LMD18400 datasheet(13/18 Pages) NSC

13 / 18 page

Applications Information (Continued)

TLH11026 – 23

FIGURE 13 Switching an Inductive Load

When switched ON the worst case power dissipation is

PD(ON) e IPeak2 c RON where IPeak e

VCC

RON a RS

The steady-state ON current of the inductor should be kept

less than 1A per power switch

The additional power dissipation during turn-off as the in-

ductor is de-energized and the voltage across the inductor

is clamped to b5V can be found by

PD(OFF) e

(VCC a 5V) c IPeak

for the time interval tClamp which is the time required for

the inductor current to fall to zero

tClamp e

IPeak c L

The size of the inductor will determine the time duration for

this additional power dissipation interval Even though the

peak current is kept less than 1A the switch during this

interval will see a voltage across it of VCC a 5V with no

power limit protection If the inductor is too large the time

interval may be long enough to heat the die temperature to

170 C thereby shutting OFF all other loads on the pack-

age

The total average power dissipation during a full ONOFF

switching cycle of an inductive load will be

PD(tot) e IPeak2 RON tON a

IPeak2L(VCC a 5V)

( 1

tON a tOFF

Due to the common cut-off of all loads forced by thermal

shutdown the thermal time constants of the package be-

come a concern

Figure 14 provides an indication of the

time it takes to heat the die to thermal shutdown with a step

increase in package power dissipation from an initial junc-

tion temperature of a25 C This data was measured using

a PC board layout providing a thermal resistance from junc-

tion to ambient of approximately 35 CW Less heatsinking

will of course result in faster thermal shutdown of the pow-

er switches

TLH11026 – 24

FIGURE 14 Approximate time required for the die to

reach the 170 C thermal shutdown point from 25 C for

different total package power dissipation levels

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