MP1474 – SYNCHRONOUS STEP-DOWN CONVERTER
APPLICATION INFORMATION
Setting the Output Voltage
Where ΔIL is the inductor ripple current.
The external resistor divider sets the output
voltage (see Typical Application on page 1). The
feedback resistor R1 also sets the feedback loop
bandwidth with the internal compensation
capacitor (see Typical Application on page 1).
Choose R1 around 40kꢀ. R2 is then given by:
Choose the inductor ripple current to be
approximately 30% of the maximum load current.
The maximum inductor peak current is:
IL
IL(MAX) ILOAD
2
R1
Use a larger inductor for improved efficiency
under light-load conditions—below 100mA.
R2
V
OUT
1
Selecting the Input Capacitor
0.807V
The input current to the step-down converter is
discontinuous, therefore requires a capacitor is to
supply the AC current to the step-down converter
while maintaining the DC input voltage. Use low
ESR capacitors for the best performance. Use
ceramic capacitors with X5R or X7R dielectrics
for best results because of their low ESR and
small temperature coefficients. For most
applications, use a 22µF capacitor.
The T-type network—as shown in Figure 5—is
highly recommended when VOUT is low.
Cf
R1
Rt
8
FB
VOUT
R2
Since C1 absorbs the input switching current, it
requires an adequate ripple current rating. The
RMS current in the input capacitor can be
estimated by:
Figure 5: T-Type Network
Table 1 lists the recommended T-type resistors
value for common output voltages.
Table 1: Resistor Selection for Common Output
Voltages
VOUT
VIN
VOUT
VIN
IC1 ILOAD
1
VOUT
(V)
R1 (kΩ) R2 (kΩ) Rt (kΩ) Cf(pF) L(μH)
The worse case condition occurs at VIN = 2VOUT
,
where:
1.0
1.2
1.8
2.5
3.3
5
20.5
30.1
40.2
40.2
40.2
40.2
84.5
61.9
32.4
19.1
13
82
82
33
33
16
16
15
15
15
15
15
15
2.2
2.2
4.7
4.7
5.5
5.5
ILOAD
IC1
2
For simplification, choose an input capacitor with
an RMS current rating greater than half of the
maximum load current.
7.68
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
capacitors, add a small, high quality ceramic
capacitor (e.g. 0.1μF) placed as close to the IC
as possible. When using ceramic capacitors,
make sure that they have enough capacitance to
provide sufficient charge to prevent excessive
voltage ripple at input. The input voltage ripple
caused by capacitance can be estimated as:
Selecting the Inductor
Use a1µH-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, use an inductor with a DC
resistance less than 15mꢀ. For most designs,
the inductance value can be derived from the
following equation.
ILOAD
VOUT
VOUT
VOUT (V VOUT
)
IN
V
1
IN
L1
fS C1
V
IN
V
IN
V IL fOSC
IN
MP1474 Rev. 1.0
9/26/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
11
MPS [ MONOLITHIC POWER SYSTEMS ]
