MP1497 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP1497 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 Typical Application on page 1).

The feedback resistor (R1) sets the feedback

loop bandwidth in conjunction with the internal

compensation capacitor. R2 is then:

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 resistors

value for common output voltages.

Table 1: Resistor Values 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.

For highest efficiency, select an inductor with a

DC resistance less than 15mΩ. For most

designs, calculate the inductance value with:

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, therefore requires a capacitor

supply the AC current to the step-down

converter while maintaining the DC input

voltage. Use low-ESR capacitors for the best

performance, such as ceramic capacitors with

X5R or X7R dielectrics that have low ESR and

small

temperature

coefficients.

For

most

applications, use a 22µF capacitor.

The input capacitor (C1) requires an adequate

ripple current rating because it absorbs the

input switching current. Estimate the RMS

current in the input capacitor as:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

1

For simplification, choose the 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, include 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 prevent excessive input voltage ripple.

Estimate the input voltage ripple caused by the

capacitance as:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR

electrolytic

capacitors.

Low

ESR

capacitors are preferred to keep the output