MP2136 – 2.5V TO 6V INPUT, 2MHz, 1A SYNCHRONOUS STEP-DOWN CONVERTER
APPLICATION INFORMATION
Output Voltage Setting
The external resistor divider sets the output
voltage. The feedback resistor R1 also sets the
feedback loop bandwidth with the internal
compensation capacitor.
Choose R1 around 300kΩ for optimal transient
response. R2 is then given by:
R2
=
R1
V
OUT
−
1
0 .6 V
Table 1—Resistor Selection vs. Output
Voltage Setting
V
OUT
1.2V
1.5V
1.8V
2.5V
R1
300k
Ω
(1%)
300k
Ω
(1%)
300k
Ω
(1%)
300k
Ω
(1%)
R2
300k
Ω
(1%)
200k
Ω
(1%)
150k
Ω
(1%)
95.3k
Ω
(1%)
Input Capacitor Selection
The input capacitor reduces the surge current
drawn from the input and switching noise from
the device. The input capacitor impedance at
the switching frequency shall be less than input
source impedance to prevent high frequency
switching current passing to the input. Ceramic
capacitors with X5R or X7R dielectrics are
highly recommended because of their low ESR
and small temperature coefficients. The input
capacitor absorbs the input switching current,
so it requires an adequate ripple current rating.
The RMS current through the input capacitor
can be estimated by:
I
CIN
=
I
LOAD
×
V
OUT
V
×
(1
−
OUT
)
V
IN
V
IN
The worse case condition occurs at V
IN
=2V
OUT
，
where:
Inductor Selection
A 0.68µH to 10µH inductor with DC current
rating at least 25% higher than the maximum
load current is recommended for most
applications. For best efficiency, the inductor
DC resistance shall be <200mΩ. For most
designs, the inductance value can be derived
from the following equation:
L
=
V
OUT
×
(
V
IN
−
V
OUT
)
V
IN
× ∆
I
L
×
f
OSC
I
CIN
=
I
LOAD
2
For simplification, choose the input capacitor
whose RMS current rating greater than half of
the maximum load current.
For most applications, a 4.7µF capacitor is
sufficient.
Output Capacitor Selection
The output capacitor keeps output voltage
ripple small and ensures regulation loop stable.
The output capacitor impedance shall be low at
the switching frequency. Ceramic capacitors
with X5R or X7R dielectrics are recommended.
The output ripple
∆V
OUT
is approximately:
∆
V
OUT
≤
⎞
V
OUT
×
(
V
IN
−
V
OUT
)
⎛
1
⎟
× ⎜
ESR
+
⎜
V
IN
×
f
OSC
×
L
8
×
f
OSC
×
C3
⎟
⎝
⎠
Where
∆I
L
is the inductor ripple current. Choose
inductor ripple current approximately 30% of the
maximum load current, 1A.
The maximum inductor peak current is:
I
L(MAX )
=
I
LOAD
+
∆
I
L
2
Under light load conditions below 100mA, larger
inductance is recommended for improved
efficiency.
For most applications, a 10uF capacitor is
sufficient.
MP2136 Rev. 0.9
8/23/2010
www.MonolithicPower.com
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9
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