MP9840 – 36V, 3.5A, LOW IQ, SYNC STEP-DOWN CONVERTER

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider connected to FB

sets the output voltage (see Figure 5).

RFB1

Vout

FB

MP9840

RFB2

Figure 5: Feedback Network

Choose RFB1 to be around 40

kΩ. R

FB2 can then

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

VOUT (V)

RFB1

(kΩ)

RFB2

(kΩ)

3.3

41.2 (1%)

13 (1%)

5

68.1 (1%)

13 (1%)

Selecting the Inductor

A 1µH to 10µH inductor with a DC current rating

at least 25% higher than the maximum load

current is recommended for most applications.

For higher efficiency, choose an inductor with a

lower DC resistance. A larger-value inductor

results in less ripple current and a lower output

ripple voltage, but also has a larger physical

size,

higher

series

resistance,

and

lower

saturation current. A good rule for determining

the inductor value is to allow the inductor ripple

current

to

be

approximately

30%

of

the

maximum load current. The inductance value

can then be calculated with Equation (4):

=

× −

×∆

OUT

OUT

SW

L

IN

VV

L

(1

)

fI

V

(4)

Where

∆I

L is the peak-to-peak inductor ripple

current.

Choose the inductor ripple current to be

approximately 30%

of the maximum

load

current. The maximum inductor peak current

can be calculated with Equation (5):

=

+

× −

×

OUT

OUT

LP

LOAD

SW

IN

VV

I

I

(1

)

2f

L

V

(5)

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.

Since CIN absorbs the input switching current, it

requires an adequate ripple current rating. The

RMS current in the input capacitor can be

estimated with Equation (6):

=

×

× −

OUT

OUT

CIN

LOAD

IN

IN

VV

I

I

(1

)

VV

(6)

The worst-case condition occurs at VIN = 2VOUT,

shown in Equation (7):

= LOAD

CIN

I

I

2

(7)

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 the input.

MP9840 Rev. 1.01

www.MonolithicPower.com

21

1/23/2018

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