MP1475S – HIGH-EFFICIENCY, 3A, 16V, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER

MP1475S Rev. 1.0

www.MonolithicPower.com

14

1/8/2015

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© 2015 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see “Typical Application” on page 1).

Choose R1 around 40.2kΩ; R2 is then given by:

OUT

R1

R2

V

1

0.807V

=

−

The T-type network is recommended highly

Figure 7. T-Type Network

Table 1 lists the recommended T-type resistor

values for common output voltages.

Table 1. Resistor Selection for Common Output

Voltages

(8)

(V)

R1 (kΩ)

R2 (kΩ) Rt (kΩ)

1.0

20.5

84.5

82

1.2

30.1

61.9

82

1.8

40.2

32.4

33

2.5

40.2

19.1

33

3.3

40.2

13

16

5

40.2

7.68

16

Notes:

8) The recommended parameters are based on a 500kHz

switching frequency; a different input voltage, output-inductor

value, and output-capacitor value may affect the selection of

R1, R2, and Rt. For additional component parameters, please

refer to the “Typical Application Circuits” section on pages 17

and 18.

Selecting the Inductor

For most applications, use a1µH to 22µH

inductor with a DC current rating at least 25%

higher than the maximum load current. For

highest efficiency, use an inductor with a DC

resistance less than 15mΩ. For most designs,

the inductance value is derived from the

following equation:

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

Choose

an

inductor-ripple

current

approximately 30% of the maximum load

current. The maximum inductor peak current is

calculated by the following equation:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Use a larger inductor for improved efficiency

under light-load conditions (below 100mA).

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore it requires a capacitor

to supply the AC current while maintaining the

DC input voltage. Use low ESR capacitors for

optimum 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.

Since C1 absorbs the input-switching current, it

requires an adequate ripple-current rating. The

RMS current in the input capacitor is estimated

by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

1

For simplification, choose an input capacitor

that has a RMS current rating greater than half

of the maximum load current.

The input capacitor can be electrolytic, tantalum,

or ceramic. When using electrolytic or tantalum

capacitors,

a

small,

high-quality

ceramic

capacitor (e.g. 0.1μF) should be placed as

close to the IC as possible. When using

ceramic capacitors, ensure that they have

enough capacitance to provide sufficient charge