MP8772

– 17V, 12A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8772 Rev. 1.0

www.MonolithicPower.com

17

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 be 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.

MP8772

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.