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UCC28083D Datasheet(PDF) 6 Page - Texas Instruments

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Part No. UCC28083D
Description  8-PIN CURRENT MODE PUSH-PULL PWM CONTROLLERS WITH PROGRAMMABLE SLOPE COMPENSATION
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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UCC28083D Datasheet(HTML) 6 Page - Texas Instruments

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UCC28083, UCC28084, UCC28085, UCC28086
UCC38083, UCC38084, UCC38085, UCC38086
SLUS488E − SEPTEMBER 2002 − REVISED JULY 2009
6
www.ti.com
detailed pin descriptions
CTRL: The error voltage is typically generated by a secondary-side error amplifier and transmitted to the
primary-side referenced UCC3808x by means of an opto-coupler. CTRL has an internal divider ratio of 0.45 to
maintain a usable range with the minimum VDD of 4.1 V. The UCC38083/UCC38084 family features a built-in
full-cycle soft start while the UCC38085/6 does not.
For the UCC38083/4, soft-start is implemented as a clamp at the input to the PWM comparator. This causes
the output pulses to start near 0% duty cycle and increase until the clamp exceeds the CTRL voltage.
ISET: Program the slope compensation current ramp by connecting a resistor, RSET, from ISET to ground. The
voltage of the ISET pin tracks the 1.5-V internal oscillator ramp, as shown in Figure 1.
ISET
IRAMP
OUTA
OUTB
I RAMP, peak = 5 x ISET, peak
1
2
3
4
UCC38083
8
7
6
5
1
mF
VDD
RT
165 k
W
220
mF
10 k
W
RF
1 k
W
I
RAMP
VCS
CTRL VDD
ISET
CS
RT
OUTA
OUTB
GND
Figure 1. Full Duty Cycle Output
The compensating current source, ISLOPE, at the CS pin is proportional to the ISET current, according to the
relation:
I
SLOPE +
5
I
SET
The ramping current due to ISLOPE develops a voltage across the effective filter impedance that is normally
connected from the current sense resistor to the CS input. In order to program a desired compensating slope
with a specific peak compensating ramp voltage at the CS pin, use the RSET value in the following equation:
RSET
+ V
OSC(peak)
5
RF
RAMP VOLTAGE HEIGHT
Where V
OSC(peak) +
1.5 V
Notice that the PWM Latch drives an internal MOSFET that will discharge an external filtering capacitor on the
CS pin. Thus, ISLOPE will appear to terminate when the PWM comparator or the cycle-by-cycle current limit
comparator sets the PWM latch. The actual compensating slope is not affected by premature termination of the
switching cycle.
(1)
(2)


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