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LT1720 Datasheet(PDF) 7 Page - Linear Technology |
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LT1720 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 28 page  7 LT1720/LT1721 Input Voltage Considerations The LT1720/LT1721 are specified for a common mode range of –100mV to 3.8V when used with a single 5V supply. In general the common mode range is 100mV below ground to 1.2V below VCC. The criterion for this common mode limit is that the output still responds correctly to a small differential input signal. Also, if one input is within the common mode limit, the other input signal can go outside the common mode limits, up to the absolute maximum limits (a diode drop past either rail at 10mA input current) and the output will retain the correct polarity. When either input signal falls below the negative common mode limit, the internal PN diode formed with the sub- strate can turn on, resulting in significant current flow through the die. An external Schottky clamp diode between the input and the negative rail can speed up recovery from negative overdrive by preventing the sub- strate diode from turning on. When both input signals are below the negative common mode limit, phase reversal protection circuitry prevents false output inversion to at least – 400mV common mode. However, the offset and hysteresis in this mode will increase dramatically, to as much as 15mV each. The input bias currents will also increase. When both input signals are above the positive common mode limit, the input stage will become debiased and the output polarity will be random. However, the internal hysteresis will hold the output to a valid logic level, and because the biasing of each comparator is completely independent, there will be no impact on any other com- parator. When at least one of the inputs returns to within the common mode limits, recovery from this state will take as long as 1 µs. The propagation delay does not increase significantly when driven with large differential voltages. However, with low levels of overdrive, an apparent increase may be seen with large source resistances due to an RC delay caused by the 2pF typical input capacitance. Input Protection The input stage is protected against damage from large differential signals, up to and beyond a differential voltage equal to the supply voltage, limited only by the absolute maximum currents noted. External input protection cir- cuitry is only needed if currents would otherwise exceed these absolute maximums. The internal catch diodes can conduct current up to these rated maximums without latchup, even when the supply voltage is at the absolute maximum rating. The LT1720/LT1721 input stage has general purpose internal ESD protection for the human body model. For use as a line receiver, additional external protection may be required. As with most integrated circuits, the level of immunity to ESD is much greater when residing on a printed circuit board where the power supply decoupling capacitance will limit the voltage rise caused by an ESD pulse. Input Bias Current Input bias current is measured with both inputs held at 1V. As with any PNP differential input stage, the LT1720/ LT1721 bias current flows out of the device. With a differential input voltage of even just 100mV or so, there will be zero bias current into the higher of the two inputs, while the current flowing out of the lower input will be twice the measured bias current. With more than two diode drops of differential input voltage, the LT1720/ LT1721’s input protection circuitry activates, and current out of the lower input will increase an additional 30% and there will be a small bias current into the higher of the two input pins, of 4 µA or less. See the Typical Performance curve “Input Current vs Differential Input Voltage.” High Speed Design Considerations Application of high speed comparators is often plagued by oscillations. The LT1720/LT1721 have 4mV of internal hysteresis, which will prevent oscillations as long as parasitic output to input feedback is kept below 4mV. APPLICATIONS INFORMATION |
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