sip12203 Vishay, sip12203 Datasheet - Page 9

• Manufacturer Part Number: sip12203
• Description: Triple Step Down Controller Ic For 2 Synchronous And 1 Linear Power Rails
• Manufacturer: Vishay

This peak current varies with inductance tolerance and other
errors, and the rated saturation level varies over
temperature. So a sufficient design margin is required when
choosing current ratings. A high-frequency core material,
such as ferrite, should be chosen, the core loss could lead to
serious efficiency penalties. The DCR should be kept as low
as possible to reduce conduction losses.
Input Capacitor Selection
To minimize input voltage ripple caused by the step-down
conversion, and interference of large voltage spikes from
other circuits, a low-ESR input capacitor is required to filter
the input voltage. The input capacitor should be rated for the
maximum RMS input current of:
It is common practice to rate for the worst-case RMS ripple
that occurs when the duty cycle is at 50 %:
Compensation
The SiP12203 uses voltage mode control in conjunction with
a high frequency transconductance error amplifier. The volt-
age feedback loop is compensated at the COMP pin, which
is the output node of the error amplifier. The feedback loop is
generally compensated with an RC + C (one pole, one zero)
network from COMP to AGND. Loop stability is affected by
the values of the inductor, the output capacitor, the output
capacitor ESR, and the error amplifier compensation net-
work.
The ideal bode plot for a compensated system would be gain
that rolls off at a slope of - 20 dB/decade, crossing 0 dB at
the desired bandwidth and a phase margin greater than 90°
for all frequencies below the 0 dB crossing.
Document Number: 69986
S-82337-Rev. B, 22-Sep-08
I
I
RMS
RMS
=
=
I
O
I
.
O
V
MAX
.MAX
2
OUT
R1
R2
V
V
FB
IN
O
0.6 V
(
1
GM
-
V
V
IN
O
C1
R3
)
Compensation
PWM Comp
C2
500 kHz
OSC
ΔV osc
Output Capacitor Selection
The output capacitor affects output voltage ripple due to 2
reasons: the capacitance and the effective series resistance
(ESR). The selection of the output capacitor is primarily
determined by the capacitor ESR required minimizing
voltage ripple and current ripple. The relationship between
output ripple · V
Multiple capacitors placed in parallel may be needed to meet
the ESR requirements. However if the ESR is too low it may
cause stability problems.
MOSFET Selection
The key selection criteria for the MOSFETs include
maximum specifications of on-resistance, drain source
voltage, gate source voltage and current, and total gate
charge Q
important to carefully balance on-resistance and gate
charge. In typical MOSFETs, the lower the on-resistance, the
higher the gate charge. The power loss of a MOSFET
consists of conduction loss, gate charge loss and crossover
loss. For lower-current applications, gate charge loss
becomes a significant factor. In this case low gate charge
MOSFETs, such as Vishay Siliconix's LITTLE FOOT family
devices, are desirable.
The compensation network used with the error amplifier
must provide enough phase margin at the 0 dB crossover
frequency for the overall open-loop transfer function to be
stable.
compensation pole and zero to stabilize the SiP12203.
The inductor and output capacitor values are usually
determined by efficiency, voltage and current ripple
requirements. The inductor and the output capacitor create a
double pole and a - 180° phase change at the frequency of:
Δ
f
P
V
(
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LC
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)
=
I
PP
The
G
2
. The voltage ratings are fairly straightforward. It is
V
•
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ESR
following
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L
1
, capacitance C
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C
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guidelines
f
1
•
O
C
Vishay Siliconix
and its ESR is:
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C
ESR
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will
SiP12203
www.vishay.com
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V
OUT
the
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