MIC24053 Datasheet, PDF (21/32 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

The maximum value of ESR is calculated with
Equation 5-7.
EQUATION 5-7:
ESR_COUT

_-----V_{----O----U----T-----P---P---}
I _{LPP}
Where:
ΔV_OUT(PP) = Peak-to-peak output voltage
ripple
ΔI_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---^--_f^P---_SP---_W-^----_-----8--_
2
+
I
LPP

ESR_COUT
²
Where:
D = Duty cycle
C_OUT = Output capacitance value
f_SW = Switching frequency
As described in the Theory of Operation subsection in
the Functional Description section, the MIC24053
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 used for the output
capacitors, a ripple injection method should be applied
to provide enough feedback voltage ripple. Please refer
to the Ripple Injection subsection for more 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 5-9:
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
2
RMS

ESR_COUT
MIC24053
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 on
the input supply. 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 primarily depends 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 the 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
The power dissipated in the input capacitor is:
EQUATION 5-13:
P_{DISS ON }
=
I
CIN
1
RMS

ESR_CIN
5.4 Ripple Injection
The V_FB ripple required for proper operation of the
MIC24053 g_m amplifier and error comparator is 20 mV
to 100 mV. However, the output voltage ripple is
generally designed as 1% to 2% of the output voltage.
For a low output voltage, such as 1V, the output voltage
ripple is only 10 mV to 20 mV, and the feedback voltage
ripple is less than 20 mV. If the feedback voltage ripple
is so small that the g_m amplifier and error comparator
cannot sense it, then the MIC24053 will lose control
and the output voltage is not regulated. To have some
amount of V_FB ripple, a ripple injection method is
applied for low output voltage ripple applications.
 2015 Microchip Technology Inc.
DS20005668A-page 21