MP2307DN Monolithic Power Systems, MP2307DN Datasheet - Page 7

• Manufacturer Part Number: MP2307DN
• Description: 3A, 23V, 340khz Synchronous Rectified Step-down Converter
• Manufacturer: Monolithic Power Systems

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:
Thus the output voltage is:
R2 can be as high as 100kΩ, but a typical value
is 10kΩ. Using the typical value for R2, R1 is
determined by:
For example, for a 3.3V output voltage, R2 is
10kΩ, and R1 is 26.1kΩ. Table 1 lists
recommended resistance values of R1 and R2
for standard output voltages.
Table 1—Recommended Resistance Values
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.
MP2307 Rev. 1.7
3/14/2006
VOUT
1.8V
2.5V
3.3V
12V
5V
R
1
=
TM
10
V
MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER
OUT
V
.
81
FB
=
×
=
(
. 0
V
V
9.53kΩ
16.9kΩ
26.1kΩ
44.2kΩ
925
121kΩ
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
OUT
OUT
R1
×
R
−
1
R
R
. 0
+
1
2
R
925
R
+
2
2
R
2
)
(kΩ)
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
R2
© 2006 MPS. All Rights Reserved.
www.MonolithicPower.com
The inductance value can be calculated by:
Where V
input voltage, f
∆I
Choose an inductor that will not saturate under
the maximum inductor peak current, calculated
by:
Where I
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
Manufacturers.
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.
L
Part Number
is the peak-to-peak inductor ripple current.
MBRS130
SK13
B130
example
Table 2—Diode Selection Guide
LOAD
I
OUT
LP
=
is the load current.
L
I
is the output voltage, V
LOAD
=
S
f
is the switching frequency, and
S
Voltage/Current
Schottky
V
+
×
OUT
2
∆
30V, 1A
30V, 1A
30V, 1A
I
×
V
Rating
L
OUT
f
S
×
×
⎛
⎜ ⎜
⎝
1
L
−
×
diodes
V
⎛
⎜ ⎜
⎝
V
1
OUT
−
IN
V
V
⎞
⎟ ⎟
⎠
OUT
International
IN
Diodes, Inc.
Diodes, Inc.
Rectifier
Vendor
and
⎞
⎟ ⎟
⎠
IN
is the
their
7