MP8768 – 8A, 28V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFET

MP8768 Rev. 1.0

www.MonolithicPower.com

9

12/12/2013

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider R1 and R2 is used

to set the output voltage (see the schematic on

front page). 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 31.6kΩ for

optimal transient response. R2 is then given by:

OUT

R1

R2

V

1

0.808V

=

−

The T-type network is highly recommended when

Vo is low, as Figure 3 shows.

FB 1

R2

R1

Rt

VOUT

Figure 3— 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

10 (1%)

33.2 (1%)

80.6 (1%)

1.2

10 (1%)

20.5 (1%)

80.6 (1%)

1.5

10 (1%)

11.8 (1%)

80.6 (1%)

1.8

10 (1%)

8.2 (1%)

80.6 (1%)

2.5

31.6 (1%)

14.7 (1%)

0

3.3

31.6 (1%)

10 (1%)

0

5

31.6 (1%)

6.04 (1%)

0

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

×

Δ

×

−

×

=

Choose inductor current to be approximately

30% if the maximum load current, 8A. The

maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, larger

inductance

is

recommended

for

improved

efficiency.

Synchronous MOSFET

The external synchronous MOSFET is used to

supply current to the inductor when the internal

high-side switch is off. It reduces the power loss

significantly when compared against a Schottky

rectifier.

Table 2 lists example synchronous MOSFETs

and manufacturers.

Table 2—Synchronous MOSFET Selection

Guide

Part No.

Manufacture

FDS6670AS

Fairchild

Si7112

Vishay

Si7114

Vishay

AM4874

Analog Power

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