TM
MP2305 – 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
APPLICATIONS INFORMATION
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output voltage to FB
pin. The voltage divider divides the output
voltage down to the feedback voltage by the
ratio:
⎛
⎜
⎝
⎞
⎟
⎟
⎠
VOUT
VOUT
VIN
⎜
ILP = ILOAD
+
× 1−
2× fS ×L
Where ILOAD is the load current.
R2
The choice of which style inductor to use mainly
depends on the price vs. size requirements and
any EMI requirements.
VFB = VOUT
R1+ R2
Where VFB is the feedback voltage and VOUT is
the output voltage.
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 1 lists example
Schottky diodes and their Manufacturers.
Thus the output voltage is:
R1+ R2
VOUT = 0.923 ×
R2
R2 can be as high as 100kΩ, but a typical value
is 10kΩ. Using the typical value for R2, R1 is
determined by:
R1 = 10.83 × (VOUT − 0.923) (kꢀ)
Table 1—Diode Selection Guide
For example, for a 3.3V output voltage, R2 is
10kꢀ, and R1 is 26.1kꢀ.
Voltage/Current
Part Number
B130
Rating
30V, 1A
30V, 1A
30V, 1A
Vendor
Inductor
Diodes, Inc.
Diodes, Inc.
The inductor is required to supply constant
current to the output load while being driven by
the switched input voltage. A larger value
inductor will result in less ripple current that will
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 the inductance to use is to allow
the peak-to-peak ripple current in the inductor
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:
SK13
MBRS130
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:
⎛
⎜
⎝
⎞
VOUT
VOUT
VIN
⎜
⎟
⎟
L =
× 1−
fS × ∆IL
⎠
Where VOUT is the output voltage, VIN is the
input voltage, fS is the switching frequency, and
∆IL is the peak-to-peak inductor ripple current.
⎛
⎞
⎟
VOUT
VOUT
⎜
IC1 = ILOAD
×
× 1−
⎜
⎝
⎟
⎠
V
V
IN
IN
MP2305 Rev. 1.7
4/17/2006
www.MonolithicPower.com
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