Datasheet
VDG−03172
RAMP
COMP
SW
VIN
VIN
SW
COMP
RAMP
V
PEAK
V
VALLEY
T
2
t
ON1
> t
ON2
and d
1
> d
2
t
ON2
t
ON1
d +
t
ON
T
T
1
PROGRAMMING SOFT START
t
START
w 2p L C
OUT
Ǹ
(5)
C
SS
+ t
SS
I
SS
V
FB
(6)
TPS40077
SLUS714D – JANUARY 2007 – REVISED APRIL 2009 .....................................................................................................................................................
www.ti.com
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 (C
SS
) 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-C
OUT
time constant or:
Note that there is a direct correlation between t
START
and the input current required during start-up. The lower
t
START
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, C
SS
, can be found from:
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