MP2351DQ MPS [Monolithic Power Systems], MP2351DQ Datasheet - Page 5

• Manufacturer Part Number: MP2351DQ
• Description: 1.5A, 23V, 1.4MHz Step-Down Converter
• Manufacturer: MPS [Monolithic Power Systems]

APPLICATION INFORMATION
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:
Where V
the output voltage.
Thus the output voltage is:
R2 can be as high as 100kΩ, but a typical value
is 10kΩ. Using that value, R1 is determined by:
For example, for a 3.3V output voltage, R2 is
10kΩ, and R1 is 17kΩ.
Inductor
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:
Where V
switching frequency and ∆I
inductor ripple current.
MP2351 Rev. 1.5
1/6/2006
FB
IN
is the feedback voltage and V
R
TM
L
is the input voltage, f
1
V
=
V
=
OUT
FB
f
. 8
S
V
18
×
OUT
=
=
∆I
. 1
V
×
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
OUT
L
23
(
V
×
×
OUT
⎛
⎜ ⎜
⎝
R
1
R
1
−
L
1
R
+
R
is the peak-to-peak
V
−
+
2
V
R
2
OUT
. 1
R
IN
2
2
23
⎞
⎟ ⎟
⎠
)
S
© 2006 MPS. All Rights Reserved.
is the
www.MonolithicPower.com
OUT
MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
is
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
Where, I
Output Rectifier Diode
The output rectifier diode supplies the current to
the inductor when the high-side switch is off. To
reduce losses due to the diode forward voltage
and recovery times, use a Schottky diode.
Choose a diode whose maximum reverse
voltage rating is greater than the maximum
input voltage, and whose current rating is
greater than the maximum load current.
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.
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:
The worst-case condition occurs at V
where:
For simplification, choose the input capacitor
whose RMS current rating greater than half of
the maximum load current.
LOAD
I
LP
I
C
1
=
is the load current.
=
I
LOAD
I
LOAD
+
I
C
2
×
1
×
V
=
OUT
f
V
S
I
V
LOAD
OUT
×
IN
2
L
×
×
⎛
⎜
⎜
⎝
⎛
⎜ ⎜
⎝
1
1
−
−
V
V
V
OUT
V
IN
OUT
IN
⎞
⎟
⎟
⎠
IN
⎞
⎟ ⎟
⎠
= 2V
OUT
5
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