MP1470B – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
APPLICATION INFORMATION
Setting the Output Voltage
peak current is calculated with Equation (3):
The external resistor divider sets the output
voltage. Also, the feedback resistor (R1) sets
the feedback loop bandwidth through the
internal compensation capacitor (see the
Typical Application circuits). Refer to Table 1 to
choose R1 and R2 using Equation (1):
R1
ΔIL
IL(MAX) = ILOAD
+
(3)
2
Under light-load conditions (below 100mA), use
a larger inductor to improve efficiency.
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 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.
R2 =
(1)
V
a
OUT
− 1
0.8V
Use a T-type network (see Figure 4).
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 Equation (4):
Figure 4: T-Type Network
Table 1 lists the recommended T-type resistor
values for common output voltages.
⎛
⎞
⎟
Table 1: Resistor Selection for Common Output
Voltages
VOUT
VIN
VOUT
VIN
⎜
IC1 = ILOAD
×
× 1−
(4)
⎜
⎝
⎟
⎠
VOUT (V)
1.05
1.2
R1 (kΩ)
R2 (kΩ)
Rt (kΩ)
The worst-case condition occurs at VIN = 2VOUT
as shown in Equation (5):
,
10 (1%)
32.4 (1%) 300 (1%)
20.5 (1%) 41.2 (1%) 249 (1%)
40.2 (1%) 32.4 (1%) 120 (1%)
40.2 (1%) 19.1 (1%) 100 (1%)
ILOAD
1.8
IC1
=
(5)
2
2.5
For simplification, choose an input capacitor
with an RMS current rating greater than half the
maximum load current.
3.3
40.2 (1%)
13 (1%)
75 (1%)
5
40.2 (1%) 7.68 (1%) 75 (1%)
Selecting the Inductor
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 Equation (6):
Use a 1µH to 10µH inductor with a DC current
rating at least 25% 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 Equation (2):
VOUT ×(V − VOUT
)
IN
(2)
L1 =
V × ΔIL × fOSC
IN
⎛
⎞
⎟
⎠
ILOAD
VOUT
VOUT
(6)
ΔV
=
×
× 1−
⎜
IN
Where ΔIL is the inductor ripple current. Choose
an inductor current approximately 30% of the
maximum load current. The maximum inductor
fS ×C1
V
IN
V
IN
⎝
Selecting the Output Capacitor
The output capacitor (C2) maintains the DC
output voltage. Use ceramic, tantalum, or low
MP1470B Rev. 1.0
8/25/2015
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2015 MPS. All Rights Reserved.
13
MPS [ MONOLITHIC POWER SYSTEMS ]
