SP6134 Datasheet, PDF (9/14 Pages) Sipex Corporation – Dual Supply Synchronous Buck Controller

APPLICATIONS INFORMATION: Continued
Output Capacitor Selection
The required ESR (Equivalent Series Resis-
tance) and capacitance drive the selection of the
type and quantity of the output capacitors. The
ESR must be small enough that both the resis-
tive 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 addi-
tional current demanded by the load until the
SP6134CU adjusts the inductor current to the
new value.
Therefore the capacitance must be large enough
so that the output voltage is help up while the
inductor current ramps up or down to the value
corresponding to the new load current. Addi-
tionally, the ESR in the output capacitor causes
a step in the output voltage equal to the current.
Because of the fast transient response and inher-
ent 100% and 0% duty cycle capability provided
by the SP6134CU when exposed to output load
transient, the output capacitor is typically cho-
sen 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 maxi-
mum allowable ESR required to maintain a
specified output voltage ripple can be calculated
by:
RESR ≤ ∆V_OUT
I_PK-PK
where:
∆V_OUT = Peak to Peak Output Voltage Ripple
I_PK-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:
F_S = Switching Frequency
D = Duty Cycle
C_OUT = 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 cur-
rent 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:
I_CIN(rms) = I_{OUT(max) √D(1 - D)}
The worse case occurs when the duty cycle D is
50% and gives an RMS current value equal to
I_OUT/2.
Select input capacitors with adequate ripple
current rating to ensure reliable operation.
The power dissipated in the input capacitor is:
P = I R ²
C IN
CIN ( rms) ESR( CIN)
This can become a significant part of power
losses in a converter and hurt the overall energy
transfer efficiency. The input voltage ripple
primarily depends on the input capacitor ESR
and capacitance. Ignoring the inductor ripple
current, the input voltage ripple can be deter-
mined by:
∆ V_IN
= I _out(max)
R_{E SR(CIN )}
+
I _{OUT (MAX} V_{) OUT} (V_IN
F_SC _INV_IN ²
−V_OUT )
( ) ^{∆VOUT =}
I_PP (1 – D)
C_OUTF_S
2
+ (I_PPR_ESR)²
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SP6134 Dual Supply, Synchronous Buck Controller
9
© Copyright 2004 Sipex Corporation