MP4433 - 36V, 3A, LOW IQ, SYNCHRONOUS STEP-DOWN CONVERTER

MP4433 Rev. 1.01

www.MonolithicPower.com

22

1/12/2018

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider connected to FB

sets the output voltage (see Figure 5).

Vout

FB

MP4433

Figure 5: Feedback Network

be calculated with Equation (3):

1

0.8V

V

R

R

OUT

FB1

FB2





(3)

Table

1

lists the recommended feedback

resistor values for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

3.3

41.2 (1%)

13 (1%)

5

68.1 (1%)

13 (1%)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous

and

therefore

requires

a

capacitor to supply AC current to the converter

while maintaining the DC input voltage. For the

best performance, use low ESR capacitors.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients.

For most applications, use a 4.7µF to 10µF

capacitor. It is strongly recommended to use

another lower-value capacitor (e.g.: 0.1µF) with

a small package size (0603) to absorb high-

frequency switching noise. Place the smaller

capacitor as close to VIN and GND as possible.

requires an adequate ripple current rating. The

RMS current in the input capacitor can be

estimated with Equation (4):





 

OUT

OUT

CIN

LOAD

IN

IN

VV

I

I

(1

)

VV

(4)

shown in Equation (5):

CIN

I

I

2

(5)

For simplification, choose an input capacitor

with an 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, add a small, high-quality ceramic

capacitor (e.g.:

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 input. The input voltage ripple

caused by capacitance can be estimated with

Equation (6):







 



LOAD

OUT

OUT

IN

SW

IN

IN

IN

I

V

V

V

(1

)

f

C

V

V

(6)

Selecting the Output Capacitor

The output capacitor maintains the DC output

voltage. Use ceramic, tantalum, or low-ESR

electrolytic capacitors. For best results, use low

ESR capacitors to keep the output voltage

ripple low. The output voltage ripple can be

estimated with Equation (7):





 







OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V

(1

) (R

)

f

L

V

8f

C

(7)

equivalent series resistance (ESR) value of the

output capacitor.

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency, and the capacitance causes the

majority of the output voltage ripple.