SC415 Semtech Corporation, SC415 Datasheet - Page 17

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

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Applications Information (continued)
With a peak voltage V
above 1.8V upon load release), the required capacitance
is,
The above requirements (323μF, 6.4mΩ) can be met using
a single 330μF 6mΩ capacitor.
Note that output voltage ripple is often higher than
expected due to the ESL (inductance) of the capacitor.
See the Stability Considerations section.
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 750mV 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 down-slope 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 formula can be used to
calculate the needed capacitance for a given dILOAD/dt.
Example: Load dI/dt = 2.5A/μsec
This would cause the output current to move from 10A
to zero in 4μsec.
Rate of change of Load current = dILOAD/dt
IMAX = maximum DC load current = 10A
COUT
COUT
IL
IL
C
C
C
OUT
OUT
OUT
PEAK
PEAK
= 204 μF
= IL
= 12.1× (1.5μH×12.1/1.8 - 10/(2.5/1μsec)
= IMAX + 1/2 × I
= 10 + 1/2 × 4.2 = 12.1A
MIN
MIN
_________________________________
________________________________
PEAK
= 323μF
= 1.5μH × (10 + 1/2 • 4.2)
× (L ×IL
PEAK
2 × (1.98 - 1.8)
2 × (V
PEAK
of 1.98V (180mV or 10% rise
RIPPLEMAX
/ V
PEAK
OUT
- V
- I
OUT
MAX
)
/dILOAD /dt)
2
/ (1.98
2
- 1.8
2
)
Note that 204μF is less than the 330μF needed to meet
the harder (instantaneous) transient load release.
Stability Considerations
Unstable operation shows up in two related but distinctly
diff erent ways: fast-feedback loop instability due to
insuffi cient ESR and double-pulsing.
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. 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 ringing after
the initial step is a sign that the ESR should be increased.
SC415 ESR Requirements
The on-time control used in the SC415 regulates the
valley of the output ripple voltage. This ripple voltage
consists of a term generated by the ESR of the output
capacitor and a term based on the capacitance charging
and discharging during the switching cycle. A minimum
ESR is required to generate the required ripple voltage for
regulation. For most applications the minimum ESR ripple
voltage is dominated by PCB layout and the properties of
the output capacitors, typically SP or POSCAP devices.
For stability the ESR zero of the output capacitor should
be lower than one-third the switching frequency. The
formula for minimum ESR is:
For applications using ceramic output capacitors, the ESR
is generally too small to meet the above criteria. In these
cases it is possible to create a ripple voltage ramp that
mimics the ESR ramp.
ESRMIN = 3 / (2 × π × COUT × FREQ)
SC415
17