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LM25115 데이터시트(PDF) 10 Page - Texas Instruments

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부품명 LM25115
상세설명  LM25115 Secondary Side Post Regulator Controller
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Phase signal
RAMP pin
HS pin
12V
12V
6V
6V
Main Output = 3.3V
Secondary Output = 2.5V
PWM Threshold
LM25115
SNVS418A – NOVEMBER 2005 – REVISED APRIL 2013
www.ti.com
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
10
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