MP1484 Просмотр технического описания (PDF) - Monolithic Power Systems
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MP1484 Datasheet PDF : 11 Pages
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MP1484 – 3A, 18V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER
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:
VFB
=
VOUT
R2
R1+ R2
Thus the output voltage is:
VOUT
=
0.925
×
R1 + R2
R2
The inductance value can be calculated by:
L
=
VOUT
fS × ΔIL
× ⎜⎜⎝⎛1 −
VOUT
VIN
⎟⎟⎠⎞
Where VOUT is the output voltage, VIN is the
input voltage, fS is the switching frequency, and
ΔIL is the peak-to-peak inductor ripple current.
Choose an inductor that will not saturate under
the maximum inductor peak current, calculated
by:
ILP
= ILOAD
+
VOUT
2 × fS × L
× ⎜⎜⎝⎛1−
VOUT
VIN
⎟⎟⎠⎞
R2 can be as high as 100kΩ, but a typical value
is 10kΩ. Using the typical value for R2, R1 is
determined by:
R1 = 10.81× (VOUT − 0.925) (kΩ)
Where ILOAD is the load current.
The choice of which style inductor to use mainly
depends on the price vs. size requirements and
any EMI constraints.
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
VOUT
1.8V
2.5V
3.3V
5V
12V
R1
9.53kΩ
16.9kΩ
26.1kΩ
44.2kΩ
121kΩ
R2
10kΩ
10kΩ
10kΩ
10kΩ
10kΩ
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.
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
B130
SK13
MBRS130
Voltage/Current
Rating
30V, 1A
30V, 1A
30V, 1A
Vendor
Diodes, Inc.
Diodes, Inc.
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.
MP1484 Rev. 0.9
www.MonolithicPower.com
6
10/23/2008
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© 2008 MPS. All Rights Reserved.
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