MP1470 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
ΔIL
APPLICATION INFORMATION
IL(MAX) = ILOAD
+
2
Setting the Output Voltage
Under light-load conditions (below 100mA), use
a larger inductor to improve efficiency.
The external resistor divider sets the output
voltage. The feedback resistor R1 also sets the
feedback-loop bandwidth through the internal
compensation capacitor (see the Typical
Application circuit). Choose R1 around 10kꢀ,
and R2with:
Selecting the Input Capacitor
The input current to the step-down converter is
discontinuous, and therefore requires
capacitor to both supply the AC current to the
step-down converter and maintain the DC input
voltage. For the best performance, use low
ESR capacitors, such as ceramic capacitors
with X5R or X7R dielectrics and small
temperature coefficients. A 22µF capacitor is
sufficient for most applications.
a
R1
R2 =
V
OUT
− 1
0.8V
Use a T-type network for when VOUT is low.
RT
R1
VOUT
FB
The input capacitor (C1) requires an adequate
ripple current rating because it absorbs the
input switching. Estimate the RMS current in
the input capacitor with:
R2
Figure 6: T-Type Network
⎛
⎞
⎟
VOUT
VIN
VOUT
VIN
⎜
IC1 = ILOAD
×
× 1−
⎜
⎝
⎟
⎠
Table 1 lists the recommended T-type resistors
value for common output voltages.
The worst-case condition occurs at VIN = 2VOUT
where:
,
Table 1: Resistor Selection for Common Output
Voltages
V
OUT (V)
1.05
1.2
R1 (kΩ)
R2 (kΩ)
Rt (kΩ)
ILOAD
IC1
=
10(1%)
32.4(1%) 300(1%)
2
20.5(1%) 41.2(1%) 249(1%)
40.2(1%) 32.4(1%) 120(1%)
40.2(1%) 19.1(1%) 100(1%)
For simplification, choose an input capacitor
with an RMS current rating greater than half the
maximum load current.
1.8
2.5
3.3
40.2(1%)
13(1%)
75(1%)
The input capacitor can be electrolytic, tantalum,
or ceramic. Place a small, high-quality, ceramic
capacitor (0.1μF) as close to the IC as possible
when using electrolytic or tantalum capacitors.
When using ceramic capacitors, make sure that
they have enough capacitance to provide
sufficient charge to prevent excessive input
voltage ripple. Estimate the input voltage ripple
caused by the capacitance with:
5
40.2(1%) 7.68(1%) 75(1%)
Selecting the Inductor
Use a 1µH-to-10µH inductor with a DC current
rating of at least 25% percent higher than the
maximum load current for most applications.
For highest efficiency, select an inductor with a
DC resistance less than 15mꢀ. For most
designs, derive the inductance value from the
following equation.
⎛
⎞
⎟
⎠
ILOAD
VOUT
VOUT
ΔV
=
×
× 1−
⎜
IN
fS ×C1
V
IN
V
IN
⎝
VOUT ×(V − VOUT
)
IN
L1 =
Selecting the Output Capacitor
V × ΔIL × fOSC
IN
The output capacitor (C2) maintains the DC
output voltage. Use ceramic, tantalum, or low-
ESR electrolytic capacitors. Use low ESR
capacitors to limit the output voltage ripple.
Estimate the output voltage ripple with:
Where ΔIL is the inductor ripple current. Choose
an inductor current approximately 30% of the
maximum load current. The maximum inductor
peak current is:
MP1470 Rev. 1.02
8/27/2013
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
11
MPS [ MONOLITHIC POWER SYSTEMS ]
