MP8771 – 18V, 10A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8771 Rev. 1.11

www.MonolithicPower.com

17

2/24/2020

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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.

Figure 7: Feedback Network

Table 1 lists the recommended resistor values

for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages

VOUT

(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

)

FI

V

 



(4)

Where ∆IL is the peak-to-peak inductor ripple

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

II

(1

)

2F

L

V



 



(5)

MPS inductors are optimized and tested for use

with our complete line of integrated circuits.

Table 2 lists our power inductor

recommendations. Select a part number based

on your design requirements.

Table 2: Power Inductor Selection

Part Number

Inductor

Value

Manufacturer

Select family

series (MPL-AY)

0.56µH to

1.2µH

MPS

MPL-AY1265-R56

0.56μH

MPS

MPL-AY1265-R82

0.82μH

MPS

MPL-AY1265-1R0

1μH

MPS

MPL-AY1265-1R2

1.2μH

MPS

Visit MonolithicPower.com under Products >

Inductors for more information.

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.