LMD18400 Datasheet, PDF (12/26 Pages) National Semiconductor (TI) – Quad High Side Driver

LMD18400
SNVS094C – JUNE 1996 – REVISED APRIL 2013
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This provides a maximum transient current, and drain-to-source voltage characteristic as shown in Figure 22.
Figure 22. Maximum Output Current with Instantaneous Power Limiting
Figure 23. Driving a Lamp / Soft Turn-On of a Lamp Load
The steady state current to the load is limited by the package power dissipation, ambient temperature and the
ON resistance of the switch which has a positive temperature coefficient as shown in the Typical Performance
Characteristics.
This dynamic current limiting of the switches is beneficial when driving lamp and large capacitive loads. Lamps
require a large inrush current, on the order of 10 times the normal operating current, when first switched on with
a cold filament. The LMD18400 will limit this initial current to the level where 15W is dissipated in the switch. As
the filament warms up the voltage across the lamp increases thereby decreasing the voltage across the switch
which permits more current to fully light the lamp. With limited inrush current the lifetime of a lamp load is
increased significantly. Figure 23 illustrates the soft turn-on of a lamp load.
The same principle of increasing output current as the voltage across the load increases allows large capacitive
loads to be charged more quickly by an LMD18400 driver than as opposed to a driver with a fixed 1A current
limit protection scheme. Figure 24 shows the output response while driving a large capacitive load.
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