MP8861 – 2.85V - 18V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8861 Rev. 1.1

www.MonolithicPower.com

29

5/28/2020

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APPLICATION INFORMATION

Setting the Output Voltage in a FB Control

Loop

The MP8861 can be controlled by the FB loop.

The output voltage can be set by the external

resistor dividers. The FB loop reference voltage

is a default value (0.72V) and can be

programmed by the I2C.

The FB loop network is shown in Figure 11.

Figure 11: FB Loop Network

Choose R1 and R2 with Equation (4):





OUT

R1

R2

V

1

0.72V

(4)

Table 6 lists the recommended feedback

resistor values for common output voltages.

Table 6: Resistor Selection for Common Output

Voltages (8)

VOUT (V)

R1 (kΩ)

R2 (kΩ)

Rt (kΩ)

L (μH)

1.0

80.6

205

10

1.5

1.2

80.6

121

10

1.5

1.5

80.6

74.4

10

1.5

1.8

80.6

53.6

10

1.5

2.5

80.6

32.4

10

2.2

3.3

80.6

22.6

10

2.2

5

80.6

13.7

10

3.3

NOTE:

6)

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 components’ parameters, please refer to

the Typical Application Circuits on page 33-35.

Output Voltage Dynamic Scale

The output voltage dynamic scaling can be

done only via the I2C. Refer to Figure 12 and

follow the steps below.

1) Write GO_BIT (reg01[6]) to 1.

2) Write reg00 to set the reference voltage by

Output

Reference

(reg00

[6:0])

simultaneously. When the command is

finished, GO_BIT auto-resets to 0 to prevent

false operation of the VOUT scaling.

Repeat the above two steps if the output

voltage needs to be changed to a different

voltage.

Figure 12: Output Voltage Dynamic Scale Flow

Chart

Selecting the Inductor

Use a 0.47µH to 5µH 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

resistance less than 5mΩ. For most designs,

the inductance value can be derived from

Equation (5):

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f





 

(5)

Where ∆IL is the inductor ripple current.

Choose the inductor ripple current to be

approximately 30% of the maximum load

current.