MP24971 –1.5A, 50V, 100kHz, 5V FIXED OUTPUT STEP-DOWN CONVERTER

MP24971 Rev. 1.01

www.MonolithicPower.com

10

1/6/2012

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

© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Line Drop Compensation

Long traces to the circuit load induce a voltage

described as:

DROP

OUT

TRACE

VI

R



(1)

Where

TRACE

TRACE1

TRACE2

RR

R



as seen in

LOAD

OUT

OUT

TRACE

VV

I

R





(2)

Supplying an accurate and consistent load

voltage over current-dependant trace resistance

necessitates line-drop compeonsation.

MP24971 offers a compensation method that

adjusts the FB voltage slightly according to the

load current.

described by:

OUT

FB

OUT

SENSE

FB

TOP

BOT

C

VV

I

R

6

V

RR

R







(3)

OUT

SENSE

TOP

OUT

C

IR

6 R

V5V

R

 



(4)

OUT

SENSE

TOP

LOAD

C

OUT

TRACE

IR

6 R

V5V

R

IR

 





(5)

current, balance the equation below:

OUT

SENSE

TOP

OUT

TRACE

C

IR

6 R

IR

R

 



(6)

SENSE

C

TRACE

R6 300k

R

R







(7)

tested or evaluated

SW

ISP

ISN

0.8V

VOS

Load

ERROR

AMPLIFIER

600mV

X 6

Hiccup

Current Limit

10000kO

10pF

Internal Block

VOC

Figure 2—Output Line Drop Compensation

Selecting the Inductor

For most applications, use a 10µH to 47µH

inductor with a DC current rating of at least

125% of the maximum load current. For best

efficiency, select an inductor with a DC

resistance less than 200mΩ. For most designs,

the inductance value can be determined by.

OSC

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L













(8)

Choose inductor current ripple to equal

approximately 30% of the maximum load

current, 1.5A. Then maximum inductor peak

current is:

2

I

I

I

L

LOAD

)

MAX

(

L







(9)

Under light-load conditions below 100mA, use a

larger inductor to improve efficiency.