MP8774

– 18V, 12A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8774 Rev. 1.0

www.MonolithicPower.com

18

1/15/2018

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© 2018 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

An external resistor divider is used to set the

output voltage. First, choose a value for R2. R2

should be chosen reasonably, since a small R2

leads to considerable quiescent current loss,

while a large R2 makes FB noise-sensitive. R2

is recommended to between 2 -

100kΩ.

Typically, set the current through R2 to be

below 250µA for a good balance between

system

stability

and

no-load

loss.

Then

determine R1 with Equation (3):

OUT

REF

REF

VV

R1

R2

V





(3)

The feedback circuit is shown in Figure 7.

MP8774

FB

R1

R2

VOUT

Rt

Cf

Figure 7: Feedback Network

Table 1 lists the recommended resistor values

for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

(V)

R1

(kΩ)

R2

(kΩ)

L

(μH)

Cf

(pF)

Rt

(kΩ)

1.0

20

30

0.56

56

1

1.2

20

20

0.56

56

1

1.5

20

13

0.56

56

1

1.8

20

10

0.82

56

1

2.5

20

6.34

0.82

56

1

3.3

20

4.42

1

56

1

5

20

2.7

1.2

56

1

Selecting the Inductor

An inductor is necessary for supplying constant

current to the output load while being driven by

the switched input voltage. A larger-value

inductor results in less ripple current and a

lower output ripple voltage but also has a larger

physical footprint, higher series resistance, and

lower saturation current. A good rule for

determining the inductance value is to design

the peak-to-peak ripple current in the inductor

to be in the range of 30 - 40% of the maximum

output current, and ensure that the peak

inductor current is below the maximum switch

current limit. The inductance value can be

calculated with Equation (4):

OUT

OUT

SW

L

IN

VV

L

(1

)

F

I

V



 



(4)

Where

current.

The inductor should not saturate under the

maximum inductor peak current, where the

peak inductor current can be calculated with

Equation (5):

OUT

OUT

LP

OUT

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 step-down

converter

while

maintaining

the

DC

input

voltage. For the best performance, use ceramic

capacitors placed as close to VIN as possible.

Capacitors

with

X5R

and

X7R

ceramic

dielectrics are recommended because they are

fairly stable with temperature fluctuations.

The capacitors must also have a ripple current

rating greater than the maximum input ripple

current of the converter. The input ripple current

can be estimated with Equation (6):

OUT

OUT

CIN

OUT

IN

IN

VV

I

I

(1

)

VV





 

(6)

shown in Equation (7):

OUT

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 capacitance value determines the

input voltage ripple of the converter. If there is

an input voltage ripple requirement in the

system, choose an input capacitor that meets

the specification.