TPS40054PWPR datasheet(13/35 Pages) TI

13 / 35 page

( )

SS / SD

START

START w 2

(seconds)

TPS40054

TPS40055

TPS40057

www.ti.com

SLUS593H – DECEMBER 2003 – REVISED JULY 2012

PROGRAMMING SOFT START

The TPS4005x uses a closed-loop soft-start system to ensure a controlled ramp of the output during startup. The

reference voltage used for the startup is derived in the following manner. A capacitor (CSS/SD) is connected to the

SS/SD pin. There is a ramped voltage generated at this pin by charging CSS/SD with a current source. A value of

0.85 V is subtracted from the voltage at the SS/SSD pin and is applied to a non-inverting input of the error

amplifier. This is the effective soft-start ramp voltage, VSSRMP. The error amplifier also has the 0.7-V reference

(VFB) voltage applied to a non-inverting input. The structure of the error amplifier input stage is such that the

lower of VFB or VSSRMP becomes the dominant voltage that the error amplifier uses to regulate the FB pin. This

provides a clean, closed-loop startup while VSSRMP is lower than VFB and a precision reference regulated supply

as VSSRMP climbs above VFB. 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 12.

where

•

tSTART is the startup ramp time in s

(12)

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 described in more detail in the section titled,

Programming the Current Limit which follows. The soft-start capacitance, CSS/SD, is described in Equation 13.

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.

where

•

ISS/SD is the soft-start charge current (typical value is 2.35 μA)

•

VFB is the feedback reference voltage (typical value is 0.7 V)

(13)

Submit Documentation Feedback

Product Folder Link(s): TPS40054 TPS40055 TPS40057

TPS40054PWPR Datasheet(PDF) 13 Page - Texas Instruments