MP4570 – 3A, 55V, SYNCHRONOUS STEP DOWN CONVERTER

MP4570 Rev. 1.01

www.MonolithicPower.com

17

1/4/2017

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Switching Frequency

The MP4570 has an externally adjustable

CHARACTERISTICS section for more detailed

values.

1000

47.5

900

56

800

63.4

700

73.2

600

84.5

500

102

400

133

300

178

200

261

100

523

Setting the Output Voltage

A resistive voltage divider from the output voltage

to FB pin sets the output voltage. The voltage

divider divides the output voltage down to the

feedback voltage by the ratio:

FB

OUT

R2

V=V

R1+R2



Thus the output voltage is:

OUT

FB

R1+R2

V=V

R2



For example, the value for R1 can be 10kΩ. With

this value, R2 is:

kΩ

1

V

10

R2



So for a 3.3V output voltage, R1 is 10kΩ, and R1

is 4.32kΩ.

Inductor

The inductor provides constant current to the

output load while being driven by the switched

input voltage. A larger-value inductor will result in

lower ripple current that will lower the output

ripple voltage. However, a larger inductor value

will be physically larger, have higher series

resistance, or lower saturation current.

To determine the inductance, allow the inductor’s

peak-to-peak ripple current to approximately

equal 30% of the maximum switch current limit.

Make sure that the peak inductor current is less

than the maximum switch current limit. The

inductance value can be calculated by:





















IN

OUT

L

SW

OUT

V

V

1

∆I

f

V

L1

the peak-to-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

SW

OUT

LOAD

LP

V

V

1

L1

f

2

V

I

I

Input Capacitor

The input current to the step-down converter is

discontinuous and requires a capacitor to supply

the AC current to the step-down converter while

maintaining the DC input voltage. Use capacitors

with low equivalent series resistances (ESR) for

the best performance. Ceramic capacitors are

best, but tantalum or low-ESR electrolytic

capacitors may also suffice.

For simplification, choose the input capacitor with

an RMS current rating greater than half of the

maximum load current. The input capacitor (C1)

can be electrolytic, tantalum, or ceramic.