MIC26950 MICREL, MIC26950 Datasheet - Page 17

• Manufacturer Part Number: MIC26950
• Description: 12A Hyper Speed Control Synchronous DC-DC Buck Regulator
• Manufacturer: MICREL

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Micrel, Inc.
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 11:
where:
D = duty cycle
C
f
As described in the “Theory of Operation” subsection in
“Functional Description”, the MIC26950 requires at least
20mV peak-to-peak ripple at the FB pin to make the g
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 12:
The power dissipated in the output capacitor is:
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 upon the input
capacitor’s ESR. The peak input current is equal to the
SW
September 2010
ΔV
OUT
= switching frequency
OUT(pp)
= output capacitance value
=
I
P
C
DISS(C
OUT
⎛
⎜
⎜
⎝
C
(RMS)
OUT
OUT
ΔI
)
×
=
L(PP)
=
f
ΔI
SW
I
C
L(PP)
12
OUT
×
8
(RMS)
⎞
⎟
⎟
⎠
2
+
2
(
×
ΔI
ESR
L(PP)
C
×
OUT
ESR
IN
C
should
OUT
(11)
(12)
(13)
)
2
m
17
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
MIC26950 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
the MIC26950 will lose control and the output voltage is
not regulated. In order to have some amount of V
ripple, a ripple injection method is applied for low output
voltage ripple applications.
The applications are divided into three situations
according to the amount of the feedback voltage ripple:
1) Enough ripple at the feedback voltage due to the large
ESR of the output capacitors.
As shown in Figure 5a, the converter is stable without
any ripple injection. The feedback voltage ripple is:
where ΔI
current ripple.
2) Inadequate ripple at the feedback voltage due to the
small ESR of the output capacitors.
The output voltage ripple is fed into the FB pin through a
feedforward capacitor C
Figure 5b. The typical C
100nF. With the feedforward capacitor, the feedback
voltage ripple is very close to the output voltage ripple:
ΔV
FB
FB(pp)
L(pp)
m
ripple required for proper operation of the
amplifier and error comparator can’t sense it,
ΔV
P
m
I
CIN(RMS)
=
is the peak-to-peak value of the inductor
DISS(CIN)
amplifier and error comparator is 20mV to
IN
R1
= I
R2
+
L(pk)
R2
≈
= I
I
OUT(max)
CIN(RMS)
×
× ESR
ff
ESR
ff
in this situation, as shown in
value is between 1nF and
C
CIN
2
×
OUT
× ESR
D
×
×
ΔI
(1
CIN
L
(pp)
M9999-091710-C
−
D)
MIC26950
(14)
(15)
(16)
(17)
FB