signs and terminations easier to implement. Typical setup and circuit waveforms of the UCC3583 system application are shown in Figure 1. Figure 2 shows waveforms
for a single ended output rectifier application of the
UCC3583 shown on page 1. The UCC3583 can also be
used in half bridge rectifier applications as shown by the
circuit and waveforms depicted in Figures 3 and 4. Referencing the IC to the positive output terminal creates a requirement for a floating bias voltage for the IC which can
be referenced to the same positive voltage terminal. Possible implementations of deriving the floating bias voltage
are shown in Figure 5.
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APPLICATION INFORMATION (cont.)
For the circuit shown in Figure 5a, CC1 is charged when
the transformer voltage is positive and the synchronous
switch is on. During the off period of Q-SYNC, the charge
is transferred to CC2 through diode DC2. Diode DC3
charges CC2 during the blocking interval of Q-SYNC.
This method is preferable when the transformer positive
voltage is high enough to generate the required bias voltage. For the circuit shown in Figure 5b, CC1 is charged
during the period when reverse (reset) voltage appears
across the secondary. The charge on CC1 is transferred
to CC2 through DC2 when Q-SYNC turns on. This
method is preferable when the reverse voltage is high
enough to generate the required bias voltage. The series
resistor should be chosen to handle the required voltage
drop at full IC operating current when the zener clamp
across VDD and COM is activated.
The following is a description of the major functional
blocks of the UCC3583. Refer to Figure 6 (Typical Application Circuit) for component designations.
UVLO and Start Up
The UCC3583 has an internal undervoltage lockout circuit which keeps the internal circuitry inactive until VDD
exceeds the upper threshold (9V). Once the chip is activated, VDD has to be above the lower UVLO threshold
(8.4V) for it to remain functional. The IC requires a low
startup current of only 100µA when VDD is under the
UVLO threshold. VDD has an internal clamp of 14V
which can sink up to 10mA. Measures must be taken not
to exceed this current. The internal reference (REF) is
brought up when the UVLO on threshold is exceeded.
The soft start pin provides an effective means to start the
IC in a controlled manner. An internal current of 10µA
starts discharging a capacitor connected to SS when the
UVLO conditions have been removed. The voltage on
SS controls the duty cycle of the output during the discharge period.
Synchronizing Circuit and Oscillator
UCC3583 is primarily intended for synchronizable operation where its switching frequency is determined by the
secondary pulse of the power transformer. However, it
has an internal oscillator which allows it to operate in
free-running mode when an external synchronization
pulse is not available. The switching frequency is determined by resistor R
RAMP and capacitor CT connected from RAMP to GND.
The frequency is given by:
=
+
CH DIS
and
T connected between REF and
=••
CH T T
UCC1583
UCC2583
UCC3583
•
T
=
DIS
RAMP dis
The values of RT and CT are also dictated by the fact
that the ramp is discharged through an internal impedance of 2k. The value of R
ensure that the internal discharge current is the current
through R
T during the entire discharge period. This re-
sults in making the value of CT relatively small for a desired frequency of operation.
When the synchronizing signal is available, the oscillator
frequency should be programmed to be lower than the
synchronizing frequency to ensure proper operation. A
large difference in self-running and synchronizing frequencies leads to smaller ramp amplitude and higher
noise sensitivity. The ramp capacitor is discharged when
the synchronization signal arrives and begins charging
when the low threshold is crossed.
There are two methods to synchronize to the secondary
pulse. One method is to use the rising edge of the secondary pulse, which reduces the maximum duty cycle
available. Subsequently, the post regulator switch cannot
be turned on during the C
method is to use the falling edge of the secondary pulse
for synchronization. This method is preferable because it
allows a slower discharge of the ramp capacitor without
affecting the maximum available duty cycle of the post
regulator. The UCC3583 SYNC input needs to reach a
fixed threshold (1.0V typical) for synchronization to take
effect. Hence the IC is usable with either method of synchronization. However, the UCC3583 oscillator configuration is better suited for synchronization to the falling
edge. A recommended method to implement the synchronization is shown in Figure 6. By connecting SYNC
to a resistive divider between REF and the secondary
terminal S2, the synchronization is achieved whenever
the voltage on S2 goes from a negative value to zero. R
and RBshould be selected so that the voltage on the
SYNC pin varies from 0V to 1V. Placement of a Schottky
diode from SYNC to COM prevents the voltage at SYNC
from going negative. The internal hysteretic SYNC comparator has an inverting input set to 0.5V with about
±0.5V hysteresis.
PWM Comparator
The UCC3583 uses a leading edge PWM scheme. In a
leading edge PWM, the output pulse (gate signal) is
turned on when the error amplifier crosses the PWM
ramp and turned off by the clock/oscillator. Leading edge
modulation is naturally provided by magamp type post
regulators and is an essential feature for post regulators.
Without the leading edge modulation in a multiple output
()
RAMP p p
()
ť
T needs to be at least 50k to
T discharge time. The other
T
A
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APPLICATION INFORMATION (cont.)
converter with post regulation on one or more outputs,
the primary current shape does not remain monotonic
and can lead to instability when the primary current is
used for current mode control or current limiting. When
compared to conventional trailing edge PWMs, the leading edge modulation leads to a phase inversion that
needs to be accounted for in the feedback loop. For the
UCC3583, this inversion is automatically provided since
the sensed voltage at the power supply output negative
terminal has a negative polarity with respect to the chip
common. Thus, UCC3583 does not require inverting
buffers which would otherwise be needed.
Error Signal Generation and Current Limiting
The PWM comparator in the UCC3583 is controlled by
three parallel loops with only one of them in effect at a
time. During normal operation, the voltage error amplifier
output is fed to the PWM comparator. The voltage error
amplifier can be compensated using commonly used
feedback techniques to achieve the desired dynamic performance. The ouput drive capability of the voltage amplifier is limited to 100µA, so appropriately high
impedances should be used to utilize the full output
swing of the amplifier. During startup, the soft start ca-
UCC1583
UCC2583
UCC3583
pacitor controls the pulse width. The third control loop is
provided by the average current amplifier. By sensing the
instantaneous inductor current and filtering/averaging it
with the current error amplifier, accurate current limiting
is achieved. This loop is in effect only during the overcurrent mode and provides a more accurate and noise free
control of the maximum output current compared to conventional peak current limiting circuits. The current limit is
set by programming the voltage at ILIM based on the
current sense resistor chosen. In addition, the current
limit can be made proportional to the output voltage in order to limit the power dissipation under short circuit conditions. This is implemented by inserting a bias voltage
on CS which is proportional to the output voltage.
Gate Drive Circuit
The gate drive circuit of the UCC3583 provides high current drive capability and is very easy to implement as a
result of tying the chip common to the source of the
switching device. Turn on current is higher (1.5A) as fast
turn on is essential for low losses and effective operation.
During the turn off, the drain voltage disappears, so turn
off time can be slower without increasing switching
losses.
Figure 1. UCC3583 SSPR system application and typical waveforms.
6
UDG-98195
Page 7
APPLICATION INFORMATION (cont.)
UCC1583
UCC2583
UCC3583
Note: All waveforms are referenced to chip common.
Figure 2. Single ended post regulator waveforms.
Figure 3. Half-bridge synchronous post regulator application.
UDG-96141-1
UDG-96142-1
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APPLICATION INFORMATION (cont.)
UCC1583
UCC2583
UCC3583
Figure 4. Half-bridge synchronous post regulator to waveforms.
UDG-96143-1
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APPLICATION INFORMATION (cont.)
UCC1583
UCC2583
UCC3583
Figure 5. Possible implementation for floating bias voltage generation.
R
A
R
B
UDG-96175-1
Figure 6. Typical application circuit.
UNITRODE CORPORATION
7 CONTINENTALBLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 • FAX (603) 424-3460
UDG-96072-2
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