MP1471 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP1471 Rev. 1.01

www.MonolithicPower.com

12

8/27/2013

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage. The feedback resistor R1 also sets the

feedback-loop bandwidth through the internal

compensation

capacitor

(see

the

Typical

Application circuit). Choose R1 around 10kΩ,

and R2 by:

OUT

R1

R2

V

1

0.8V

=

−

Figure 6: T-Type Network

Table 1 lists the recommended T-type resistors

value for common output voltages.

Table 1

Voltages

(V)

R1

(kΩ)

R2

(kΩ)

Rt

(kΩ)

(μH)

(μF)

1.05

10

32.4

150

2.2

44

1.2

20.5

41.2

120

2.2

44

1.8

40.2

32.4

75

3.3

44

2.5

40.2

19.1

59

4.7

44

3.3

40.2

13

40.2

6.8

44

5

40.2

7.68

24.9

6.8

44

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, derive the inductance value from the

following equation.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

an inductor current approximately 30% of the

maximum load current. The maximum inductor

peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light-load conditions (below 100mA), use

a larger inductance for improved efficiency.

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous,

and

therefore

requires

a

capacitor to both supply the AC current to the

step-down converter and maintain the DC input

voltage. Use low ESR capacitors for the best

performance, such as ceramic capacitors with

X5R or X7R dielectrics of their low ESR and

small

temperature

coefficients.

A

22µF

capacitor is sufficient for most applications.

The input capacitor (C1) requires an adequate

ripple current rating because it absorbs the

input switching. 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. Place a small, high-quality, ceramic

capacitor (0.1μF) as close to the IC as possible

when using electrolytic or tantalum capacitors.

When using ceramic capacitors, make sure that

they have enough capacitance to provide

sufficient charge to prevent excessive input

voltage ripple. Estimate the input voltage ripple

caused by the capacitance with:

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. Use low ESR

capacitors to limit the output voltage ripple.

Estimate the output voltage ripple with: