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LT1721CS Datasheet(PDF) 9 Page - Linear Technology |
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LT1721CS Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  9 LT1720/LT1721 The exact amount of hysteresis will vary from part to part as indicated in the specifications table. The hysteresis level will also vary slightly with changes in supply voltage and common mode voltage. A key advantage of the LT1720/ LT1721 is the significant reduction in these effects, which is important whenever an LT1720/LT1721 is used to de- tect a threshold crossing in one direction only. In such a case, the relevant trip point will be all that matters, and a stable offset voltage with an unpredictable level of hyster- esis, as seen in competing comparators, is of little value. The LT1720/LT1721 are many times better than prior com- parators in these regards. In fact, the CMRR and PSRR tests are performed by checking for changes in either trip point to the limits indicated in the specifications table. Because the offset voltage is the average of the trip points, the CMRR and PSRR of the offset voltage is therefore guaranteed to be at least as good as those limits. This more stringent test also puts a limit on the common mode and power supply dependence of the hysteresis voltage. Additional hysteresis may be added externally. The rail-to- rail outputs of the LT1720/LT1721 make this more pre- dictable than with TTL output comparators due to the LT1720/LT1721’s small variability of VOH (output high voltage). To add additional hysteresis, set up positive feedback by adding additional external resistor R3 as shown in Figure 3. Resistor R3 adds a portion of the output to the threshold set by the resistor string. The LT1720/LT1721 pulls the outputs to the supply rail and ground to within 200mV of the rails with light loads, and to within 400mV with heavy loads. For the load of most circuits, a good model for the APPLICATIONS INFORMATION Figure 4. Model for Additional Hysteresis Calculations – + 1/2 LT1720 1720/21 F04 R2 ′ VREF VTH R3 VCC 2 VAVERAGE = R1 voltage on the right side of R3 is 300mV or VCC – 300mV, for a total voltage swing of (VCC – 300mV) – 300mV = VCC – 600mV. With this in mind, calculation of the resistor values needed is a two-step process. First, calculate the value of R3 based on the additional hysteresis desired, the output voltage swing, and the impedance of the primary bias string: R3 = (R1 R2)(VCC – 0.6V)/(additional hysteresis) Additional hysteresis is the desired overall hysteresis less the internal 3.5mV hysteresis. The second step is to recalculate R2 to set the same average threshold as before. The average threshold before was set at VTH = (VREF)(R1)/(R1 + R2). The new R2 is calculated based on the average output voltage (VCC/2) and the simplified circuit model in Figure 4. To assure that the comparator’s noninverting input is, on average, the same VTH as before: R2 ′ = (VREF – VTH)/(VTH/R1 + (VTH – VCC/2)/R3) For additional hysteresis of 10mV or less, it is not uncom- mon for R2 ′ to be the same as R2 within 1% resistor tolerances. This method will work for additional hysteresis of up to a few hundred millivolts. Beyond that, the impedance of R3 is low enough to effect the bias string, and adjustment of R1 may also be required. Note that the currents through the R1/R2 bias string should be many times the input currents of the LT1720/LT1721. For 5% accuracy, the current must be at least 120 µA(6µA IB ÷ 0.05); more for higher accuracy. Figure 3. Additional External Hysteresis – + 1/2 LT1720 INPUT 1720/21 F03 R2 VREF R3 R1 |
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