MIC24051 Datasheet, PDF (21/34 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

5.2 Output Capacitor Selection
The type of the output capacitor is usually determined
by its equivalent series resistance (ESR). Voltage and
RMS current capability are two other important factors
for selecting the output capacitor. Recommended
capacitor types are ceramic, low-ESR aluminum
electrolytic, OS-CON and POSCAP. The output
capacitor’s ESR is usually the main cause of the output
ripple. The output capacitor ESR also affects the
control loop from a stability point of view.
The maximum value of ESR is calculated:
EQUATION 5-7:
Where:
ESR_COUT

-^---^V----^O----^U---^T---^--^P----^P---^
I _{L  P P }
∆V_OUT(PP) Peak-to-peak output voltage ripple
∆I_L(PP) Peak-to-peak inductor current ripple
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 5-8:
EQUATION 5-8:
V_{OUTPP} =


-_C----_O---_U--^--_T--^I--^L_---^--^P_f--_S-^P--_W-^----_-----8-_
2
+
 I _{L  P P }

ESR_COUT²
Where:
C_OUT Output capacitance value
f_SW Switching frequency
As described in the Theory of Operation section, the
MIC24051 requires at least 20 mV peak-to-peak ripple
at the FB pin to make the g_m amplifier and the error
comparator behave properly. Also, the output voltage
ripple should be in phase with the inductor current.
Therefore, the output voltage ripple caused by the
output capacitors value should be much smaller than
the ripple caused by the output capacitor ESR. If
low-ESR capacitors, such as ceramic capacitors, are
selected as the output capacitors, a ripple injection
method should be applied to provide the enough
feedback voltage ripple. Please refer to the Ripple
Injection section for more details.
MIC24051
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for
aluminum electrolytic or OS-CON. The output capacitor
RMS current is calculated in Equation 5-9:
EQUATION 5-9:
I_{COUTRMS}
=
_----I_{--L-----P---P----}
12
The power dissipated in the output capacitor is:
EQUATION 5-10:
P_{DISSCOUT} = I_{COUTRMS}  ESR_COUT
5.3 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:
EQUATION 5-11:
V_IN = I_{LPK}  ESR_CIN
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:
EQUATION 5-12:
I_{CINRMS}  I_{OUTMAX}  D  1 – D
 2016 Microchip Technology Inc.
DS20005658A-page 21