MP4541

– 80V, 0.8A, SYNCHRONOUS STEP-DOWN CONVERTER

MP4541 Rev. 1.0

MonolithicPower.com

14

5/28/2021

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider sets the output

voltage (VOUT). First, choose a value for the

resistor (R2). If R2 is too low, there can be

considerable IQ loss, while a higher-value R2 can

make FB noise sensitive. It is recommended to

choose an R2 value between 10

kΩ and 100kΩ.

R1 can be calculated with Equation (3):

OUT

REF

REF

VV

R1

R2

V





(3)

Where VREF is the reference voltage (typically

1V).

Figure 2 shows the typical feedback circuit.

MP4541

FB

R1

R2

VOUT

Figure 2: Feedback Network

Selecting the Inductor

A larger-value inductor provides less ripple

current, which results in a lower output voltage

ripple. However, a larger-value inductor has a

larger

physical

footprint,

higher

series

resistance, and lower saturation current. The

recommended inductance value is to design the

peak-to peak ripple current in the inductor to be

in the range of 0.3A to 0.4A. The inductance can

be calculated with Equation (4):

OUT

OUT

SW

L

IN

VV

L

(1

)

f

I

V



 



(4)

Where ΔIL is the peak-to-peak inductor ripple

current.

The inductor should not saturate under the

maximum peak inductor current (ILP). ILP can be

calculated with Equation (5):

OUT

OUT

LP

OUT

SW

IN

VV

I

I

(1

)

2 f

L

V





 



(5)

Selecting the Input Capacitor

The step-down converter has a discontinuous

input current, and requires a capacitor to supply

AC

current

to

the

converter

while

also

maintaining the DC input voltage. For the best

results, it is recommended to use ceramic

capacitors placed as close to VIN as possible.

Ceramic

capacitors

with

X5R

and

X7R

dielectrics are recommended due to their

stability amid temperature fluctuations.

The capacitors must have a ripple current rating

that exceeds the

converter’s maximum input

ripple current (ICIN). ICIN can be estimated with

Equation (6):

OUT

OUT

CIN

OUT

IN

IN

VV

I

I

(1

)

VV





 

(6)

The worst-case scenario occurs at VIN = 2 x VOUT,

calculated with Equation (7):

OUT

CIN

I

I

2



(7)

For simplification, choose an input capacitor with

an RMS current rating greater than half the

maximum load current.

The

input

capacitance

determines

the

converter’s input voltage ripple. If there is an

input voltage ripple requirement in the system,

choose an input capacitor that meets the

relevant specifications.

The input voltage ripple can be estimated with

Equation (8):

OUT

OUT

OUT

IN

SW

IN

IN

IN

I

V

V

V

(1

)

f

C

V

V







 



(8)

The worst-case scenario occurs at VIN = 2 x VOUT,

calculated with Equation (9):

OUT

IN

SW

IN

I

1

V

4 f

C



 



(9)

Selecting the Output Capacitor

An output capacitor is required to maintain the

DC output voltage. It is recommended to use

ceramic or POSCAP capacitors.