LT1432CS8 datasheet(13/28 Pages) LINER

13 / 28 page

LT1432

APPLICATI

I FOR ATIO

100mV/DIV

µs/DIV

Figure 14. PC trace Current Limit Sense Resistor

with Kelvin Contacts

Time scale has been expanded to 50

µs/DIV. The spike

consists of several switching cycles of the LT1271 as

shown in the lower trace. In the upper trace, the output

filter has smoothed the switching frequency content of the

spike, but the actual spike amplitude is only modestly

reduced. Increasing the output filter constants from 10

µH

and 220

µF to 20µH and 330µF would eliminate most of

the spike.

Current Limiting

The LT1432 has true switching current limit with a sense

voltage of 60mV. This low sense voltage is used to

maintain high efficiency with normal loads and to make it

possible to use the printed circuit board trace material as

the sense resistor. The sense resistor value must take

ripple current into account because the LT1432 limits on

the peak of the inductor ripple current. Errors in the sense

resistor must also be allowed for.

I1.2 * +

SENSE

MAX

RIP

()

RSENSE = Required sense resistor

VSENSE = 60mV

IMAX = Maximum load current, including any surge

longer than 50

µs

* 1.2 is a fudge factor for errors in RSENSE and VSENSE.

= 1/2 Peak to Peak Inductor Ripple Current

VV – V

2V (f)(L)

RIP

OUT

()

f = Frequency

L = Inductance

Use VIN maximum

Example: IMAX = 2A, f = 60kHz, maximum VIN = 15V,

L = 50

µH;

515 – 5

2 15 60E

50E

0.55A

60mV

2A 1.2 + 0.55A

0.02

RIP

3–6

SENSE

()

() 





()

=Ω

The formula for RSENSE shows a 1.2 multiplier term in the

denominator which makes typical current limit 20% above

full load current. This accounts for small errors in the PCB

trace resistance. Trace resistance errors are kept to a

minimum by using internal traces (on multilayer boards)

KEEP THIS DISTANCE SHORT

FOR BEST LOAD REGULATION.

TO LOAD

}

TO VOUT

TO VLIM

“W”

“L” IS MEASURED FROM

POINT “X” TO POINT “Y”

FROM

INDUCTOR

LT 432 F14

“X”

“Y”

Figure 13

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