MP4575

– 5A, 55V, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4575 Rev. 1.0

www.MonolithicPower.com

17

8/16/2016

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© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Switching Frequency

The MP4575 has an externally adjustable

the Typical Characteristics section on page 8

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 sets the output voltage. The

voltage divider divides the output voltage down

to the feedback voltage by the ratio shown in

Equation (3):

FB

OUT

R2

V =V

R1+R2



(3)

Calculated the output voltage with Equation (4):

OUT

FB

R1+R2

V

=V

R2



(4)

For example, if R1

is 10kΩ, then R2 can be

calculated with Equation (5):

k

Ω

1

V

10

R2



(5)

So for a 3.3V output voltage, R1

is 10kΩ, and

R1 is 4.32

kΩ.

Selecting the Inductor

The inductor provides a constant current to the

output load while being driven by the switched

input voltage. A larger-value inductor results in

a lower ripple current and lower output ripple

voltage, but also is physically larger, has a

higher series resistance, and lower saturation

current.

To

determine

the

inductance,

allow

the

inductor’s peak-to-peak ripple current to equal

approximately 30% of the maximum switch

current limit. Ensure that the peak inductor

current is less than the maximum switch current

limit. The inductance value can be calculated

with Equation (6):





















IN

OUT

L

SW

OUT

V

V

1

ΔI

f

V

L1

(6)

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor

current

can

be

calculated

with

Equation (7):

























IN

OUT

SW

OUT

LOAD

LP

V

V

1

L1

f

2

V

I

I

(7)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous and requires a capacitor to

supply 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 preferred, but tantalum

or low-ESR electrolytic capacitors may also be

sufficient.

For simplification, choose an 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.

When using electrolytic or tantalum capacitors,

place a small, high-quality, ceramic capacitor

(0.1μF) as close to the IC as possible. When

using ceramic capacitors, ensure that they have

enough capacitance to provide a sufficient

charge to prevent excessive voltage ripple at

the input. The input voltage ripple caused by

capacitance can be approximated with Equation

(8):