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EL1056ACM

Monolithic High-Speed Pin Driver

Elantec -> Intersil 

EL1056AC EL1056C

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 50X to

75X and 12 pF to 22 pF for the series capacitor

The effect of the snubber is to ‘‘de-Q’’ resonances

at the output

Figure 2 ISR Pin Circuitry

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1056 – 27

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