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LM1946 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM1946 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 12 page Application Hints (Continued) Typical values are R1 e 62k g5% R2 e 12k g5% Iset e 20 mA 25 C Vthrshld e 20 mA (62k b 12k) e 100 mV 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 IL (Amps) e 062 a 0069 Vbattery TLH8707 – 10 Iset e VCC b 14 R4 a Vref b 14 R3 Iset e VCC R4 a Vref R3 b 14 1R3a 1R4J 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 |
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