SC415 Semtech Corporation, SC415 Datasheet - Page 18

• Manufacturer Part Number: SC415
• Description: Dual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

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Applications Information (continued)
This virtual ESR ramp is created by integrating the voltage
across the inductor, and coupling the signal into the FB
pin as shown in Figure 9.
Double-Pulsing
Double-pulsing occurs because the ripple waveform seen
at the FB pin is either too small, or because the FB and
VOUT ripple waveform is very noisy and prone to cause
premature triggering of the FB comparator. Both are
discussed below.
Increasing FB Ripple
If the ripple waveform at FB is too small, the FB waveform
will be susceptible to switching noise. Note that under
normal conditions the FB voltage is within 10-20mV of the
750mV trip point. Noise can couple into the FB point from
either side1 or side1, or even from an external circuit. This
causes the FB comparator to trigger too quickly after the
330nsec minimum off -time has expired. Double-pulsing
will result in higher ripple voltage at the output but in
most cases is harmless.
A way to remedy this is to couple more ripple into FB from
VOUT. Note that the feedback resistor divider attenuates
the FB ripple. This can be compensated by placing a small
capacitor in parallel with the top resistor, which eff ectively
increases the ripple that appears at FB.
DL
DH
RL
CC
To FB
Figure 9
L
CL
R2
R1
COUT
The schematic with added capacitor C as shown in Figure
10.
This capacitor should be left out until confi rmation that
double-pulsing exists. It is best to leave a spot on the PCB
in case it is needed.
FB/VOUT Ripple Waveform
Because the constant on-time control method triggers a
DH pulse whenever the FB waveform reaches the 750mV
trip point, it is important that the VOUT and FB ripple
waveforms are well shaped. This waveform will depend
on the output capacitors.
The idealized circuit has an inductor, a capacitor COUT
with series ESR. The charging and discharging of COUT is
generally much smaller than the ripple voltage due to ESR,
so the resulting ripple waveform is generally determined
by ESR only.
To FB
C
R2
R1
Figure 10
VOUT
COUT
SC415
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