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MIC261203ZA_14 Datasheet, PDF (19/29 Pages) Micrel Semiconductor – Synchronous DC-to-DC Buck Regulator
Micrel, Inc.
The maximum value of ESR is calculated using Equation 9.
MIC261203-ZA
The power dissipated in the output capacitor is:
ESR COUT
≤
ΔVOUT(pp)
ΔIL(PP)
Eq. 9
PDISS(COUT )
=
ICOUT
2
(RMS)
× ESR COUT
Eq. 12
where:
ΔVOUT(pp) = peak-to-peak output voltage ripple
ΔIL(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:
ΔVOUT(pp) =
( ) 
ΔIL(PP)
 COUT × fSW
 2
× 8 
+
ΔIL(PP) × ESR COUT
2
Eq. 10
where:
D = duty cycle
COUT = output capacitance value
fSW = switching frequency
As described in the “Theory of Operation” subsection in
Functional Description, the MIC261203-ZA 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 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.
ICOUT (RMS)
=
ΔIL(PP)
12
Eq. 11
Input Capacitor Selection
The input capacitor for the power stage input VIN should
be selected for ripple current rating and voltage rating.
Tantalum input capacitors may fail when subjected to
high inrush currents which are 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:
∆VIN = IL(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:
ICIN(RMS) ≈ IOUT(max) × D × (1− D)
Eq. 14
The power dissipated in the input capacitor is:
PDISS(CIN) = ICIN(RMS)2 × ESRCIN
Eq. 15
Ripple Injection
The VFB ripple required for proper operation of the
MIC261203-ZA gm 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 gm amplifier and error comparator can’t
sense it, then the MIC261203-ZA will lose control and the
output voltage is not regulated.
July 22, 2014
19
Revision 1.1