LMD18400 datasheet(9/18 Pages) NSC

9 / 18 page

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

VOUT (in Power Limit) e

VCC b

0VCC2 b 60 RL

This provides a maximum transient current and drain-to-

source voltage characteristic as shown in

Figure 5

TLH11026 – 14

FIGURE 5 Maximum Output Current with

Instantaneous Power Limiting

Driving a Lamp

TLH11026 – 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

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

TLH11026 – 16

FIGURE 7 Driving a Large Capacitive Load

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