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

LM5115A

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SNVS467B – DECEMBER 2006 – REVISED MARCH 2013

Figure 12. Line Feed-Forward Diagram

Figure 13. Line Feed-Forward Waveforms

SYNC pin resistor is therefore:

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.

Error Amplifier and Soft-Start (FB, CO, COMP & TRK/SS)

An internal wide bandwidth error amplifier is provided within the LM5115A for voltage feedback to the PWM

controller. The amplifier’s inverting input is connected to the FB pin. The output of the auxiliary converter is

regulated by connecting a voltage setting resistor divider between the output and the FB pin. Loop compensation

networks are connected between the FB pin and the error amplifier output (COMP). The amplifier has two non-

inverting inputs. The first non-inverting input connects to a 0.75V bandgap reference while The second non-

inverting input connects to the TRK/SS pin and it has 15 µA pull-up current source. The TRK/SS pin can be tied

to an external resistor divider from the master output for tracking, or it can be tied to a capacitor for soft-start .

TRK/SS is the reference input to the amplifier when the voltage applied to the pin is < 0.75V. For higher inputs,

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Product Folder Links: LM5115A