Datasheet

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Soft Start

VSENSE

VCOMP

FAULT

Soft-Start

gmv

= -30uA

for VSENSE < 4.25V

during Soft-Start

PRECHARGE

source for

rapid pre-charge

of VCOMP prior

to Soft-Start

FAULT

END OF SS

(LATCHED)

UCC28019A

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.................................................................................................................................................. SLUS828B – DECEMBER 2008 – REVISED APRIL 2009

Soft Start controls the rate of rise of VCOMP in order to obtain a linear control of the increasing duty cycle as a

function of time. VCOMP, the output of the voltage loop transconductance amplifier, is pulled low during UVLO,

IBOP, and OLP (Open-Loop Protection)/STANDBY. Once the fault condition is released, an initial pre-charge

source rapidly charges VCOMP to about 1.9 V. After that point, a constant 30 µ A of current is sourced into the

compensation components causing the voltage on this pin to ramp linearly until the output voltage reaches 85%

of its final value. At this point, the sourcing current decreases until the output voltage reaches 99% of its final

rated voltage. The Soft-Start time is controlled by the voltage error amplifier compensation capacitor values

selected, and is user programmable based on desired loop crossover frequency. Once the output voltage

exceeds 99% of rated voltage, the pre-charge source is disconnected and EDR is no longer inhibited.

Figure 19. Soft Start

Product Folder Link(s): UCC28019A