MIC261203 Datasheet, PDF (20/31 Pages) Micrel Semiconductor – 28V, 12A Hyper Light Load™ Synchronous DC/DC Buck Regulator

Micrel, Inc.
The maximum value of ESR is calculated:
ESR _COUT
≤
ΔV_OUT(pp)
ΔI_L(PP)
Eq. 9
where:
ΔVOUT(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 by
Equation 10:
ΔV_OUT(pp) =
( ) _{⎜⎜⎝⎛}
C
ΔI_L(PP)
OUT ^{× f}SW
× 8 ^{⎟⎟⎠⎞2}
+
ΔI_L(PP) × ESR _COUT
2
Eq. 10
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 MIC261203
requires at least 20mV 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” 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 by Equation 11:
I_{COUT (RMS)}
=
ΔI_L(PP)
12
Eq. 11
The power dissipated in the output capacitor is:
P_{DISS(COUT )} = I_{COUT (RMS)} ² × ESR _COUT
Eq. 12
MIC261203
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) × ESR_CIN
Eq. 13
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. 14
The power dissipated in the input capacitor is:
P_DISS(CIN) = I_CIN(RMS)2 × ESR_CIN
Eq. 15
Ripple Injection
The V_FB ripple required for proper operation of the
MIC261203 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 MIC261203 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.
July 2011
20
M9999-071311-A