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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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LTC6255 Fiches technique(PDF) 19 Page - Linear Technology |
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LTC6255 Fiches technique(HTML) 19 Page - Linear Technology |
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19 / 30 page  LTC6752/LTC6752-1/ LTC6752-2/LTC6752-3/ LTC6752-4 19 6752fc For more information www.linear.com/LTC6752 applicaTions inForMaTion Conversely, when processing small or fast differential sig- nals, hysteresis may need to be eliminated. The LTC6752-1/ LTC6752-2/LTC6752-3 provide a hysteresis pin, LE/ HYST, that can be used to increase the internal hysteresis, completely remove it, or enable the output to latch. For these 3 options of the LTC6752, the internal hysteresis is disabled when the LE/HYST pin voltage is above 1.7V. Although eliminating hysteresis does reduce the voltage gain of the comparator to a finite value, in many cases it will be high enough (typically 6000V/V) to process small input signals. The output will latch when the LE/HYST pin voltage is below 0.3V. The internal hysteresis will increase as the voltage of the pin is adjusted from its default open circuit value of 1.25V to 800mV. The LE/HYST pin can be modeled as a 1.25V voltage source in series with a 20k resistor. The simplest method to increase the internal hysteresis is to connect a single resistorasshowninFigure4betweentheLE/HYSTpinand VEE to adjust hysteresis. Figure 5 shows how hysteresis typically varies with the value of the resistor. In addition to adjusting hysteresis using the LE/HYST pin, additional hysteresis can be added using positive feedback from the output back to the positive input, as shown in Figure 6. The offset (with respect to the input signal) and hysteresis become VOS_FB = VDD +VEE ( ) 2 R1 R1 +R2 +VREF R2 R1 +R2 −VOS − VOH 2 R1 R1 +R2 +VOL R2 R1 +R2 → (1) VHYST_FB = VDD −VEE ( ) R1 R1 +R2 +VOL R2 R1 +R2 + VOH R1 R1 +R2 +VHYST → (2) VOS_FB and VHYST_FB denote the values of offset and hysteresis with positive feedback present. VHYST denotes the hysteresis of the device without positive feedback. For light loads, VOH (output swing high) and VOL (output swing low) are typically a few mV (typically are less than 10mV for a 500µA load). On a 3.0V total supply with VEE = 0V, an increase in hysteresis of approximately 300mV can be obtained with VREF = 1.25V, R2 = 4.53kΩ , R1 = 511Ω, with an induced offset of approximately 1.275V. Figure 4. Adjusting Hysteresis Using an External Resistor at the LE/HYST Pin Figure 5. Hysteresis vs Control Resistor Figure 6. Additional Hysteresis Using Positive Feedback 6752 F04 – + VCC VDD Q VEE LE/HYST R –IN +IN 6752 F06 – + VCC VREF SIGNAL VDD Q VEE R2 R1 50 CONTROL RESISTANCE (k) 30 230 430 130 330 6752 F05 480 180 380 80 280 0 5 10 15 20 25 30 35 40 45 VCC = VDD = 2.5V VCM = 0.3V TA = 25°C CONTROL RESISTOR CONNECTED BETWEEN LE/HYST PIN AND VEE |
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