MP2497DN datasheet(9/14 Pages) MPS

9 / 14 page

MP2497 – 3A, 50V, 100kHz STEP-DOWN CONVERTER

MP2497 Rev. 1.12

www.MonolithicPower.com

12/22/2011

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

APPLICATION INFORMATION

Setting the Output Line Drop Compensation

Figure 2 shows the block of output line drop

compensation.

If the trace to the load is long, there is a voltage

drop between VOUT and VLOAD. VOUT (voltage at

output terminator) is not equal to VLOAD(voltage

at load). The voltage drop can be described by:

DROP

OUT

TRACE

=×

(1)

Where, the RTRACE is the resistance of the

output line. (

TRACE

TRACE1

TRACE2

)

Then, the VLOAD is:

LOAD

OUT

TRACE

=−

(2)

To keep an accurate and constant load voltage,

the output line drop compensation is necessary.

MP2497 offers a compensation method, by

adjusting the FB voltage slightly according to

the load current.

The relation between VOUT and VFB can be

described by:

OUT

SENSE

400k

−×

(3)

Where, VFB is 0.8V.

Then, the VOUT can be calculated by:

OUT

SENSE

OUT

6 R1

V(1

) 0.8V

400k

×× ×

(4)

The VLOAD is determined by:

OUT

SENSE

LOAD

OUT

TRACE

6 R1

V(1

) 0.8V

400k

×× ×

−×

(5)

To maintain the VLOAD is not variable with load

current. The equation below should be satisfied:

OUT

SENSE

OUT

TRACE

6 R1

400k

×× ×

=×

(6)

Simplify the formula above, we can get:

TRACE

SENSE

400k

×Ω

(7)

In the formula above, RSENSE is known. And

RTRACE can be tested or evaluated. So, we can

select a proper top feedback resistor R1

according to the RTRACE and RSENSE to

compensate the output line voltage drop.

ISP

ISN

RSENSE

0.8V

Load

VOUT

VLOAD

ERRPR

AMPLIFIER

600mV

X 6

Hiccup

Current Limit

10000kO

10pF

400kO

Internal Block

RTRACE

RTRACE1

RTRACE2

Figure 2—Output Line Drop Compensation

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see the typical application circuit

on the front page). The feedback resistor R1 is

decided by output line drop compensation. R2

is then given by:

OUT −

(8)

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