MP8867 – SYNCHRONOUS STEP-DOWN CONVERTER

MP8867 Rev.1.01

www.MonolithicPower.com

21

2/21/2017

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APPLICATION INFORMATION

Setting the Output Voltage in a FB Control

Loop

The reference voltage and external resistor

dividers set the output voltage through FB. The

feedback resistors R1 and R3 also set the

feedback loop bandwidth with the internal

compensation capacitor. Choose the R1 value

1

V

V

R1

R2

REF

OUT





(3)

NOTE:

10) VREF is 0.6V when the power is up or EN is on. After the

I

The T-type network (see Figure 9) is highly

recommended.

Figure 9: T-Type Network

Table 1 lists the recommended feedback

resistor values for common output voltages.

Table 1: Resistor Selection for Common Output

(11)

(V)

R1

(kΩ)

R2

(kΩ)

R3

(kΩ)

C6

(pF)

L

(µH)

0.9

80.6

(1%)

162

(1%)

51

(1%)

22

1.5

1.0

80.6

(1%)

120

(1%)

51

(1%)

22

1.5

1.2

80.6

(1%)

80.6

(1%)

40.2

(1%)

22

1.5

2.5

60.4

(1%)

19.1

(1%)

30

(1%)

22

2.2

3.3

60.4

(1%)

13.3

(1%)

20

(1%)

33

3.3

5

60.4

(1%)

8.25

(1%)

20

(1%)

33

3.3

NOTE:

11) The recommended parameters are based on a 12V input

voltage and a 22µFx4 output capacitor. Different input

voltages and output capacitor values may affect the

selection of R1, R2, R3, and C6. For additional component

parameters, please refer to the Typical Application Circuits

section on page 26.

Loop

In addition to setting the output voltage through

this case, the output voltage is the set reference

voltage. Refer to Table 1 for additional details

about the output voltage settings.

Output Voltage Dynamic Scale

To set the output voltage to dynamic scaling

during normal operation, refer to Figure 10 and

follow the steps below:

1. Write the GO_bit (reg01[6]) to “1.”

2. Write reg00 to select the feedback loop

by setting V_BOOT (reg00[7]) and set

the reference voltage output reference

(reg00[0:6])

simultaneously.

If

the

reference adjustment is within 50mV,

GO_BIT

will

not

reset

to

“0”

automatically. If this is the case, set

GO_BIT to “0” manually.

Repeat these steps if dynamic scaling is

needed for other voltages.

Voltage Scaling

Start

FB Control Loop

(FB Sense Feedback)

(VOUT Direct Sense

Feedback)

Write reg00(Set

“V_BOOT”=1 &

“Vref” Code)

Write reg00(Set

“V_BOOT”=0 &

“Vref” Code)

Loop Selection

Write reg01[6]

“GO_Bit”=1

Figure 10: Output Voltage Dynamic Scale Flow

Chart