MP8865

– SYNCHRONOUS STEP-DOWN CONVERTER

MP8865 Rev. 1.03

www.MonolithicPower.com

22

1/18/2018

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© 2018 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage in the FB Control

Loop

The reference voltage and the external resistor

divider can set the output voltage through FB.

feedback loop bandwidth with the internal

compensation capacitor. Choose the value of

1

V

V

R1

R2

REF

OUT





Notes:

9) VREF is 0.6V when powering up or EN is on. After the

Set V_BOOT=1 to enable the FB control loop.

The

T-type

network

(see

Fig.

10)

is

Figure 10. T-Type Network

Table 3 lists the recommended T-type resistor

values for common output voltages.

TABLE 3. Resistor Selection for Common Output

(V)

R1

(kΩ)

R2

(kΩ)

(kΩ)

Cff

(pF)

(pF)

L

(uH)

0.9

Connect FB to VCC

1

1

34(1%)

51(1%)

33(1%)

15

10

1

1.2

34(1%)

34(1%)

33(1%)

10

10

1

1.8

34(1%) 16.5(1%) 15(1%)

10

15

1.5

2.5

34(1%) 10.7(1%) 15(1%)

15

22

1.8

3.3

34(1%)

7.5(1%)

15(1%)

15

33

2.7

5

34(1%) 4.64(1%) 7.5(1%)

15

47

2.7

Note:

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

voltage and a 22µFx2 output capacitor. A different input

voltage and output capacitor value may affect the selection of

R1, R2, RT, Cff, and CFB. For additional component

parameters, please refer to

the “Typical Application Circuits”

section on page 26.

Loop

In addition to setting the output voltage through

voltage through VOUT by setting V_BOOT=0.

In this case, the output voltage is the set

reference voltage. Please refer to Table 2 on

page 20 for additional details concerning the

output voltage setting.

Output Voltage Dynamic Scale

To dynamic scale the output voltage during

normal operation, refer to Fig. 11 and follow the

steps below:

Step1: Write the GO_Bit (reg01[6])

to “1;”

Step2: Write reg00 to select the feedback loop

by setting V_BOOT(reg00[7]). Set the reference

voltage using the output reference (reg00[0:6])

simultaneously. If the reference adjustment is

within 50mV, GO_BIT will NOT auto re-set to

“0.” If this is the case, manually set GO_BIT to

“0.”

Repeat the two steps above to dynamic scale to

another voltage.

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 11. Output Voltage Dynamic Scale Flow

Chart