MIC28510_12 Datasheet, PDF (20/30 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
Input Capacitor Selection
The input capacitor for the power stage input V_IN should
be selected for ripple current rating and voltage rating.
Tantalum input capacitors may fail when subjected to
high inrush currents, caused by turning the input supply
on. A tantalum input capacitor’s voltage rating should be
at least two times the maximum input voltage to
maximize reliability. Aluminum electrolytic, OS–CON,
and multilayer polymer film capacitors can handle the
higher inrush currents without voltage de–rating. The
input voltage ripple will primarily depend on the input
capacitor’s ESR. The peak input current is equal to the
peak inductor current, so:
ΔV_IN = I_L(pk) × C_ESR
Eq. 14
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determined at the maximum output current. Assuming
the peak–to–peak inductor current ripple is low:
I_CIN(RMS)  I_OUT(MAX ) D  (1 D)
Eq. 15
The power dissipated in the input capacitor is:
P_DISS(CIN) = I_CIN(RMS)2 × C_ESR
Eq. 16
Ripple Injection
The V_FB ripple required for proper operation of the
MIC28510 g_m amplifier and error comparator is 20mV to
100mV. However, the output voltage ripple is generally
designed as 1% to 2% of the output voltage. For a low
output voltage, such as a 1V, the output voltage ripple is
only 10mV to 20mV, and the feedback voltage ripple is
less than 20mV. If the feedback voltage ripple is so small
that the g_m amplifier and error comparator can’t sense it,
then the MIC28510 will lose control and the output
voltage is not regulated. In order to have some amount
of V_FB ripple, a ripple injection method is applied for low
output voltage ripple applications.
MIC28510
The applications are divided into three situations
according to the amount of the feedback voltage ripple:
1) Enough ripple at the feedback voltage due to the large
ESR of the output capacitors.
As shown in Figure 7a, the converter is stable without
any ripple injection. The feedback voltage ripple is:
ΔV_FB(pp)

R2
R1  R2
 ESR_COUT
 ΔI_L(pp)
Eq. 17
where ΔI_L(pp) is the peak–to–peak value of the inductor
current ripple.
2) Inadequate ripple at the feedback voltage due to the
small ESR of the output capacitors.
The output voltage ripple is fed into the FB pin through a
feedforward capacitor C_FF in this situation, as shown in
Figure 7b. The typical C_FF value is between 1nF and
22nF.
With the feedforward capacitor, the feedback voltage
ripple is very close to the output voltage ripple:
ΔV_FB(pp)  ESR  ΔI_{L (pp)}
Eq. 18
3) Virtually no ripple at the FB pin voltage due to the very
low ESR of the output capacitors.
Figure 7a. Enough Ripple at FB
March 2012
Figure 7b. Inadequate Ripple at FB
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M9999-030912-A