MIC45205 Datasheet, PDF (18/31 Pages) Micrel Semiconductor – 26V/6A DC-to-DC Power Module

Micrel, Inc.
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 7:
( ) ΔV_OUT(PP) =

ΔI_L(PP)
C_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
the Functional Description, the MIC45205 requires at
least 20mV peak-to-peak ripple at the FB pin to make the
gm 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 “Ripple Injection”
subsection in the Application Information section for more
details.
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
MIC45205
Input Capacitor Selection
The input capacitor for the power stage input PVIN
should be selected for ripple current rating and voltage
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:
∆V_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-case RMS capacitor current.
Equation 13 should be used to calculate the input
capacitor. Also it is recommended to keep some margin
on the calculated value:
C_IN
≈
I_OUT(MAX) × (1 −
f_SW × dV
D)
Eq. 13
Where:
dV = The input ripple
f_SW = Switching frequency
April 15, 2014
18
Revision 1.0