MP4568

―60V, 100mA, SYNCHRONOUS STEP-DOWN CONVERTER

MP4568 Rev. 1.01

www.MonolithicPower.com

13

8/18/2013

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© 2013 MPS. All Rights Reserved.

Figure 4

– f

Setting the Output Voltage

The output voltage is set using a resistive voltage

divider from the output voltage to FB pin. To get

the desired output voltage, divider resistor can be

chosen through below formula:

1

-

V

V

R2

R1

REF

OUT



The current flows into divider resistor would

increase the supply current, especially at no load

and light load condition. The Vin supply current

caused

by

the

feedback

resistors

can

be

calculated from:

η

1

V

V

R2

R1

V

I

IN

OUT

OUT

IN_FB









To reduce this current, resistors in the megohm

range are recommended. The recommended

value of the feedback resistors are shown in

Table 1.

Table 1

—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

3.3

1200

523

5

1200

300

Under Voltage Lock Out Point Setting

MP4568 has internal fix under voltage lock out

(UVLO) threshold: rising threshold is about 4V

while trailing threshold is about 3.5V. For the

application needs higher UVLO point, can use

external resistor divider between EN and VIN as

shown in Figure 5 to get higher equivalent UVLO

threshold.

VIN

EN

IN

R4

2M

R5

Figure 5

– Adjustable UVLO using EN pin

The UVLO threshold can be computed from

below two equations.

TH_Rising

EN

2M//R5

R4

(1

UVLO

TH_Rising







)

TH_Falling

EN

2M//R5

R4

(1

UVLO

TH_Falling







)

Furthermore, ENHY pin can be used to get

additional UVLO hysteresis as shown in Figure 6.

VIN

EN

IN

R4

2M

R5

ENHY

R6

Figure 6

– Adjustable UVLO using EN &

ENHY pin

The ENHY pin is an open drain output that goes

low when EN falls below its falling threshold 1.2V.

The UVLO rising threshold in Figure 6 is same to

the one in Figure 5. But the falling threshold is

lowered by adding R6 and the detailed value can

be calculated by below formula.

TH_Rising

EN

R6)

2M//(R5

R4

[1

UVLO

TH_Falling









]

Inversely, when the special UVLO threshold is

determined,

resistor

divider

also

can

be

calculated by above equations. Just note again

the current flows through this divider. To reduce

the current, resistors in the megohm range are

recommended.