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The automatic power isolation circuit disclosed permits a monolithic driver like that described in the preceding article to meet strict power-off specification requirements.
English (United States)
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Automatic Power Isolation Circuit
The automatic power isolation circuit disclosed permits a monolithic driver
like that described in the preceding article to meet strict power-off specification
The outputs of the driver (with six drivers per chip) must present 100 K ohms
or more with both +V and -V supplies off and with a negative voltage applied to
the output terminals. The purpose of the specification is to prevent a charge of a
digital bit (zero to one) to a computer, when power to the peripheral equipment is
lost or turned off. The circuit causes an open circuit to exist from all three power
supply terminals of the monolithic circuit, that is, +V,-V and ground, when the +V
and -V supplies are off. Q1 becomes saturated when the +V supply is turned
on, causing +V, -V saturation to exist on the driver terminal. When +V is off, Q1
is turned off (due to loss of base drive) and the chip +V terminal can be pulled
negative (by the output connection and the chip parasitic devices) with only the
small leakage current following.
Q5 and R5 perform a similar function in the -V circuit.
Q2, Q3, Q4, R2, R3, and R4 generate a virtual ground potential for the chip
ground pin. Q2 and Q3 are connected as a differential pair, with Q4 in the
feedback loop such that the base of Q2 is the power of off, the ground terminal
presents a high impedance when a negative potential is applied to the output
The resulting circuit provides an automatic open circuit when...