TPS40050 datasheet(13/30 Pages) TI

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TPS40050

TPS40051

TPS40053

SLUS540F − DECEMBER 2002 − REVISED JUNE 2004

www.ti.com

APPLICATION INFORMATION

Energy in the capacitor is described in equation (10).

C +

V2 (Joules)

where:

+ V

* V

Volts2

where:

D Vf is the final peak capacitor voltage

D Vi is the initial capacitor voltage

Substituting equation (9) into equation (8), then substituting equation (11) into equation (10), then setting

equation (10) equal to equation (8), and then solving for CO yields the capacitance described in equation (12).

O +

* I

* V

(Farads)

PROGRAMMING SOFT START

TPS4005x uses a closed-loop approach to ensure a controlled ramp on the output during start-up. Soft-start

is programmed by charging an external capacitor (CSS) via an internally generated current source. The voltage

on CSS is fed into a separate non-inverting input to the error amplifier (in addition to FB and 0.7-V VREF). The

loop is closed on the lower of the CSS voltage or the internal reference voltage ( 0.7-V VREF). Once the CSS

voltage rises above the internal reference 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-CO time constant as

described in equation (13).

START w 2p

(seconds)

There is a direct correlation between tSTART and the input current required during start-up. The faster tSTART,

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

titled, Programming the Current Limit which follows. The soft-start capacitance, CSS, is described in

equation (14).

For applications in which the VIN supply ramps up slowly, (typically between 50 ms and 100 ms) it may be

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

tripping. The soft-start time should be longer than the time that the VIN supply transitions between 6 V and 7 V.

SS +

2.3

0.7 V

START

(Farads)

(10)

(11)

(12)

(13)

(14)

TPS40050 Datasheet(PDF) 13 Page - Texas Instruments

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