LM5111 datasheet(9/10 Pages) NSC

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Thermal Performance (Continued)

We know that the junction temperature is given by

J =PD x

JA +TA

Or the rise in temperature is given by

RISE =TJ −TA =PD x

For SOIC-8 package

JA is estimated as 170˚C/W for the

conditions of natural convection.

Therefore T

RISE is equal to

RISE = 0.236 x 170 = 40.1˚C

CONTINUOUS CURRENT RATING OF LM5111

The LM5111 can deliver pulsed source/sink currents of 3A

and 5A to capacitive loads. In applications requiring continu-

ous load current (resistive or inductive loads), package

power dissipation, limits the LM5111 current capability far

below the 5A sink/3A source capability. Rated continuous

current can be estimated both when sourcing current to or

sinking current from the load. For example when sinking, the

maximum sink current can be calculated as:

where R

DS(on) is the on resistance of lower MOSFET in the

output stage of LM5111.

Consider T

J(max) of 125˚C and

JA of 170˚C/W for an SO-8

package under the condition of natural convection and no air

flow. If the ambient temperature (T

A) is 60˚C, and the RD-

(on) of the LM5111 output at T

J(max) is 2.5

Ω, this equation

yields I

SINK(max) of 391mA which is much smaller than 5A

peak pulsed currents.

Similarly, the maximum continuous source current can be

calculated as

where V

DIODE is the voltage drop across hybrid output stage

which varies over temperature and can be assumed to be

about 1.1V at T

J(max) of 125˚C. Assuming the same param-

eters as above, this equation yields I

SOURCE(max) of 347mA.

LM5111 Datasheet(PDF) 9 Page - National Semiconductor (TI)

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