MP1472 – 2A, 18V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
MP1472 Rev. 1.0 www.MonolithicPower.com 8
9/2/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
Optional Schottky Diode
During the transition between high-side switch
and low-side switch, the body diode of the low-
side power MOSFET conducts the inductor
current. The forward voltage of this body diode is
high. An optional Schottky diode may be
paralleled between the SW pin and GND pin to
improve overall efficiency. Table 2 lists example
Schottky diodes and their Manufacturers.
Table 2—Diode Selection Guide
Part Number Voltage/Current
Rating Vendor
B230 30V, 2A Diodes, Inc.
SL23 30V, 2A Vishay, Inc.
MBRS230 30V, 2A
International
Rectifier
Input Capacitor
The input current to the step-down converter is
discontinuous, therefore a capacitor is required to
supply the AC current to the step-down converter
while maintaining the DC input voltage. Use low
ESR capacitors for the best performance.
Ceramic capacitors are preferred, but tantalum or
low-ESR electrolytic capacitors may also suffice.
Choose X5R or X7R dielectrics when using
ceramic capacitors.
Since the input capacitor (C1) absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input
capacitor can be estimated by:
IN
OUT
IN
OUT
LOAD1C V
V
1
V
V
II
The worst-case condition occurs at VIN = 2VOUT,
where IC1 = ILOAD/2. For simplification, choose the
input capacitor whose RMS current rating greater
than half of the maximum load current.
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
capacitors, a small, high quality ceramic
capacitor, i.e. 0.1F, should be placed as close
to the IC as possible. When using ceramic
capacitors, make sure that they have enough
capacitance to provide sufficient charge to
prevent excessive voltage ripple at input. The
input voltage ripple for low ESR capacitors can
be estimated by:
IN
OUT
IN
OUT
S
LOAD
IN V
V
1
V
V
f1C
I
V
Where C1 is the input capacitance value.
Output Capacitor
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:
2Cf8
1
R
V
V
1
Lf
V
V
S
ESR
IN
OUT
S
OUT
OUT
Where C2 is the output capacitance value and
RESR is the equivalent series resistance (ESR)
value of the output capacitor.
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:
IN
OUT
2
S
OUT
OUT V
V
1
2CLf8
V
V
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:
ESR
IN
OUT
S
OUT
OUT R
V
V
1
Lf
V
V
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP1472 can be optimized for a wide range of
capacitance and ESR values.
Compensation Components
MP1472 employs current mode control 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 pole-zero combination to control the
characteristics of the control system.