MP8712 – 18V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8712 Rev. 1.2

MonolithicPower.com

19

3/19/2021

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APPLICATION INFORMATION

Setting the Output Voltage

The MP8712 output voltage can be set by the

external resistor dividers. The reference voltage

is fixed at 0.6V.

The feedback network is shown in Figure 5.

Figure 5: Feedback Network

Choose R1 and R2 using Equation (3):

1

6

.

0

1

2





V

V

R

R

OUT

(3)

Table 1 lists the recommended feedback

resistor values for common output voltages.

Table 1: Resistor Selection for Common Output

Voltages (7)

VOUT (V)

R1 (kΩ)

R2 (kΩ) Rt (kΩ)

L (μH)

1.0

80.6

120

10

1.5

1.2

80.6

80.6

10

1.5

1.5

80.6

53.6

10

1.5

1.8

80.6

40.2

10

1.5

2.5

80.6

25.5

10

2.2

3.3

80.6

17.8

10

2.2

5

80.6

11

10

3.3

NOTE:

7)

The recommended parameters are based on a 12V input

voltage and 22µFx4 output capacitor. Different input voltage

and output capacitor values may affect the selection of R1

and R2. For other component parameters, please refer to

the Typical Application Circuits on page 22 to page 25.

Selecting the Inductor

For most applications, use a 0.47µH to 5µH

inductor with a DC current rating at least 25%

higher than the maximum load current. For the

highest efficiency, use an inductor with a DC

resistance less than 5mΩ.

For most designs, the inductance value can be

derived from Equation (4):

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f





 

(4)

Where ∆IL is the 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):

2

I

I

I

L

LOAD

)

MAX

(

L







(5)

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 because of

their

low

ESR

and

small

temperature

coefficients. For most applications, use two

22µF capacitors. 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 (6):

























IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(6)

The worst-case condition occurs at VIN =

2VOUT, shown in Equation (7):

2

I

I

LOAD

1

C



(7)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current. The input capacitor

can be electrolytic, tantalum, or ceramic. When

using electrolytic or tantalum capacitors, add a

small, high-quality ceramic capacitor (e.g.:

0.1μF) placed as close to the IC as possible.

When using ceramic capacitors, ensure that

they have enough capacitance to provide a