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

R

SYNC

C

RAMP

LM5115

SNVS343D – MARCH 2005 – REVISED JULY 2006

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Synchronization (SYNC) and Feed-Forward (RAMP)

The pulsing “phase signal” from the main converter synchronizes the PWM ramp and gate drive outputs of the

LM5115. The phase signal is the square wave output from the transformer secondary winding before rectification

(Figure 1). A resistor connected from the phase signal to the low impedance SYNC pin produces a square wave

connected to the RAMP begins to charge. The current source that charges the RAMP capacitor is equal to 3

capacitor until the next rising edge of the phase signal. The RAMP capacitor is discharged to ground by a low

impedance (100

Ω) n-channel MOSFET. The input impedance at SYNC pin is 2.5kΩ which is normally much less

than the external SYNC pin resistance.

Figure 4. Line Feed-Forward Diagram

The RAMP and SYNC functions illustrated in Figure 4 provide line voltage feed-forward to improve the regulation

of the auxiliary output when the input voltage of the main converter changes. Varying the input voltage to the

main converter produces proportional variations in amplitude of the phase signal. The main channel PWM

controller adjusts the pulse width of the phase signal to maintain constant volt*seconds and a regulated main

output as shown in Figure 5. The variation of the phase signal amplitude and duration are reflected in the slope

the LM5115 is automatically adjusted to regulate the auxiliary output voltage with virtually no change in the PWM

threshold voltage. Transient line regulation is improved because the PWM duty cycle of the auxiliary converter is

immediately corrected, independent of the delays of the voltage regulation loop.

Figure 5. Line Feed-forward Waveforms

SYNC pin resistor is therefore:

10

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