LMD18400 Datasheet, PDF (9/18 Pages) National Semiconductor (TI) – Quad High Side Driver

Applications Information (Continued)
tinually monitored and limited to 15W by varying the gate
voltage and therefore the ON resistance of the switch Basi-
cally the ON resistance will be as low as possible until 15W
is being dissipated To maintain 15W the ON resistance
increases to reduce the load current This results in a de-
crease of the output voltage For resistive loads the output
voltage when in power limit will be
V_OUT
(in
Power
Limit)
e
V_CC
b
0VCC2
2
b
60
R_L
This provides a maximum transient current and drain-to-
source voltage characteristic as shown in Figure 5
TL H 11026 – 14
FIGURE 5 Maximum Output Current with
Instantaneous Power Limiting
Driving a Lamp
This dynamic current limiting of the switches is beneficial
when driving lamp and large capacitive loads Lamps re-
quire 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 cu-
rent the lifetime of a lamp load is increased significantly
Figure 6 illustrates the soft turn-on of a lamp load
The same principle of increasing output current as the volt-
age 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 7 shows the output response while driving a
large capacitive load
Thermal Protection
The die temperature of the LMD18400 is continually moni-
tored Should any conditions cause the die temperature to
rise to a170 C all of the power switches are turned OFF
automatically to reduce the power dissipation It is important
to realize that the thermal shutdown affects all four of the
switches together That is if just one switch load is enough
to heat the die to the thermal shutdown threshold all of the
other switches regardless of their power dissipation condi-
tions will be switched OFF All of the switches will be re-en-
abled when the die temperature has cooled to approximate-
ly a160 C Until the high temperature forcing conditions
have been removed the switches will cycle ON and OFF
thus maintaining an average die temperature of a165 C
The LMD18400 will signal that excessive temperatures exist
through several diagnostic output signals (see Diagnostics)
Driving a Large
Capacitive Load
TL H 11026 – 15
FIGURE 6 Soft Turn-On of a Lamp Load
The steady state current to the load is limited by the pack-
age power dissipation ambient temperature and the ON re-
sistance of the switch which has a positive temperature co-
efficient as shown in the Typical Performance Characteris-
tics
TL H 11026 – 16
FIGURE 7 Driving a Large Capacitive Load
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