TPS40077PWPR datasheet(16/43 Pages) TI

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VDG−03172

RAMP

COMP

VIN

COMP

RAMP

VPEAK

VVALLEY

tON1 > tON2 and d1 > d2

tON2

tON1

d +

PROGRAMMING SOFT START

START w 2

OUT

(5)

SS + tSS

(6)

TPS40077

SLUS714 – JANUARY 2007

APPLICATION INFORMATION (continued)

Figure 23. Voltage Feed-Forward and PWM Duty Cycle Waveforms

TPS40077 uses a closed-loop approach to ensure a controlled ramp on the output during start-up. Soft start is

programmed by connecting an external capacitor (CSS) from the SS pin to GND. This capacitor is charged by a

fixed current, generating a ramp signal. The voltage on SS is level-shifted down approximately 1 V and fed into

a separate noninverting input to the error amplifier. The loop is closed on the lower of the level-shifted SS

voltage or the 700-mV internal reference voltage. Once the level-shifted SS voltage rises above the internal

reference voltage, output-voltage regulation is based on the internal reference. To ensure a controlled ramp-up

of the output voltage, the soft-start time should be greater than the L-COUT time constant or:

Note that there is a direct correlation between tSTART and the input current required during start-up. The lower

tSTART is, the higher the input current required during start-up, because the output capacitance must be charged

faster. For a desired soft-start time, the soft-start capacitance, CSS, can be found from:

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