LMD18400 Datasheet, PDF (17/26 Pages) National Semiconductor (TI) – Quad High Side Driver

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LMD18400
SNVS094C – JUNE 1996 – REVISED APRIL 2013
Maximum Power Dissipated and Junction to Ambient
Thermal Resistance vs Size
Figure 29. Recommended PC Board Layout to Reduce the Thermal Resistance from Junction-to-Ambient
Figure 30. Switching an Inductive Load
When switched ON, the worst case power dissipation is:
(6)
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 inductor is de-energized and the voltage across the
inductor is clamped to −5V, can be found by:
(7)
for the time interval, t_CLAMP. which is the time required for the inductor current to fall to zero:
(8)
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 V_CC + 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 package.
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