Datasheet

TPS40055-EP
www.ti.com SGLS310D –JULY 2005–REVISED FEBRUARY 2012
(16)
where:
I
SINK
is the current into the ILIM pin and is 7.5 µA, minimum
I
OC
is the overcurrent setpoint which is the dc output current plus one-half of the peak inductor current
V
OS
is the overcurrent comparator offset and is –20 mV, maximum
SYNCHRONIZING TO AN EXTERNAL SUPPLY
The TPS40055 can be synchronized to an external clock through the SYNC pin. Synchronization occurs on the
falling edge of the SYNC signal. The synchronization frequency should be in the range of 20% to 30% higher
than its programmed free-run frequency. The clock frequency at the SYNC pin replaces the master clock
generated by the oscillator circuit. Pulling the SYNC pin low programs the TPS40055 to freely run at the
frequency programmed by R
T
.
The higher synchronization must be factored in when programming the PWM ramp generator circuit. If the PWM
ramp is interrupted by the SYNC pulse, a UVLO condition is declared and the PWM becomes disabled. Typically
this is of concern under low-line conditions only. In any case, R
KFF
needs to be adjusted for the higher switching
frequency. In order to specify the correct value for R
KFF
at the synchronizing frequency, calculate a dummy value
for RT that would cause the oscillator to run at the synchronizing frequency. Do not use this value of RT in the
design.
(17)
Use the value of R
T(dummy)
to calculate the value for R
KFF
.
(18)
This value of R
KFF
ensures that UVLO is not engaged when operating at the synchronization frequency.
R
T(dummy)
is in kΩ
Loop Compensation
Voltage-mode buck-type converters are typically compensated using Type III networks. Since the TPS40055
uses voltage feedforward control, the gain of the PWM modulator with voltage feedforward circuit must be
included. The modulator gain is described in Figure 11, with V
IN
being the minimum input voltage required to
cause the ramp excursion to cover the entire switching period as described in Equation 19.
(19)
Duty cycle (D) varies from 0 to 1 as the control voltage (V
C
) varies from the minimum ramp voltage to the
maximum ramp voltage (V
S
). Also, for a synchronous buck converter, D = V
O
/ V
IN
. To get the control voltage to
output voltage modulator gain in terms of the input voltage and ramp voltage:
(20)
Copyright © 2005–2012, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Link(s): TPS40055-EP