MP8652 – 15A, 14V, HIGH FREQUENCY STEP-DOWN WITH SYNCHRONOUS GATE DRIVER

MP8652 Rev. 0.9

www.MonolithicPower.com

9

10/26/2009

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see the Typical Application

Circuit on the front page). The feedback resistor

R1 also sets the feedback loop bandwidth with

the internal compensation capacitor (see Figure

1). Choose R1 to be around 40.2kΩ for optimal

transient response. R2 is then given by:

1

V

8

.

0

V

1

R

2

R

=

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

1.8

40.2 (1%)

32.4 (1%)

2.5

40.2 (1%)

19.1 (1%)

3.3

40.2 (1%)

13 (1%)

5

40.2 (1%)

7.68 (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

7mΩ. 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% of the maximum load current, 15A. The

maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 200mA, 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 MOSFETSelection

Guide

Manufacture

Part No.

Siliconix

si7336ADP

IR

1RFH7932P6F

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 are preferred, but tantalum or low-ESR

electrolytic capacitors may also suffice.

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 using electrolytic or tantalum

capacitors, 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

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V

Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended.

Low

ESR

capacitors

are

preferred to keep the output voltage ripple low.