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LM1946N Arkusz danych(PDF) 8 Page - National Semiconductor (TI) |
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LM1946N Arkusz danych(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Application Hints (Continued) TLH8707 – 7 FIGURE 10 Using the LM1946 as an Over-Current Limit Detector and R4 again allow the system designer to tailor the thresh- old limit to the VI characteristics of each particular system The input threshold voltage is determined by and directly proportional to Iset into pin 20 R3 from the on-chip refer- ence voltage provides a current and threshold that is inde- pendent of the supply voltage VCC R4 provides a current directly proportional to supply These resistors allow thresh- olds to be either independent of or directly proportional to supply voltage or anything in between For example the values in Figure 10 are tailored to match the VI characteris- tics of the bulb filament used in earlier examples However if the load had purely resistive characteristics Iset and the threshold would be set with R4 only eliminating R3 Like- wise if the load current was independent of supply such as in many systems powered by a voltage regulator Iset would be better set by R3 only eliminating R4 Further details on this and how to handle variations with ambient temperature with resistor and thermistor combinations are discussed in detail in previous sections Compensation for temperature variations however is rarely necessary since short circuit or over-current values are usually much greater than the nomi- nal value For example if the load in Figure 10 represented a DC motor the circuit could be used to detect the motor stall condition Stall current through the sense resistor Rs would typically be five times the nominal running current By setting the threshold at three times the nominal current val- ue enough margin exists that minor variations due to tem- perature can be ignored The variation in stall current due to battery or supply voltage can be significant however Being approximately proportional Iset would best be set in this case by R4 only WINDOW DETECTOR The availability of more than one comparator per IC allows many other applications One is the current sense window detector Many times it is useful to know that a certain cur- rent is within both an upper and lower limit Using two of the LM1946 comparators and the circuit of Figure 11 will ac- complish this In this particular case high and low limits are approximately 3A and 1A respectively The outputs can be kept separate or wired-or as shown to a single output load as a simple out-of-bounds detector Vthrshld-lo e Iset (R10 b R11) Vthrshld-hi e Iset (R13 b R12) TLH8707 – 8 FIGURE 11 Current Limit Window Detector COMPARATOR INPUT STAGE The LM1946 IC consists of five specially designed compara- tor input circuits to monitor the IR drop across the wiring harness or the sense resistor between the battery and the light bulb These comparators have been designed to ac- commodate a wide range of input signals without damage to the IC or the load circuitry The inputs can easily withstand a common mode voltage above the positive supply since the inputs are the emitters of two matched PNP devices (see Figure 12 ) This is vital in a system which must operate in the conditions present under the hood of an automobile The inputs can also survive when taken well below ground If a negative voltage is present at the inputs of the compara- tor the two emitter-base PNP junctions become reverse bi- ased and block any current flow in or out of the device To disable an unused comparator it is recommended that the inputs be connected to ground 8 |
Podobny numer części - LM1946N |
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Podobny opis - LM1946N |
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