Datasheet LT1720, LT1721 (Analog Devices) - 10

LT1720/LT1721
APPLICATIONS INFORMATION
The exact amount of hysteresis will vary from part to part model for the voltage on the right side of R3 is 300mV or as indicated in the specifi cations table. The hysteresis level VCC – 300mV, for a total voltage swing of (VCC – 300mV) will also vary slightly with changes in supply voltage and – 300mV = VCC – 600mV. common mode voltage. A key advantage of the LT1720/ With this in mind, calculation of the resistor values needed LT1721 is the signifi cant reduction in these effects, which is a two-step process. First, calculate the value of R3 based is important whenever an LT1720/LT1721 is used to detect on the additional hysteresis desired, the output voltage a threshold crossing in one direction only. In such a case, swing, and the impedance of the primary bias string: the relevant trip point will be all that matters, and a stable offset voltage with an unpredictable level of hysteresis, R3 = (R1 || R2)(VCC – 0.6V)/(additional hysteresis) as seen in competing comparators, is of little value. The Additional hysteresis is the desired overall hysteresis less LT1720/LT1721 are many times better than prior compara- the internal 3.5mV hysteresis. tors in these regards. In fact, the CMRR and PSRR tests are performed by checking for changes in either trip point to The second step is to recalculate R2 to set the same av- the limits indicated in the specifi cations table. Because the erage threshold as before. The average threshold before offset voltage is the average of the trip points, the CMRR was set at VTH = (VREF)(R1)/(R1 + R2). The new R2 is and PSRR of the offset voltage is therefore guaranteed to calculated based on the average output voltage (VCC/2) be at least as good as those limits. This more stringent and the simplifi ed circuit model in Figure 4. To assure test also puts a limit on the common mode and power that the comparator’s noninverting input is, on average, supply dependence of the hysteresis voltage. the same VTH as before: Additional hysteresis may be added externally. The R2′ = (VREF – VTH)/(VTH/R1 + (VTH – VCC/2)/R3) rail-to-rail outputs of the LT1720/LT1721 make this more For additional hysteresis of 10mV or less, it is not predictable than with TTL output comparators due to the uncommon for R2′ to be the same as R2 within 1% LT1720/LT1721’s small variability of VOH (output high resistor tolerances. voltage). This method will work for additional hysteresis of up to To add additional hysteresis, set up positive feedback a few hundred millivolts. Beyond that, the impedance of by adding additional external resistor R3 as shown in R3 is low enough to effect the bias string, and adjust- Figure 3. Resistor R3 adds a portion of the output to the ment of R1 may also be required. Note that the currents threshold set by the resistor string. The LT1720/LT1721 through the R1/R2 bias string should be many times the pulls the outputs to the supply rail and ground to within input currents of the LT1720/LT1721. For 5% accuracy, 200mV of the rails with light loads, and to within 400mV the current must be at least 120μA(6μA IB ÷ 0.05); more with heavy loads. For the load of most circuits, a good for higher accuracy. VREF VREF R3 R2a VTH R3 V R2 CC VAVERAGE = 2 + R1 R1 1/2 LT1720 + – 1/2 LT1720 – INPUT 17201 F03 17201 F04
Figure 3. Additional External Hysteresis Figure 4. Model for Additional Hysteresis Calculations
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