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LM1946N Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM1946N Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 12 page Application Hints (Continued) 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 microprocessor used When operating with a VCC supply less than 7V it is important to connect the VREF pin to VCC This forces VREF to a fixed voltage which is used for bias of internal circuitry TLH8707 – 19 FIGURE 13 TLH8707 – 14 FIGURE 14 TEST PIN The test pin is a high impedance logic input Forcing this pin high (t 2V) forces all five comparator outputs on This is 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) 001 a 15 C1 (mF) TLH8707 – 15 FIGURE 15 9 |
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