MP2308 – 18V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP2308 Rev. 1.0

www.MonolithicPower.com

11

7/9/2012

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© 2012 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 10kΩ. R2 is then given by:

OUT

R1

R2

V

1

0.8V





The T-type network 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 Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

Rt (kΩ)

1.05

3.09(1%)

10(1%)

24.9(1%)

1.2

4.99(1%)

10(1%)

24.9(1%)

1.8

10(1%)

8.06(1%)

24.9(1%)

2.5

10(1%)

4.75(1%)

24.9(1%)

3.3

10(1%)

3.16(1%)

24.9(1%)

5

10(1%)

1.91(1%)

24.9(1%)

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.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f





 

Choose

the

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

The MP2308 oscillating frequency is set by an

external resistor, Rfreq from the FREQ pin to

ground. The programmable switching frequency

range is from 300kHz to 2MHz. The value of

Rfreq can be calculated from:

freq

s

15000

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

To simplify, choose the input capacitor whose

RMS current rating is greater than half of the

maximum load current.