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EL1056AC Datasheet(PDF) 10 Page - Elantec Semiconductor |
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EL1056AC Datasheet(HTML) 10 Page - Elantec Semiconductor |
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10 / 16 page  EL1056ACEL1056C Monolithic High-Speed Pin Driver Applications Information Contd The ISR pin is connected to the emitter of a PNP transistor whose base is biased a diode below ground (see Figure 2) Thus the ISR input looks like a low impedance for positive input currents and is biased close to ground A protection diode absorbs negative currents and the input PNP will not conduct In power-down mode the PNP releases its current sink and the external circuit must not present more than 6V to the disabled ISR input or emitter-base damage to the NPN will occur within the driver A signal diode or zener can be used to clamp the ISR input for posi- tive input voltages if the voltage on the ISR resis- tor is potentially greater than 6V when the driver is in power-down mode Output Stage – Tristate Mode In tristate mode (OE low) the output transistors have their emitter-base junctions reverse-biased by a diode voltage This turn-off voltage is in fact provided by an internal buffer whose input is connected to the output pin (see Figure 3) Tran- sistors Q1 – Q4 form the output buffer in normal mode The tristate mode buffer Q5 – Q8 replicates externally impressed voltages from the output pin onto the internal schottky switch node They also turn off Q1 – Q4 by a reverse diode voltage between bases and emitters effectively boot- strapping the internal voltages so that no tran- sistor’s base-emitter junction is reverse-biased by a damaging potential Another benefit is that the capacitance seen at the output in tristate mode is reduced Because the tristate buffer’s input is connected to the output terminal the output is quite ‘‘alive’’ during tristate For instance the input bias cur- rent of the buffer is seen as the tristate ‘‘leakage’’ and its variation with applied voltage becomes tristate input impedance The tristate input current is like a current source and it can drag an output to unpredictable volt- ages It is not a danger to connect a tristated out- put that has drifted to say b6V to a logic pin of a device to be tested The tristate output current will simply comply with whatever voltage the connected part normally establishes The tristate input impedance is also quite active over frequency The output can oscillate when presented with resonant or inductive impedances To prevent this a snubber should be connected from output to ground consisting of a resistor in series with a small capacitor The snubber can also reduce the reflections of the coaxial line when driven from the far end since the line ap- pears to have an open termination during tri- state Typical values for the resistor are 50 X to 75 X and 12 pF to 22 pF for the series capacitor The effect of the snubber is to ‘‘de-Q’’ resonances at the output 1056 – 27 Figure 2 ISR Pin Circuitry 10 |
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