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LT1719IS8 Datasheet(PDF) 11 Page - Linear Technology |
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LT1719IS8 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1719 APPLICATIONS INFORMATION – + LT1719S8 1719 F05 R2 ′ VREF VTH R3 +VS 2 VAVERAGE = R1 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 (+VS/2) and the simplified circuit model in Figure 5. To assure that the comparator’s noninverting input is, on average, the same VTH as before: R2 ′ = (VREF – VTH)/(VTH/R1 + [VTH – (+VS)/2]/R3) Figure 5. Model for Additional Hysteresis Calculations Additional hysteresis may be added externally. The rail-to- rail outputs of the LT1719 make this more predictable than with TTL output comparators due to the LT1719’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 4. Resistor R3 adds a portion of the output to the threshold set by the resistor string. The LT1719 pulls the outputs to + VS 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 voltage on the right side of R3 is 300mV or +VS – 300mV, for a total voltage swing of (+VS – 300mV) – (300mV) = +VS – 600mV. – + LT1719S8 INPUT 1719 F04 R2 VREF R3 R1 Figure 4. Additional External Hysteresis 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)(+VS – 0.6V)/(additional hysteresis) Additional hysteresis is the desired overall hysteresis less the internal 4mV hysteresis. 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 LT1719. For 5% accuracy, the current must be at least 20 times the input current, more for higher accuracy. This illustration used an LT1719S8; with an LT1719S6 the same procedure is used with V+ substituted for +VS. |
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