LM25115MTSLASHNOPB datasheet(10/23 Pages) TI

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Phase signal

RAMP pin

HS pin

12V

Main Output = 3.3V

Secondary Output = 2.5V

PWM Threshold

LM25115

SNVS418A – NOVEMBER 2005 – REVISED APRIL 2013

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The RAMP and SYNC functions illustrated in Figure 11 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 12. The variation of the phase signal amplitude and duration are reflected in the slope

and duty cycle of the RAMP signal of the LM25115 (ISYNC α phase signal amplitude). As a result, the duty cycle

of the LM25115 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 12. 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 amplitude may be programmed by

selecting the proper RAMP pin capacitor value. The recommended peak amplitude of the RAMP waveform is 1V

to 1.75V. The CRAMP capacitor is chosen to provide the desired RAMP amplitude with the nominal phase signal

voltage and pulse width.

CRAMP = (3 x ISYNC x TON ) / VRAMP

where

•

CRAMP = RAMP pin capacitance

•

ISYNC = SYNC pin current current

•

TON = corresponding phase signal pulse width

•

VRAMP = desired RAMP amplitude (1V to 1.75V)

For example,

Main channel output = 3.3V. Phase signal maximum amplitude = 12V. Phase signal frequency = 250kHz

•

Set ISYNC = 150µA with phase signal at maximum amplitude (12V):

–

ISYNC = 150µA = VPHASE(max) / (RSYNC + 2.5 kΩ) = 12V / (RSYNC + 2.5 kΩ)

–

RSYNC = 12V/150µA - 2.5kΩ = 77.5kΩ

•

TON = Main channel duty cycle / Phase frequency = (3.3V/12V) / 250kHz = 1.1µs

•

Assume desired VRAMP = 1.5V

•

CRAMP = (3 x ISYNC x TON ) / VRAMP = (3 x 150µA x 1.1µs) / 1.5V

•

CRAMP = 330pF

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