MPQ9842GL datasheet(25/37 Pages) MPSIND

25 / 37 page

MPQ9842

– 36V, 2A, LOW IQ, SYNCHRONOUS STEP-DOWN CONVERTER

MPQ9842 Rev. 1.1

www.MonolithicPower.com

7/29/2022

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

APPLICATION INFORMATION

Setting the Output Voltage

For adjustable output version, the external

resistor divider connected to FB sets the output

voltage (see Figure 4).

RFB1

Vout

MPQ9842

RFB2

Figure 4: Feedback Network

Choose RFB1 first, then RFB2 can be calculated

with Equation (3):

0.8V

OUT

FB1

FB2

−

(3)

Table

lists

the

recommended

feedback

resistor values for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

VOUT (V)

RFB1

(kΩ)

RFB2 (k

Ω)

3.3

41.2 (1%)

13 (1%)

68.1 (1%)

13 (1%)

For fixed output version, connect FB pin to the

output directly.

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. An inductor with a larger

value 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

)

(4)

Where

∆IL 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

LOAD

)

(5)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous

and

therefore

requires

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

CIN

LOAD

)

(6)

The worst-case condition occurs at VIN =

2VOUT, shown in Equation (7):

= LOAD

CIN

(7)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current.

The

input

capacitor

can

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,

MPQ9842GL Datasheet(PDF) 25 Page - MPS Industries, Inc.

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