Datasheet

www.ti.com
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
(seconds)
(5)
C
SS
+ t
SS
I
SS
V
FB
(6)
TPS40075
SLUS676A – MAY 2006 – REVISED SEPTEMBER 2007
Figure 26 shows the effect of changing input voltage on the duty cycle, and how that change takes place. The
pulse width modulator (PWM) ramp input is generated using a current that is proportional to the current into the
KFF pin. The TPS40075 holds this pin at a constant 400 mV, so connecting a resistor from KFF to the input
power supply causes a current to flow into the KFF pin that is proportional to the input voltage. The slope of the
ramp signal to the PWM is therefore proportional to the input voltage. This allows the duty cycle to change with
variations in Vin without requiring much response from the error amplifier, resulting in very good line transient
response. Another benefit is essentially constant PWM gain over the entire input voltage operating range. This
makes the output control loop easier to design for a wide input range converter.
Figure 26. Voltage Feed-Forward and PWM Duty Cycle Waveforms
TPS40075 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 non-inverting 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:
where
• L is the value of the filter inductor
• C
OUT
is the value of the output capacitance
• t
START
is the output ramp up-time
For a desired soft-start time, the soft-start capacitance, C
SS
, can be found from:
Please note: 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 since the output capacitance must be charged
faster.
Copyright © 2006 – 2007, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Link(s) :TPS40075