UCC2802M datasheet(21/55 Pages) TI1

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Full Cycle

Soft Start

0.5 V

4 V

Ref

VCC

Over-Current

1.5 V

Leading Edge

Blanking

COMP

REF/2

= 5 ms

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UCC2800, UCC2801, UCC2802, UCC2803, UCC2804, UCC2805

UCC3800, UCC3801, UCC3802, UCC3803, UCC3804, UCC3805

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SLUS270E – MARCH 1999 – REVISED JUNE 2016

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amp output (COMP) to rise. When COMP gets high enough to enable the output, another fault occurs, latching

off the PWM output, but the soft-start capacitor still continues to rise to 4 V before being discharged and

permitting start of a new cycle. This means that for a severe fault, successive retries is spaced by the time

required to fully charge the soft-start capacitor. TI recommends low leakage transformer designs in high-

frequency applications to activate the overcurrent protection feature. Otherwise, the switch current may not ramp

up sufficiently to trigger the overcurrent comparator within the leading edge blanking duration. This condition

would cause continual cyclical triggering of the cycle-by-cycle current limit comparator but not the overcurrent

comparator. This would result in brief high power dissipation durations in the main converter at the switching

frequency. The intent of the overcurrent comparator is to reduce the effective retry rate under these conditions to

a few milliseconds, thus significantly lowering the short-circuit power dissipation of the converter.

Figure 28. Detailed Block Diagram for Overcurrent Protection

Figure 29. IC Behavior at Repetitive Fault

9.3.13 Soft Start

Internal soft starting of the PWM output is accomplished by gradually increasing error amplifier (E/A) output

voltage. When used in current mode control, this implementation slowly raises the peak switch current each

PWM cycle in comparison, forcing a controlled start-up. In voltage mode (duty cycle) control, this feature

continually widens the pulse width.

UCC2802M Datasheet(PDF) 21 Page - Texas Instruments