MP28372 — DUAL 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER

MP28372 Rev. 1.4

www.MonolithicPower.com

7

12/10/2007

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TM

APPLICATION INFORMATION

COMPONENT SELECTION

The MP28372 has two channels: A and B. The

following formulas are used for component

selection of both channels. Refer to

components with reference “A” for channel A,

and components with reference “B” for channel

B, respectively, as indicated in Figure 3 (i.e. –

R1A for Channel A and R1B for Channel B).

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output voltage to FB pin.

The voltage divider divides the output voltage

down to the feedback voltage by the ratio:

2

R

1

R

2

R

V

V

OUT

FB

+

=

Thus the output voltage is:

2

R

2

R

1

R

V

92

.

0

+

×

=

the output voltage

A typical value for R2 can be as high as 100kΩ,

but a typical value is 10kΩ. Using that value, R1

is determined by:

)

1

V

92

.

0

V

(

2

R

1

R

×

=

For example, for a 3.3V output voltage, R2 is

10kΩ, and R1 is 25.9kΩ.

Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor will result in less ripple current that will

result in lower output ripple voltage. However,

the larger value inductor will have a larger

physical size, higher series resistance, and/or

lower saturation current. A good rule for

determining the inductance to use is to allow

the peak-to-peak ripple current in the inductor

to be approximately 30% of the maximum

switch current limit. Also, make sure that the

peak inductor current is below the maximum

switch current limit. The inductance value can

be calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

IN

OUT

L

S

OUT

V

V

1

ΔI

f

V

1

L

peak inductor ripple current.

Choose an inductor that will not saturate under

the maximum inductor peak current.

The peak inductor current can be calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

+

=

IN

OUT

S

OUT

LOAD

LP

V

V

1

1

L

f

2

V

I

I

Output Rectifier Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode forward voltage

and recovery times, use a Schottky diode.

Choose a diode whose maximum reverse

voltage rating is greater than the maximum

input voltage, and whose current rating is

greater than the maximum load current.

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