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LMV762MM Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LMV762MM Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Application Information BASIC COMPARATOR A basic comparator circuit is used to convert analog input signals to digital output signals. The comparator compares an input voltage (V IN) at the non-inverting input to the reference voltage (V REF) at the inverting pin. If VIN is less than VREF the output (V O) is low (VOL). However, if VIN is greater than V REF, the output voltage (VO) is high (VOH). 20037028 20037031 FIGURE 1. Basic Comparator HYSTERESIS The basic comparator configuration may oscillate or produce a noisy output if the applied differential input is near the comparator's input offset voltage. This tends to occur when the voltage on one input is equal or very close to the other input voltage. Adding hysteresis can prevent this problem. Hysteresis creates two switching thresholds (one for the rising input voltage and the other for the falling input voltage). Hys- teresis is the voltage difference between the two switching thresholds. When both inputs are nearly equal, hysteresis causes one input to effectively move quickly past the other. Thus, moving the input out of the region in which oscillation may occur. Hysteresis can easily be added to a comparator in a non-in- verting configuration with two resistors and positive feedback Figure 2. The output will switch from low to high when V IN rises up to V IN1, where VIN1 is calculated by V IN1 = (VREF(R1+R2))/R2 The output will switch from high to low when V IN falls to VIN2, where V IN2 is calculated by V IN2 = (VREF(R1+R2) – VCC R1)/R2 The Hysteresis is the difference between V IN1 and VIN2. ΔV IN = VIN1 - VIN2 = ((V REF(R1+R2))/R2)-((VREF(R1+R2)) - (VCC R1))/R2) = V CC R1/R2 20037026 20037027 FIGURE 2. Non-Inverting Comparator Configuration INPUT The LMV761/LMV762 have near zero input bias current. This allows very high resistance circuits to be used without any concern for matching input resistances. This also allows the use of very small capacitors in R-C type timing circuits. This reduces the cost of the capacitors and amount of board space used. SHUTDOWN MODE The LMV761 features a low-power shutdown pin that is acti- vated by driving SD low. In shutdown mode, the output is in a high impedance state, supply current is reduced to 20nA and the comparator is disabled. Driving SD high will turn the comparator on. The SD pin should not be left unconnected due to the fact that it is a high impedance input. When left unconnected, the output will be at an unknown voltage. Also do not three-state the SD pin. The maximum input voltage for SD is 5.5V, referred to ground and is not limited by V CC. This allows the use of 5V logic to drive SD while V CC operates at a lower voltage, such as 3V. The logic threshold limits for SD are proportional to V CC. BOARD LAYOUT AND BYPASSING The LMV761/LMV762 is designed to be stable and oscillation free, but it is still important to include the proper bypass ca- pacitors and ground pickups. Ceramic 0.1 μF capacitors should be placed at both supplies to provide clean switching. Minimize the length of signal traces to reduce stray capaci- tance. www.national.com 8 |
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