TLH8707 – 13

FIGURE 12 Comparator Input Stage

THE OUTPUT SECTION

The output section of the LM1946 is different from most

automotive comparators as it employs high beta proprietary

PNP transistors which are very rugged and capable of high-

er output currents Each of the five comparator outputs is

capable of at least 20 mA of drive and are internally current

limited and protected against supply overvoltage

The

LM1946 is therefore capable of driving LED’s directly and

larger bulbs via an external grounded base NPN (see

Fig-

ures 13 and 14 ) The outputs can also be wired-or together

without harm

For use in systems with a microprocessor flag instead of a

dashboard indicator the LM1946 can be powered by a stan-

dard 5V logic supply This prevents the LM1946 output from

swinging above the microprocessor supply which might

cause latch problems Since the input common mode range

is independent of supply the inputs can still operate at any

level up to 26V Since the outputs can source current only

pull-down resistors as in

Figure 15 are required their value

depending on the input drive requirements of the particular

internal circuitry

TLH8707 – 19

FIGURE 13

TLH8707 – 14

FIGURE 14

TEST PIN

The test pin is a high impedance logic input Forcing this pin

used to test the indicator LED display (or other output load)

The usual application circuit connects this pin to the ignition

crank line During engine crank therefore the LM1946 out-

put display will light similar to the usual dashboard indica-

tors The test pin was designed to operate with the usual

transient voltages found on the crank line as long as a limit-

ing resistor (eg 30k) separates them

(Figure 1)

Minimum pulse width (ms)

TLH8707 – 15

FIGURE 15

9