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LMV7275MF Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LMV7275MF Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 16 page Application Notes BASIC COMPARATOR A comparator is often used to convert an analog signal to a digital signal. As shown in Figure 2, the comparator com- pares an input voltage (V IN) to a reference voltage (VREF). If V IN is less than VREF, the output (VO) is low. However, if VIN is greater than V REF, the output voltage (VO) is high. RAIL-TO-RAIL INPUT STAGE The LMV727X has an input common mode voltage range (V CM) of −0.1V below the V − to 0.1V above V+. This is achieved by using paralleled PNP and NPN differential input pairs. When the V CM is near V +, the NPN pair is on and the PNP pair is off. When the V CM is near V −, the NPN pair is off and the PNP pair is on. The crossover point between the NPN and PNP input stages is around 950mV from V +. Since each input stage has its own offset voltage (V OS), the VOS of the comparator becomes a function of the V CM. See curves for V OS vs. VCM in Typical Performance Characteristics sec- tion. In application design, it is recommended to keep the V CM away from the crossover point to avoid problems. The wide input voltage range makes LMV727X ideal in power supply monitoring circuits, where the comparators are used to sense signals close to ground and power supplies. OUTPUT STAGE The LMV7271 and LMV7272 have a push-pull output stage. This output stage keeps the total system power consumption to the absolute minimum. The only current consumed is the low supply current and the current going directly into the load. When the output switches, both PMOS and NMOS at the output stage are on at the same time for a very short time. This allows current to flow directly between V + and V− through output transistors. The result is a short spike of current (shoot-through current) drawn from the supply and glitches in the supply voltages. The glitches can spread to other parts of the board as noise. To prevent the glitches in supply lines, power supply bypass capacitors must be in- stalled. See section for supply bypassing in the Application Notes for details. HYSTERESIS It is a standard procedure to use hysteresis (positive feed- back) around a comparator, to prevent oscillation, and to avoid excessive noise on the output because the comparator is a good amplifier of its own noise. Inverting Comparator with Hysteresis The inverting comparator with hysteresis requires a three resistor network that is referenced to the supply voltage V CC of the comparator (Figure 3). When V IN at the inverting input is less than V A, the voltage at the non-inverting node of the comparator (V IN < VA), the output voltage is high (for sim- plicity assume V O switches as high as VCC). The three network resistors can be represented as R 1||R3 in series with R 2. The lower input trip voltage VA1 is defined as When V IN is greater than VA (VIN > VA), the output voltage is low and very close to ground. In this case the three network resistors can be presented as R 2//R3 in series with R1. The upper trip voltage V A2 is defined as The total hysteresis provided by the network is defined as ∆V A =VA1 -VA2 A good typical value of ∆V A would be in the range of 5 to 50mV. This is easily obtained by choosing R 3 as 1000 to 100 times (R 1||R2) for 5V operation, or as 300 to 30 times (R 1||R2) for 1.8V operation. LMV7271 20064025 20064017 FIGURE 2. LMV7271 Basic Comparator www.national.com 10 |
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