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PROGRAMMING SOFT START

L

(4)

2.3 mA

0.7 V

(5)

TPS40060

TPS40061

SLUS543D–DECEMBER 2002–REVISED SEPTEMBER 2004

APPLICATION INFORMATION (continued)

The impedance of the input voltage can cause the input voltage, at the TPS4006x, to sag when the converter

starts to operate and draw current from the input source. Therefore, there is voltage hysteresis that prevents

nuisance shutdowns at the UVLO point.

With RT chosen to select the operating frequency and RKFF chosen to select the start-up voltage, the amount of

hysteresis voltage is shown in Figure 5.

TPS4006x uses a closed-loop approach to ensure a controlled ramp on the output during start-up. Soft-start is

internal reference. To ensure a controlled ramp-up of the output voltage the soft-start time should be greater than

higher the input current required during start-up. This relationship is describe in more detail in the section titled,

necessary to increase the soft-start time to between approximately 2 ms and 5 ms to prevent nuisance UVLO

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