MP8869SGL datasheet(32/39 Pages) MPS

32 / 39 page

MP8869S

– 18V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8869S Rev. 1.03

www.MonolithicPower.com

1/18/2018

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

APPLICATION INFORMATION

Setting the Output Voltage in a FB Control

Loop

The MP8869S can be controlled by the FB loop.

The output voltage can be set by the external

resistor dividers. The FB loop reference voltage

default

value

(0.72V)

and

can

programmed by the I2C. The

MP8869S’s output

voltage must be below the absolute OVP

threshold (typically 6.5V).

The FB loop network is shown in Figure 11.

VOUT

Figure 11: FB Loop Network

Calculate R1 and R2 with Equation (4):

OUT

0.72V





(4)

Table

6 lists the recommended feedback

resistors value 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

1.5

1.2

80.6

121

1.5

80.6

74.4

1.5

1.8

80.6

53.6

1.5

2.5

80.6

32.4

2.2

3.3

80.6

22.6

2.2

80.6

13.7

3.3

NOTE:

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 36 to page 38.

Output Voltage Dynamic Scale

The output voltage dynamic scaling can be

done only via the I2C. Refer to Figure 11 and

follow the steps below.

1) Write GO_BIT (Reg01[6]) to 1.

2) Write Reg00 to set the reference voltage by

feedback

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 must be changed to a different voltage.

Voltage scaling

start

Write Reg01[6]

“GO_BIT”=1

Write Reg00

(“Vref”code)

Voltage scaling is done

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 l

ess than 5mΩ. For most designs,

the inductance value can be derived from

Equation (5):

OUT

OSC

)





  

(5)

Where ∆IL is the inductor ripple current.

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