MP8868 – SYNCHRONOUS STEP-DOWN CONVERTER

MP8868 Rev.1.1

www.MonolithicPower.com

21

5/15/2015

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APPLICATION INFORMATION

Setting the Output Voltage in FB control

Loop

Reference voltage and external resistor divider

and can set the output voltage through FB. The

feedback resistor R1 and R3 also sets the

feedback loop bandwidth with the internal

compensation capacitor. Choose R1 value

1

V

V

R1

R2

REF

OUT

−

=

Notes:

10) VREF is 0.6V when Power up or EN on. After MP8868 is

enabled, VREF can be programmed through I

V_BOOT=1 to enable FB control loop.

The T-type network (as shown in Figure 10) is

highly recommended.

Figure 9: T-Type Network

Table 2 lists the recommended feedback

resistors value for common output voltages.

Table 2: 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 is basing on 12V input voltage

and 22µFx4 output capacitor, different input voltage and

output capacitor value may affect the selection of R1, R2, R3,

C6. For other components’ parameters, please refer to

TYPICAL APPLICATION CIRCUITS on page 26.

Loop

Besides setting the output voltage through FB

through VOUT pin by setting V_BOOT=0. In

this case, output voltage is the set reference

voltage. Please refer to Table 1 for more details

about output voltage setting.

Output Voltage Dynamic Scale

To dynamic scale the output voltage during

MP8868 normal operating, it suggests following

below two steps and referring to the Figure 10:

Step1: Write the GO_Bit (reg01[6]) to “1”;

Step2: Write reg00 to select the feedback loop

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

voltage by Output Reference (reg00[0:6])

simultaneously. If reference adjust is within

50mV, GO_BIT won’t be auto reset to “0”, in

this case it needs manually set GO_BIT to “0”.

Repeat above two steps if need dynamic scale

to other voltage.

Voltage Scaling

Start

FB control loop

(FB pin sense feedback)

I2C control loop

(VOUT pin 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