Phase signal

RAMP pin

HS pin

12V

12V

6V

6V

Main Output = 3.3V

Secondary Output = 2.5V

PWM Threshold

SYNC

CLK

RAMP

BUFFER

2.5k

15 PA

CLK

Isync

Isync x 3

2.5k

Phase

Signal

RSYNC

CRAMP

LM25115A

www.ti.com

SNVS501B – FEBRUARY 2007 – REVISED APRIL 2013

Figure 12. Line Feed-Forward Diagram

Figure 13. Line Feed-Forward Waveforms

The recommended SYNC input current range is 50µA to 150µA. The SYNC pin resistor (RSYNC) should be

selected to set the SYNC current (ISYNC) to 150µA with the maximum phase signal amplitude, VPHASE(max). This

will ensure that ISYNC stays within the recommended range over a 3:1 change in phase signal amplitude. The

SYNC pin resistor is therefore:

RSYNC = (VPHASE(max) / 150µA) - 2.5kΩ

Once ISYNC has been established by selecting RSYNC, the RAMP signal slope/amplitude may be programmed by

selecting the proper RAMP pin capacitor value. The RAMP signal slope should be selected to provide adequate

slope compensation for the Valley current mode control scheme (Please refer to the Valley Current Mode Control

section). The recommended peak amplitude of the ramp waveform is 1.75V.

Copyright © 2007–2013, Texas Instruments Incorporated

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