MP1475 – SYNCHRONOUS STEP-DOWN CONVERTER

MP1475 Rev. 1.01

www.MonolithicPower.com

11

4/3/2014

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APPLICATION INFORMATION

Setting the Output Voltage

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 40kΩ. R2 is then given by:

OUT

R1

R2

V

1

0.807V

=

−

The T-type network—as shown in Figure 5—is

Rt

FB

8

R2

R1

Cf

VOUT

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

(V)

R1 (kΩ)

R2 (kΩ) Rt (kΩ) Cf(pF) L(μH)

1.0

20.5

84.5

82

15

2.2

1.2

30.1

61.9

82

15

2.2

1.8

40.2

32.4

33

15

3.3

2.5

40.2

19.1

33

15

3.3

3.3

40.2

13

16

15

4.7

5

40.2

7.68

16

15

4.7

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 15mΩ. For most designs,

the inductance value can be derived from the

following equation.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

Choose the inductor ripple current to be

approximately 30% of the maximum load current.

The maximum inductor peak current is:

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 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.

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:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

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 with

an 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, 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:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠