MP92265

– 18V, 5A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP92265 Rev. 1.0

www.MonolithicPower.com

12

11/29/2016

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© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see the Typical Application on page 1).

The feedback resistor (R1) sets the feedback

loop bandwidth with the internal compensation

capacitor. Choose a value for R1 first, and then

calculate R2 with Equation (2):

OUT

R1

R2

V

1

0.6V





(2)

The feedback network is highly recommended

(see Figure 4).

R1

R2

VOUT

FB

8

Figure 4: Feedback Network

Table

1

lists

recommended

resistor

and

capacitor values for common output voltages.

Table 1: Component Selection for Common

R1 (kΩ)

R2 (kΩ)

1.0

120

180

1.2

120

120

1.35

100

80.6

1.5

80.6

53.6

1.8

80.6

40.2

2.5

80.6

25.5

3.3

40.2

8.87

5

40.2

5.49

NOTE:

7)

The recommended parameters are based on a 500kHz

switching

frequency.

Different

output

inductors

and

capacitors can affect the recommended values of R1 and R2.

For the other components’ parameters, please refer to the

Typical Application Circuits on page 15.

Selecting the Inductor

Use a 1µH to 10µH inductor with a DC current

rating at least 25% higher than the maximum

load current for most applications.

For the highest efficiency, use an inductor with

a DC res

istance less than 15mΩ. For most

designs, the inductance value can be derived

from Equation (3):

OUT

IN

OUT

1

IN

L

OSC

V

(V

V

)

L

V

I

f





  

(3)

Where

Choose the inductor ripple current to be

approximately 30%

of the maximum

load

current. The maximum inductor peak current

can then be calculated with Equation (4):

2

I

I

I

L

LOAD

)

MAX

(

L







(4)

Use a larger inductor for improved efficiency

under light-load conditions below 100mA.

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 step-down

converter

while

maintaining

the

DC

input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors with X5R or

X7R dielectrics are recommended for best

results because of their low ESR and small

temperature coefficients. For most applications,

use a 22µF capacitor.

Since C1 absorbs the input switching current, it

requires an adequate ripple current rating. The

RMS current in the input capacitor can be

estimated with Equation (5):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(5)

shown in Equation (6):

2

I

I

LOAD

1

C



(6)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current.