MIC26903ZA_14 Datasheet, PDF (19/29 Pages) Micrel Semiconductor – 28V, 9A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
The maximum value of ESR is calculated using Equation 9.
MIC26903-ZA
The power dissipated in the output capacitor is:
ESR _COUT
≤
ΔV_OUT(pp)
ΔI_L(PP)
Eq. 9
P_{DISS(COUT )} = I_{COUT (RMS)2} × ESR _COUT
Eq. 12
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 10:
ΔV_OUT(pp) =
( ) _{}
ΔI_L(PP)
C_OUT × f_SW
× 8 ^{2}
+
ΔI_L(PP) × ESR _COUT
2
Eq. 10
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 derating. The input
voltage ripple primarily depends on the input capacitor’s
ESR. The peak input current is equal to the peak inductor
current, so:
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 MIC26903-ZA 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 tantalum and 20% greater for
aluminum electrolytic or OS-CON. The output capacitor
RMS current is calculated in Equation 11.
∆V_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
I_{COUT (RMS)}
=
ΔI_L(PP)
12
Eq. 11
July 22, 2014
19
Revision 1.1