MIC28304 Datasheet, PDF (27/39 Pages) Micrel Semiconductor – 70V 3A Power Module

Micrel, Inc.
The maximum value of ESR is calculated as in Equation
6:
ESR _COUT
≤
ΔV_OUT(pp)
ΔI_L(PP)
Eq. 6
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 in
Equation 7:
ΔV_OUT(pp) =
( ) _{}
C
ΔI_L(PP)
OUT ^{× f}SW
× 8 ^{2}
+
ΔI_L(PP) × ESR _COUT
2
Eq. 7
Where:
D = Duty cycle
C_OUT = Output capacitance value
f_sw = Switching frequency
As described in the “Theory of Operation” subsection in
Functional Description, the MIC28304 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 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.
MIC28304
The output capacitor RMS current is calculated in
Equation 8:
I_{COUT (RMS)}
=
ΔI_L(PP)
12
Eq. 8
The power dissipated in the output capacitor is:
P_{DISS(COUT )} = I_{COUT (RMS)2} × ESR _COUT
Eq. 9
Input Capacitor Selection
The input capacitor for the power stage input PVIN
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. 10
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.11
The power dissipated in the input capacitor is:
P_DISS(CIN) = I_CIN(RMS)2 × ESR_CIN
Eq. 12
The general rule is to pick the capacitor with a ripple
current rating equal to or greater than the calculated
worst (V_{IN_MAX}) case RMS capacitor current. Its voltage
rating should be 20% to 50% higher than the maximum
input voltage. Typically the input ripple (dV) needs to be
kept down to less than ±10% of input voltage. The ESR
also increases the input ripple.
March 25, 2014
27
Revision 1.1