SP6136 Datasheet, PDF (12/18 Pages) Sipex Corporation – Synchronous Buck Controller

APPLICATION INFORMATION
il(rms) =
√ { } ^{iout(max) x 1 + 1 •}
Ipp
2
3
iout(max)
Output Capacitor Selection
The required ESR (Equivalent Series Re-
sistance) and capacitance drive the selec-
tion of the type and quantity of the output
capacitors. The ESR must be small enough
that both the resistive voltage deviation due
to a step change in the load current and
the output ripple voltage do not exceed
the tolerance limits expected on the output
voltage. During an output load transient,
the output capacitor must supply all the ad-
ditional current demanded by the load until
the SP6136 adjusts the inductor current to
the new value.
Therefore, the capacitance must be large
enough so that the output voltage is held up
while the inductor current ramps up or down
to the value corresponding to the new load
current. Additionally, the ESR in the output
capacitor causes a step in the output voltage
equal to the current. Because of the fast tran-
sient response and inherent 100% and 0%
duty cycle capability provided by the SP6136
when exposed to output load transients, the
output capacitor is typically chosen for ESR,
not for capacitance value.
The output capacitor’s ESR, combined with
the inductor ripple current, is typically the
main contributor to output voltage ripple.
The maximum allowable ESR required to
maintain a specified output voltage ripple
can be calculated by:
RESR < ^∆Vout

Ipk-pk
where:
∆Vout = Peak to Peak Output Voltage Ripple
Ipk-pk = Peak to Peak Inductor Ripple Current
The total output ripple is a combination of
the ESR and the output capacitance value
and can be calculated as follows:
∆Vout =
.
.
√ { } ^{(Ipp•Resr)2 +}
Ipp • (1-D) ²
Cout • Fs
where:
Fs = Switching Frequency
D = Duty Cycle
Cout = output capacitance value
Input Capacitor Selection
The input capacitor should be selected for
ripple current rating, capacitance and voltage
rating. The input capacitor must meet the
ripple current requirement imposed by the
switching current. In continuous conduction
mode, the source current of the high-side
MOSFET is approximately a square wave
of duty cycle V_OUT/V_IN. Most of this current
is supplied by the input bypass capacitors.
The RMS value of input capacitor current is
determined at the maximum output current
and under the assumption that the peak to peak
inductor ripple current is low, it is given by:
.
√ ^{icin(rms) = iout(max) x D • (1-D)}
Schottky Diode Selection
When paralleled with the bottom MOSFET,
an optional Schottky diode can improve
efficiency and reduce noise. Without this
Schottky diode, the body diode of the bot-
tom MOSFET conducts the current during
the non-overlap time when both MOSFETs
are turned off. Unfortunately, the body di-
Oct 31-06 Rev L
SP6136 Synchronous Buck Controller
12
© 2006 Sipex Corporation