SC412A Datasheet, PDF (13/22 Pages) Semtech Corporation – Synchronous Buck Controller

SC412A
POWER MANAGEMENT
Layout Guidelines
These requirements (650μF, 10.8mΩ) can be met using
two capacitors, 330μF 20mΩ.
If the load release is relatively slow, the output capacitance
can be reduced. At heavy loads during normal switching,
when the FB pin is above the 0.75V reference, the DL output
is high and the low-side mosfet is on. During this time, the
voltage across the inductor is approximately - VOUT. This
causes a downslope or falling di/dt in the inductor. If the
load di/dt is not much faster than the di/dt in the inductor,
then the inductor current can track change in load current,
and there will be relatively less overshoot from a load
release. The following can used to calculate the needed
capacitance for a given dILOAD/dt:
Peak inductor current,
ILPEAK = I_LOADMAX + 1/2 • I_RIPPLEMAX
ILPEAK = 10 + 1/2 • 4.05 = 12.02A
seen at the FB input or because the ESR is too low, caus-
ing insufficient voltage ramp in the FB signal. This causes
the error amplifier to trigger prematurely after the 350ns
minimum off-time has expired. double-pulsing will result
in higher ripple voltage at the output, but in most cases
is harmless. In some cases, however, double-pulsing can
indicate the presence of loop instability, which is caused
by insufficient ESR.
One simple way to solve this problem is to add some trace
resistance in the high current output path. A side effect of
doing this is output voltage droop with load. Another way
to eliminate doubling-pulsing is to add a small (e.g. 10pF)
capacitor across the upper feedback resistor divider net-
work, (this capacitor is shown in Figure 6). This capacitance
should be left out until confirmation that double-pulsing
exists. Adding this capacitance will add a zero in the trans-
fer function and should eliminate the problem. It is best to
leave a spot on the PCB in case it is needed.
C
Rate of change of Load current = dILOAD/dt
IMAX = maximum load release = 10A
COUT = ILPEAK • (L •ILPEAK / VOUT - IMAX/dILOAD/dt)
2 • (VPEAK - VOUT)
VOUT
R1
R2
To FB pin
Example: Load dI/dt = 2.5A/usec
This would cause the output current to move from 10A to
zero in 4μsec.
Figure 6.
COUT = 12.45•(0.7μH•12.45/1.15 - 10/(2.5/1μsec)
2 •(1.23 - 1.15)
COUT = 278 μF
Loop instability can cause oscillations at the output as a
response to line or load transients. These oscillations can
trip the over-voltage protection latch or cause the output
voltage to fall below the tolerance limit.
Stability Considerations
Unstable operation shows up in two related but distinctly
different ways: double-pulsing and fast-feedback loop
instability. double-pulsing occurs due to switching noise
The best way for checking stability is to apply a zero-to-
full load transient and observe the output voltage ripple
envelope for overshoot and ringing. Over one cycle of ring-
ing after the initial step is a sign that the ESR should be
increased.
© 2006 Semtech Corp.
13
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