MIC45116 Datasheet, PDF (23/42 Pages) Microchip Technology – 20V/6A DC/DC Power Module

5.0 APPLICATION INFORMATION
5.1 Output Capacitor Selection
The type of the output capacitor is usually determined
by the application and its equivalent series resistance
(ESR). Voltage and RMS current capability are two
other important factors for selecting the output
capacitor. Recommended capacitor types are MLCC,
OS-CON and POSCAP. The output capacitor’s ESR is
usually the main cause of the output ripple. The
MIC45116 requires ripple injection and the output
capacitor ESR affects the control loop from a stability
point of view.
Equation 5-1 shows how the maximum value of ESR is
calculated.
EQUATION 5-1:
ESR_COUT

-^---^V----^O----^U---^T---^--^P----^P---^
I _{L  P P }
Where:
∆V_OUT(PP)
∆I_L(PP)
Peak-to-peak output voltage ripple
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 5-2:
EQUATION 5-2:
V_{OUTPP} =


-_C----_O---_U--^--_T--^I--^L_---^--^P_f--_S-^P--_W-^----_-----8-_
2
+
 I _{L  P P }

ESR_COUT²
Where:
D
C_OUT
f_SW
Duty cycle
Output capacitance value
Switching frequency
As described in the Theory of Operation subsection in
the Functional Description, the MIC45116 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 enough feedback voltage ripple. Please refer
to Ripple Injection subsection for more details.
MIC45116
The output capacitor RMS current is calculated in
Equation 5-3:
EQUATION 5-3:
I_{COUTRMS}
=
_----I_{--L-----P---P----}
12
The power dissipated in the output capacitor is:
EQUATION 5-4:
P_{DISSCOUT}
=
I_C
O
U
T

R
M
2
S

ESR_COUT
5.2 Input Capacitor Selection
The input capacitor for the power stage input PV_IN
should be selected for ripple current rating and voltage
rating.
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 5-5:
I_{CINRMS}  I_{OUTMAX}  D  1 – D
The power dissipated in the input capacitor is:
EQUATION 5-6:
P_{DISSCIN}
=
I_C
I
N
R
M
S
2


ESR_CIN
The general rule is to pick the capacitor with a ripple
current rating equal to or greater than the calculated
worst-case RMS capacitor current.
Equation 5-7 should be used to calculate the input
capacitor. Also it is recommended to keep some margin
on the calculated value:
EQUATION 5-7:
C_IN

-^I--^O---^U----^T---^--^M----^A---^X----^---^-----^--1----–-----^D-----^
f_SW  dV
Where:
dV
Input ripple
f_SW
Switching frequency
 2016 Microchip Technology Inc.
DS20005571A-page 23