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MIC26603 Datasheet, PDF (20/30 Pages) Micrel Semiconductor – 28V, 6A Hyper Light Load Synchronous DC/DC Buck Regulator
Micrel, Inc.
The maximum value of ESR is calculated:
ESR COUT
≤
ΔVOUT(pp)
ΔIL(PP)
Eq. 9
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) =
( ) ⎜⎜⎝⎛
C
ΔIL(PP)
OUT × fSW
× 8 ⎟⎟⎠⎞2
+
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 MIC26603 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 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 in Equation 11:
ICOUT (RMS)
=
ΔIL(PP)
12
Eq. 11
MIC26603
The power dissipated in the output capacitor is:
PDISS(COUT ) = ICOUT (RMS) 2 × ESR COUT
Eq. 12
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, 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:
ΔVIN = IL(pk) × ESRCIN
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
MIC26603 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 MIC26603 will lose control and the output
voltage is not regulated. In order to have some amount
of VFB ripple, a ripple injection method is applied for low
output voltage ripple applications.
July 2011
20
M9999-071311-A