MP9151

– 20V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP9151 Rev. 1.0

www.MonolithicPower.com

13

3/20/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider is used to set 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 to be

around 10k

Ω. R2 is then given by:

OUT

R1

R2

V

1

0.795V

The T-type network is highly recommended

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 (k

Ω)

R2 (k

Ω)

(k

Ω)

(pF)

L

(

μH)

1

2.61

(1%)

10 (1%)

47

33

1

1.2

5.1 (1%)

10 (1%)

39

33

1

1.8

10 (1%)

7.87 (1%)

5.1

56

1.5

2.5

10 (1%)

4.64 (1%)

5.1

56

1.5

3.3

10 (1%)

3.16 (1%)

0

56

2.2

5

10 (1%)

1.91 (1%)

0

56

2.2

Selecting the Inductor

A 1µH to 22µH inductor with a DC current rating

of at least 25% percent higher than the

maximum load current is recommended for

most applications. For highest efficiency, the

inductor DC resistance should be less than

15m

Ω. For most designs, the inductance value

can be derived from the following equation.

OUT

IN

OUT

1

IN

L

OSC

V

(V

V

)

L

V

I

f

Where

ΔI

Choose inductor current to be approximately

30%

of

the

maximum

load

current.

The

maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Under light load conditions below 100mA, larger

inductance

is

recommended

for

improved

efficiency.

Setting the Switching Frequency

to GND sets the MP9151 oscillating frequency

be calculated from:

FREQ

1.1

s

28000

R

(k )

f (kHz)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the AC current to the step-down

converter

while

maintaining

the

DC

input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors with X5R or

X7R

dielectrics

are

highly

recommended

because

of

their

low

ESR

and

small

temperature coefficients. For most applications,

a 22µF capacitor is sufficient.

Since the input capacitor (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

C

For simplification, choose the input capacitor

whose RMS current rating greater than half of

the maximum load current.