MP1496 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP1496 Rev. 1.05

www.MonolithicPower.com

11

12/26/2012

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see the Typical Application on page 1).

The feedback resistor R1—in conjunction with

the internal compensation capacitor—also sets

the feedback loop bandwidth. R2 is then given

by:

OUT

R1

R2

V

1

0.807V

=

−

The T-type network shown in Figure 4 is highly

recommended.

FB

8

RT

R2

R1

VOUT

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

R1 (kΩ)

R2 (kΩ)

Rt (kΩ)

1.0

20.5

82

82

1.2

30.1

60.4

82

1.8

40.2

32.4

56

2.5

40.2

19.1

33

3.3

40.2

13

33

5

40.2

7.68

33

Selecting the Inductor

Use a 1µH-to-10µH inductor with a DC current

rating of at least 25% percent higher than the

maximum load current for most applications.

Select an inductor with a DC resistance less

than 15mΩ for highest efficiency. 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 an 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 inductance for improved light-load

efficiency.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous,

and

therefore

requires

a

capacitor to supply the AC current to the step-

down converter while maintaining the DC-input

voltage. Use low-ESR capacitors for best

performance, especially ceramic capacitors with

X5R or X7R dielectrics for their low ESR and

small

temperature

coefficients.

For

most

applications, use a 22µF capacitor.

Since the input capacitor (C1) absorbs the input

switching current, it requires an adequate ripple

current rating. Estimate the RMS current in the

input capacitor with:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

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 the

maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, place a small, high quality ceramic

capacitor—e.g. 0.1μF—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 by:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠