MP28255 – 4A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP28255 Rev. 1.01

www.MonolithicPower.com

11

3/11/2011

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© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

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 40.2kΩ for

optimal transient response. R2 is then given by:

1

V

V

R1

R2

FB

OUT

−

=

The T-type network is highly recommended when

Vo is low, as Figure 6 shows.

FB 1

R2

R1

Rt

VOUT

Figure 6— 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)

L

(uH)

(uF, Ceramic)

1.05

4.99

16.5 24.9

1-4.7

47

1.2

4.99

10.2 24.9

1-4.7

47

1.5

4.99

5.76 24.9

1-4.7

47

1.8

4.99

4.02 24.9

1-4.7

47

2.5

40.2

19.1

0

1-4.7

47

3.3

40.2

13

0

1-4.7

47

5

40.2

7.68

0

1-4.7

47

Note:

The above feedback resistor table applies to a specific load

capacitor condition as shown in the table 1. Other capacitive loading

conditions will require different values.

Selecting the Inductor

A 1µH to 10µH 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 15mΩ. For

most designs, the inductance value can be

derived from the following equation.

OSC

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

∆

×

−

×

=

Choose

inductor

ripple

current

to

be

approximately 30% if the maximum load current,

4A. 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.

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

For simplification, choose the input capacitor

whose RMS current rating greater than half of the

maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When electrolytic or tantalum

capacitor is used, a small, high quality ceramic

capacitor, i.e. 0.1µF, should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to