TPS40054_12 Datasheet, PDF (13/35 Pages) Texas Instruments – WIDE-INPUT SYNCHRONOUS BUCK CONTROLLER

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TPS40054
TPS40055
TPS40057
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 (C_SS/SD) is connected to the
SS/SD pin. There is a ramped voltage generated at this pin by charging C_SS/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, V_SSRMP. The error amplifier also has the 0.7-V reference
(V_FB) voltage applied to a non-inverting input. The structure of the error amplifier input stage is such that the
lower of V_FB or V_SSRMP becomes the dominant voltage that the error amplifier uses to regulate the FB pin. This
provides a clean, closed-loop startup while V_SSRMP is lower than V_FB and a precision reference regulated supply
as V_SSRMP climbs above V_FB. To ensure a controlled ramp-up of the output voltage, the soft-start time should be
greater than the L-C_O time constant as described in Equation 12.
t_START w 2p ^ǸL C_O (seconds)
where
• t_START is the startup ramp time in s
(12)
There is a direct correlation between t_START and the input current required during start-up. The faster t_START, 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, C_SS/SD, is described in Equation 13.
For applications in which the V_IN 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 V_IN supply transitions between 6 V and 7 V.
( ) C_{SS / SD}
=
æ
ç
I_SS
/
SD
è ^VFB
ö
÷
´
t_START
ø
F
where
• I_SS/SD is the soft-start charge current (typical value is 2.35 μA)
• V_FB is the feedback reference voltage (typical value is 0.7 V)
(13)
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