TPS40052

SLUS563C − AUGUST 2003 − REVISED FEBRUARY 2005

13

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APPLICATION INFORMATION

SYNCHRONIZING TO AN EXTERNAL SUPPLY

The TPS40052 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 TPS40052 to freely run at the

frequency programmed by RT.

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, RKFF needs to be adjusted for the higher switching

frequency. In order to specify the correct value for RKFF 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.

R

T(dummy) +

1

f

SYNC

17.82

10*6

* 17 kW

Use the value of RT(dummy) to calculate the value for RKFF.

R

KFF + VIN(min) * 3.5 V

58.14

R

T(dummy) ) 1340 W

This value of RKFF ensures that UVLO is not engaged when operating at the synchronization frequency.

D RT(dummy) is in kΩ

LOOP COMPENSATION

Voltage-mode buck-type converters are typically compensated using Type III networks. Since the TPS40052

includes no voltage feedforward control, the gain of the PWM modulator must be included. The modulator gain

is described in Figure 4.

A

MOD +

V

IN

V

S

or

A

MOD(dB) + 20

log

V

IN

V

S

Duty dycle, D, varies from 0 to 1 as the control voltage, VC, varies from the minimum ramp voltage to the

maximum ramp voltage, VS. Also, for a synchronous buck converter, D = VO / VIN. To get the control voltage

to output voltage modulator gain in terms of the input voltage and ramp voltage,

D

+

V

O

V

IN

+

V

C

V

S

or

V

O

V

C

+

V

IN

V

S

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