MP8864

– SYNCHRONOUS STEP-DOWN CONVERTER

MP8864 Rev. 1.02

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 set the output voltage through FB. The

loop bandwidth with the internal compensation

capacitor. Choose the value for R1 first, R2 is

1

V

V

R1

R2

REF

OUT





(3)

NOTES:

11)

Set V_BOOT=

”1” to enable the FB control loop.

The

T-type

network

(see

Figure

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)

(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:

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

voltage and a 22µF x 2 output capacitor. Different input voltage

and output capacitor values may affect the selection of R1, R2,

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 about the output voltage

setting.

Output Voltage Dynamic Scale

To dynamic scale the output voltage during

normal operation, refer to Figure 11 and

following the steps below:

Step 1: Write GO_Bit (reg01[6])

to “1.”

Step 2: Write reg00 to select the feedback loop

by setting V_BOOT(reg00[7]) and set the

reference voltage by the output reference

(reg00[0:6]) simultaneously. If the reference

adjustment is within 50 mV, GO_BIT will not

auto re-

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

set GO_BIT to “0.” Repeat steps 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