MIC26903 MICREL [Micrel Semiconductor], MIC26903 Datasheet - Page 20

• Manufacturer Part Number: MIC26903
• Description: 28V, 9A Hyper Light Load Synchronous DC/DC Buck Regulator
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
The maximum value of ESR is calculated:
where:
Δ V
ΔI
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 10:
where:
D = duty cycle
C
f
As described in the “Theory of Operation” subsection in
the Functional Description section, the MIC26903
requires at least 20mV peak-to-peak ripple at the FB pin
to make the g
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 below:
The power dissipated in the output capacitor is:
SW
July 2011
ΔV
OUT
L(PP)
OUT(pp)
= switching frequency
OUT(pp)
= output capacitance value
= peak-to-peak inductor current ripple
= peak-to-peak output voltage ripple
I
P
C
ESR
DISS(C
=
OUT
(RMS)
⎛
⎜
⎜
⎝
C
C
m
OUT
OUT
OUT
amplifier and the error comparator
)
=
ΔI
≤
=
ΔI
×
L(PP)
I
ΔV
C
f
L(PP)
ΔI
SW
12
OUT
OUT(pp)
L(PP)
(RMS)
×
8
⎞
⎟
⎟
⎠
2
2
×
+
ESR
(
ΔI
L(PP)
C
OUT
×
ESR
C
Eq. 10
Eq. 11
Eq. 12
OUT
Eq. 9
)
2
20
Input Capacitor Selection
The input capacitor for the power stage input V
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:
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:
The power dissipated in the input capacitor is:
Ripple Injection
The V
MIC26903 g
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 g
then the MIC26903 will lose control and the output
voltage is not regulated. In order to have some amount
of V
output voltage ripple applications.
FB
FB
ripple, a ripple injection method is applied for low
m
ΔV
P
I
ripple required for proper operation of the
CIN(RMS)
DISS(CIN)
amplifier and error comparator can’t sense it,
m
IN
amplifier and error comparator is 20mV to
= I
L(pk)
≈
= I
I
OUT(max)
CIN(RMS)
× ESR
CIN
2
×
× ESR
D
×
(1
CIN
−
D)
M9999-071311-A
MIC26903
IN
should
Eq. 13
Eq. 14
Eq. 15