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APPLICATION INFORMATION
Introduction
Programming the Operating Frequency
(
)
f
5
4
RT
2
SW
3.675 10
2.824 10
R
5.355
=
+
-
(1)
Programming the soft-start Time
=
REF
SS
V
C
T
I
(2)
SLVS753B – FEBRUARY 2007 – REVISED NOVEMBER 2007
The TPS40180 is a versatile single-phase controller that can be used as a building block for a more complex
power system, or as a stand alone power supply controller. In either system, the TPS40180 provides an excellent
power conversion solution and supports such features as pre-bias startup, intelligent fault handling capability with
graceful shutdown and restart even with multiple modules sharing a common load. Remote load voltage sense
for improved load regulation where it counts, at the load, thermal shutdown, remote enable and power good
indication features help solve the problems faced by the power supply designer. To ease application to a specific
task, there are several user programmable features including closed loop soft-start time, operating frequency and
current limit level.
More complex power solutions are readily supported by the TPS40180. The device can be configured to run in a
master/slave configuration where a master can control several slaves. Several options are possible including a
single output multiple phase supply sharing phase timing information to reduce input and output ripple, a multiple
output supply that shares phase switching timing information to reduce input ripple currents and a combination
approach that has multiple outputs sharing phase information where each output can use multiple phases. Phase
information in all cases comes from a single device designated the clock master. Current sharing information is
passed from the device designated voltage loop master for each output to the slaves for that particular output rail
by connecting the COMP pin of the master to the COMP pin of the slaves. The clock master is also the voltage
loop master in one of the rails of a multiple output supply; whereas, the other rails are controlled by a voltage
loop master that is a clock slave to the single clock master device.
A resistor is connected from the RT pin to GND to select the operating frequency of the converter. The
relationship between the desired operating frequency and the timing resistance is given by
Equation 1:
where
R
RT is the timing resistance in k
f
SW is the desired switching frequency in kHz
If this is a clock master, the switching frequency above is the per-phase switching frequency.
The soft-start time is programmable by connecting a capacitor from the SS pin to GND. An internal current
source charges this capacitor providing a linear ramp voltage. This ramp voltage is the effective reference to the
error amplifier while it is less that the 700-mV internal reference. The time required for the SS pin to ramp from
GND to 700 mV is the soft-start time. For outputs that are not pre-biased, that time is given in
Equation 2.
where
t
SS is the soft-start time in seconds
C
SS is the capacitor from SS to GND in F
I
SS is the soft-start current in A, 15-A typical
If the output of the converter has a pre-existing voltage on it, the device the soft-start happens a little differently.
The SS pin current is held to a lower value than normal until the PWM becomes active. This occurs as the SS
pin voltage exceeds the FB pin voltage and the COMP pin moves up into the ramp range, causing the first pulse.
At that point, the SS pin current is shifted to 15
Copyright 2007, Texas Instruments Incorporated
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