Typical values are

25 C

For values of sense voltages greater than 100 mV the com-

parator output is off (low) Sense voltages less than 100 mV

turn the output on (high)

It’s also important that the output of the comparator be in

the ‘‘off’’ state when the inputs are taken to ground ie S1

is opened and the lamp is turned ‘‘off’’ The input section of

LM1946 has been designed to turn ‘‘off’’ when the inputs

are grounded and therefore not deliver an erroneous bulb

out indication The comparator is only activated when the

inputs are above ground by at least 3V

R1 and R2 are necessary for another reason These resis-

tors protect the input terminals of the IC from the many

transients in an automobile found on the battery line some

of which can exceed a thousand volts for a few microsec-

onds A minimum value of approximately 1 kX is therefore

recommended

COMPENSATING FOR BATTERY VOLTAGE

The current through a typical automotive lamp whether a

headlight or dashboard illumination lamp will vary as battery

voltage changes The change however is nonlinear Dou-

bling the battery voltage does not double the lamp current

TLH8707 – 21

FIGURE 5

This occurs since a higher voltage will heat the filament

more increasing its resistance and allowing less current to

flow than expected

Figure 5 shows this effect A best fit

straight line over the normal battery range of 9V to 16V for

this particular example can be given by

TLH8707 – 10

R4

a

R3

R4

a

Vref

R3

b

14

FIGURE 6

Thus in actual use the LM1946 threshold voltage should

track the variations in bulb current with respect to battery

voltage To accomplish this Iset should have a component

that varies with the battery As shown in the LM1946 circuit

schematic of

Figure 18 the Iset pin is two diode drops

above ground or approximately 14V A resistor from this

pin to the 64V reference sets the fixed component of Iset a

resistor to the battery line sets the variable component

Thus the best fit straight line in

Figure 5 can be realized

exactly with only two resistors The result is shown in

Figure

6 giving a nominal Iset of 20 mA that tracks the bulb current

as supply varies from 9V to 16V The graph of

Figure 7

shows the final result comparing a typical sense voltage

across Rs with the comparator threshold voltage as the

supply varies

COMPENSATING FOR AMBIENT

TEMPERATURE VARIATION

If the sense resistors used in a system are perfect compo-

nents with no temperature coefficient then the compensa-

tion to be subsequently detailed here is unnecessary How-

ever resistors of the very small values usually required in a

lamp monitoring system are sometimes difficult or expen-

sive to acquire A convenient alternative is the wiring har-

ness a length of wire or even a trace on a printed circuit

board All of these are of copper material and therefore can

vary by as much as 3900 ppm C The LM1946 has been

designed to accommodate a wide range of temperature

compensation techniques If the Iset current is designed to

increase or decrease with temperature nearly any tempera-

ture coefficient can be produced in the threshold voltage of

the five input pairs

TLH8707 – 22

FIGURE 7

6