TM
MP28372 — DUAL 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
For simplification, choose the input capacitor
whose RMS current rating greater than half of
the maximum load current.
MP28372 can be optimized for a wide range of
capacitance and ESR values.
Compensation Components
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
capacitors, a small, high quality ceramic
capacitor, i.e. 0.1μF, should be placed as close
to the IC as possible.
The MP28372 employs current mode control on
each channel for easy compensation and fast
transient response. The system stability and
transient response are controlled through the
COMP pin. COMP pin is the output of the
internal transconductance error amplifier. A
series capacitor-resistor combination sets a
When using ceramic capacitors, make sure that
they have enough capacitance to provide
sufficient charge prevent excessive voltage
ripple at input. The input voltage ripple caused
by capacitance can be estimated by:
pole-zero
combination
to
control
the
characteristics of the control system.
The DC gain of the voltage feedback loop is
given by:
⎛
⎜
⎝
⎞
⎟
⎟
⎠
ILOAD VOUT
VOUT
⎜
ΔV
=
×
× 1−
IN
VFB
C1
VIN
V
IN
AVDC = RLOAD × GCS × AVEA
×
VOUT
Output Capacitor
Where AVEA is the error amplifier voltage gain,
GCS is the current sense transconductance and
The output capacitor is required to maintain the
DC output voltage. Ceramic, tantalum, or low
ESR electrolytic capacitors are recommended.
Low ESR capacitors are preferred to keep the
output voltage ripple low. The output voltage
ripple can be estimated by:
RLOAD is the load resistor value.
The system has two poles of importance. One
is due to the compensation capacitor (C3) and
the output resistor of error amplifier, and the
other is due to the output capacitor and the load
resistor. These poles are located at:
⎛
⎜
⎝
⎞
⎟
⎟
⎛
⎜
⎝
⎞
⎟
⎟
⎠
VOUT
VOUT
VIN
1
⎜
⎜
ΔVOUT
=
× 1−
× RESR
+
fS × L1
8 × fS × C2
⎠
GEA
fP1
=
=
Where L1 is the inductor value, C2 is the output
capacitance value, and RESR is the equivalent
series resistance (ESR) value of the output
capacitor.
2π × C3 × AVEA
1
fP2
2π × C2× RLOAD
In the case of ceramic capacitors, the
impedance at the switching frequency is
dominated by the capacitance. The output
voltage ripple is mainly caused by the
capacitance. For simplification, the output
voltage ripple can be estimated by:
Where
transconductance.
GEA
is
the
error
amplifier
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). This zero is located
at:
⎛
⎞
⎟
⎟
⎠
VOUT
8 × fS2 × L1× C2
VOUT
⎜
ΔVOUT
=
× 1−
1
⎜
⎝
V
IN
fZ1 =
2π × C3 × R3
In the case of tantalum or electrolytic capacitors,
the ESR dominates the impedance at the
switching frequency. For simplification, the
output ripple can be approximated to:
The system may have another zero of
importance, if the output capacitor has a large
capacitance and/or a high ESR value. The zero,
due to the ESR and capacitance of the output
VOUT
VOUT
VIN
⎛
⎞
⎟
capacitor,
is
located
at:
ΔVOUT
=
× 1−
× R
ESR
⎜
fS × L1
⎝
⎠
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP28372 Rev. 1.4
12/10/2007
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8
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