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EL5181CS-T7 Datasheet(PDF) 9 Page - Elantec Semiconductor |
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EL5181CS-T7 Datasheet(HTML) 9 Page - Elantec Semiconductor |
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9 / 11 page  9 EL5181C 8ns High-Speed Comparator times greater than the input bias current of the comparator: Let the hysteresis be VH, and calculate R3: where: VO=VSD-0.8V (swing of the output) Recalculate R2 to maintain the same value of VTH: Non inverting comparator with hysteresis: R3 adds a portion of the output to the positive input. Note that the current through R3 should be much greater than the input bias current in order to minimize errors. The calculation of the resistor values as follows: Pick the value of R1. R1 should be small (less than 1kΩ) in order to minimize the propagation delay time. Choose the hysteresis VH and calculate R3: Check the current through R3 and make sure that it is much greater than the input bias current as follows: The above two methods will generate hysteresis of up to a few hundred millivolts. Beyond that, the impedance of R3 is low enough to affect the bias string and adjustment of R1 may be required. Power Dissipation When switching at high speeds, the comparator's drive capability is limited by the rise in junction temperature caused by the internal power dissipation. For reliable operation, the junction temperature must be kept below TJMAX (125°C). An approximate equation for the device power dissipa- tion is as follows. Assume the power dissipation in the load is very small: where: VS is the analog supply voltage from VS+ to VS- IS is the analog quiescent supply current per comparator VSD is the digital supply voltage from VSD to ground ISD is the digital supply current per comparator ISD strongly depends on the input switching frequency. Please refer to the performance curve to choose the input driving frequency. Having obtained the power dissipa- tion, the maximum junction temperature can be determined as follows: where: TMAX is the maximum ambient temperature θJA is the thermal resistance of the package Threshold Detector The inverting input is connected to a reference voltage and the non-inverting input is connected to the input. As the input passes the VREF threshold, the comparator's V TH V REF R 1 R 1 R 2 + ------------------- × = R 3 V O V H --------R 1 ( R 2 ) || × = R 2 1V REF ( V TH ) V TH R 1 ----------- ÷ – V TH 0.5V SD – R 3 ------------------------------------- + = + - R3 VIN VREF R1 R 3 V ( SD 0.8 ) R 1 V H -------- × – = I 0.5V SD V REF – R 3 ---------------------------------------- = P DISS V S I S V SD I SD ) × + × ( = T JMAX T MAX Θ JA P DISS × + = |
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