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LM1946 датащи(PDF) 5 Page - National Semiconductor (TI) |
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LM1946 датащи(HTML) 5 Page - National Semiconductor (TI) |
5 / 12 page Application Hints THEORY OF OPERATION UNDER-CURRENT LIMIT DETECTOR TLH8707 – 6 Lamp Fault Detector FIGURE 3 Equivalent Automotive Lamp Circuit The diagram of Figure 3 represents the typical lamp circuit found in most automobiles Switch S1 represents a dash- board switch discrete power device relay andor flasher circuit used for turn signals Sense resistor Rs can be an actual circuit component (such as a 01X 1W carbon resis- tor) or it can represent the resistance of some or all of the wiring harness The load represented here as a single bulb can just as easily be two or more bulbs in parallel such as front and rear parking lights or left and right highbeams etc One of the easiest methods to electronically monitor proper bulb operation is to sense the voltage developed across Rs by the bulb current IL If a fault occurs due to an open bulb filament the load current and sense voltage VS drop to zero (or to half their former values in the case of two bulbs wired in parallel) A comparator circuit can then monitor this sense voltage and alert the system or system user (eg power an LED) if this sense voltage drops below a predeter- mined level (defined as the threshold voltage) Typical sense voltages range from tens to hundreds of milli- volts Not only does this sense voltage vary nonlinearly with the battery voltage it may vary significantly with ambient temperature depending on the temperature coefficient (TC) of the sense resistor or wiring harness Since these nonlin- ear characteristics can vary from system to system and sometimes even within a single system provisions must be made to accommodate them There are two general meth- odologies to accomplish this The first method uses only one bulb per monitoring circuit A sense resistor is selected to give 50 – 100 mV of sense volt- age in an operational circuit and a comparator threshold detecting voltage of approximately 10 mV is set Even if component tolerances battery line variations and tempera- ture coefficients cause the sense voltage to vary 31 or more circuit operation will not be affected The second method must be used if two or more bulbs are wired in parallel and it is necessary to detect if any single lamp fails This is often desirable as it reduces the number of comparators and displays and system cost by at least a factor of two In this case the sense voltage will drop by only half (or less) of it’s original value For example a nomi- nal 100 mV drop across the sense resistor will drop to 50 mV if one of two bulbs fail Therefore a threshold detec- tion voltage between 50 and 100 mV is required (since a 10 mV threshold would alert the system only if both bulbs failed) Yet a fixed threshold of 75 mV may not work if the nominal 100 mV sense voltage can vary 31 due to the fac- tors mentioned earlier What is required is a comparator with a threshold-detecting voltage that tracks the nominal sense voltage as battery line and ambient temperature change Thus while the sense voltage may nominally be anywhere from 50 to 150 mV the threshold voltage will always be roughly 75% of it or 37 mV to 112 mV and will detect the failure of either of two bulbs The LM1946 integrated circuit contains five comparators es- pecially designed for lamp monitoring requirements Since all lamps in a system share the same battery voltage and ambient temperature accommodations for these variations need to be made only once at the IC and each threshold of the five comparators then tracks these variations SETTING THE COMPARATOR THRESHOLD VOLTAGE The threshold voltage at which the comparator output changes state is user-set in order to accommodate the many possible system designs The input bias currents are purposely high to accomplish this and are each equal to the user-set current into the Iset pin (more on this later) Typi- cally around 20 mA the effect of this across the sense resis- tor Rs compared to a typical load measured in amps is negli- gible and can be ignored However when resistors R1 and R2 (Figure 4) are added to the circuit a shift in the threshold voltage is effected This occurs since each input has been affected by different IR drops The LM1946 behaves like any other comparator in that the output switches when the input voltage at the IC pins is zero millivolts (ignoring offset voltage for the moment) If the output therefore has just switched states due to just the right threshold voltage across the sense resistor then the sum of voltages around the resistor loop should equal zero TLH8707 – 9 Vthrshld e Iset (R1 b R2) FIGURE 4 Input Bias Current Vthrshld a Iset R2 b Voffset b Iset R1 e 0 Assuming Voffset m Vthrshld Vthrshld e Iset R1 b Iset R2 Vthrshld e Iset (R1 b R2) 5 |
Аналогичный номер детали - LM1946 |
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Аналогичное описание - LM1946 |
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