MIC2125 Datasheet, PDF (22/34 Pages) Microchip Technology – 28V Synchronous Buck Controllers Featuring Adaptive ON-Time Control

MIC2125/6
5.4 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, tantalum, 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 by Equation 5-13.
EQUATION 5-13:
Where:
ESR_COUT  ^----^V-_---^O-_I--_L-^U--_--^TP--^-_P-^P--_--^P---^
∆V_OUT(PP) Peak-to-Peak Output Voltage Ripple
∆I_L(PP) Peak-to-Peak Inductor Current Ripple
The required output capacitance is calculated in
Equation 5-14.
EQUATION 5-14:
Where:
C_OUT
=
-----------------^----^I--^L---^--^P---^P---^-----------------
V_{OUTPP}  f_SW  8
C_OUT
f_SW
Output Capacitance Value
Switching Frequency
As described in the Theory of Operation subsection of
the Functional Description, the MIC2125/26 requires at
least 20 mV peak-to-peak ripple at the FB pin to ensure
that the g_m amplifier and the 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. Refer to
the Ripple Injection subsection for details.
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 5-15.
EQUATION 5-15:
I_{COUTRMS}
=
_----I_{--L-----P---P----}
12
The power dissipated in the output capacitor is:
EQUATION 5-16:
P_{DISSCOUT}
=
I_C
O
U
T
RM
2
S

ESR_COUT
5.5 Input Capacitor Selection
The input capacitor reduces peak current drawn from
the power supply and reduces noise and voltage ripple
on the input. The input voltage ripple depends on the
input capacitance and ESR. The input capacitance and
ESR values are calculated by using Equation 5-17 and
Equation 5-18.
EQUATION 5-17:
Where:
C_IN = ^I--_-^O---^U-_---^T--_--^--_V---^D_I--_N----_^--_C---^_--1-_----–-_f--_S-^D--_W---^
I_OUT
ƞ
∆V_IN(C)
Load Current
Power Conversion Efficiency
Input Ripple Due to Capacitance Value
EQUATION 5-18:
Where:
ESR_CIN
=
_----V_{----I--N-----E----S--R---}
I_{LPK}
∆V_IN(ESR)
I_L(PK)
Input Ripple Due to Capacitor ESR
Value
Peak Inductor Current
The input capacitor should be qualified for ripple
current rating and voltage rating. The RMS value of the
input capacitor current is determined at the maximum
output current. Assuming the peak-to-peak inductor
current ripple is low:
EQUATION 5-19:
I_{CINRMS}  I_{OUTMAX}  D  1 – D
The power dissipated in the input capacitor is:
EQUATION 5-20:
P_{DISSCIN}
=
I_CIN
RM
2
S

ESR_CIN
DS20005459B-page 22
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