User Manual

MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER
MP2307 Rev. 1.9 www.MonolithicPower.com 7
5/28/2008 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2008 MPS. All Rights Reserved.
APPLICATIONS INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider connected from the output
voltage to FB. The voltage divider divides the
output voltage down to the feedback voltage by
the ratio:
2R1R
2R
VV
OUTFB
+
=
Thus the output voltage is:
2R
2R1R
925.0V
OUT
+
×=
R2 can be as high as 100k, but a typical value
is 10k. Using the typical value for R2, R1 is
determined by:
)925.0V(81.101R
OUT
−×= (k)
For example, for a 3.3V output voltage, R2 is
10k, and R1 is 26.1k. Table 1 lists
recommended resistance values of R1 and R2
for standard output voltages.
Table 1—Recommended Resistance Values
VOUT R1 R2
1.8V 9.53k 10k
2.5V 16.9k 10k
3.3V 26.1k 10k
5V 44.2k 10k
12V 121k 10k
Inductor
The inductor is required to supply constant
current to the load while being driven by the
switched input voltage. A larger value inductor
will result in less ripple current that will in turn
result in lower output ripple voltage. However,
the larger value inductor will have a larger
physical size, higher series resistance, and/or
lower saturation current. A good rule for
determining inductance is to allow the peak-to-
peak ripple current to be approximately 30% of
the maximum switch current limit. Also, make
sure that the peak inductor current is below the
maximum switch current limit.
The inductance value can be calculated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
∆×
=
IN
OUT
LS
OUT
V
V
1
If
V
L
Where V
OUT
is the output voltage, V
IN
is the
input voltage, f
S
is the switching frequency, and
I
L
is the peak-to-peak inductor ripple current.
Choose an inductor that will not saturate under
the maximum inductor peak current, calculated
by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
××
+=
IN
OUT
S
OUT
LOADLP
V
V
1
Lf2
V
II
Where I
LOAD
is the load current.
The choice of which style inductor to use mainly
depends on the price vs. size requirements and
any EMI constraints.
Optional Schottky Diode
During the transition between the 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
B130 30V, 1A Diodes, Inc.
SK13 30V, 1A Diodes, Inc.
MBRS130 30V, 1A
International
Rectifier
Input Capacitor
The input current to the step-down converter is
discontinuous, therefore a capacitor is required
to supply the AC current 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 will also suffice.
Choose X5R or
X7R dielectrics when using ceramic capacitors.