Datasheet

Phase signal
RAMP pin
HS pin
12V
12V
6V
6V
Main Output = 3.3V
Secondary Output = 2.5V
PWM Threshold
LM5115
SNVS343E –MARCH 2005–REVISED MARCH 2013 www.ti.com
The RAMP and SYNC functions illustrated in Figure 13 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 14. The variation of the phase signal amplitude and duration are reflected in the slope
and duty cycle of the RAMP signal of the LM5115 (I
SYNC
α phase signal amplitude). As a result, the duty cycle of
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 14. Line Feed-forward Waveforms
The recommended SYNC input current range is 50µA to 150µA. The SYNC pin resistor (R
SYNC
) should be
selected to set the SYNC current (I
SYNC
) to 150µA with the maximum phase signal amplitude, V
PHASE(max)
. This
will ensure that I
SYNC
stays within the recommended range over a 3:1 change in phase signal amplitude. The
SYNC pin resistor is therefore:
R
SYNC
= (V
PHASE(max)
/ 150µA) - 2.5kΩ (1)
Once I
SYNC
has been established by selecting R
SYNC
, 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 C
RAMP
capacitor is chosen to provide the desired RAMP amplitude with the nominal phase signal
voltage and pulse width.
C
RAMP
= (3 x I
SYNC
x T
ON
) / V
RAMP
where
• C
RAMP
= RAMP pin capacitance
• I
SYNC
= SYNC pin current current
• T
ON
= corresponding phase signal pulse width
• V
RAMP
= desired RAMP amplitude (1V to 1.75V) (2)
For example,
Main channel output = 3.3V. Phase signal maximum amplitude = 12V. Phase signal frequency = 250kHz
• Set I
SYNC
= 150µA with phase signal at maximum amplitude (12V):
– I
SYNC
= 150µA = V
PHASE(max)
/ (R
SYNC
+ 2.5 kΩ) = 12V / (R
SYNC
+ 2.5 kΩ)
– R
SYNC
= 12V/150µA - 2.5kΩ = 77.5kΩ
• T
ON
= Main channel duty cycle / Phase frequency = (3.3V/12V) / 250kHz = 1.1µs
• Assume desired V
RAMP
= 1.5V
• C
RAMP
= (3 x I
SYNC
x T
ON
) / V
RAMP
= (3 x 150µA x 1.1µs) / 1.5V
• C
RAMP
= 330pF
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