TION
advantages
not normally
available
in modular
or discrete
component
units. The amplifiers
have thermal sensing
and shut-off
circuitry
which automatically
turns the
amplifier
off when the internal
temperature
reaches
approximately
150°C. This is accomplished
by sensing
the substrate temperature
and deactivating
the input
stage current source when the temperature
reaches a
critical level. As this happens, the output load current
limits at a safe value and the amplifier's quiescent current
decreases.
If the cause of the abnormal
power dissipation
is
continuous
(such as a short circuit across the load) the
output current may remain at a low value or oscillate
between two values d-ependi-ng on the amount of power
being dissipated and the heat sink conditions seen by the
amplifier.
In either case, the amplifier
will not sustain
internal damage and will return to normal operation
within a few seconds after the abnormal
condition
is
removed.
The incorporation
of thermal sensing and shut-off in the
amplifier will allow the use of a smaller heat sink than
would otherwise be required. This is due to the fact that
the amplifier
will protect itself and does not require a
massive heat sink for protection
under abnormal
conditions.
Another unique feature of the 3580 family is the thorough
testing of the unit receiver. In addition
to the normal
tests, all amplifiers are 100% tested for input protection at
the full rated differential voltage ( +v " -V ,( ). Each unit is
also 100% tested for output
maximum
supply voltage.
that is
In these
short circuit
to common
at
The 3581 and 3582 have a unique feature
important
in many high voltage applications.
two models the input bias current is virtually independent
of the applied common-mode
voltage. This is accom-
plished by the true cascode input stage which keeps the
drain-to-source
voltage of the input transistors
as the common-mode
voltage changes.
constant
) +300VOC
OPERATION
FROM A SINGLE SUPPLY
It may be desirable in some applications
to operate the
amplifiers from a single supply. The circuit in Figure 3
illustrates a typical application.
eo = ej [
R
O
~
RO
R
~
+R
II
N ote that there are restrictions
on the input and output
voltages (ei and e,,) which are necessary in order to keep
the amplifier circuits operating in a linear manner.
It should be noted that when the 3581 and 3582 amplifiers
are operated from a single supply, the output stage, which
is still short-circuit-current
limited
and thermally
protected,
is not protected
against short circuits
to
ground (the 3580 will still be short circuit protected under
these conditions).
When the amplifiers are operated from
a single supply, the voltage across one of the output
transistors is high enough that secondary breakdown is a
consideration.
The output current must be limited in
order to prevent damage. This can be done by keeping the
load resistor larger than 5kO for the 3582 and greater
than IkO for the 3581.
v
e.
I
+
r
':1;
l21
I~+
+10V ~ ej ~ +290V
+5V ~ eo ~ +290V
'
t RL-eo
FIGURE
3. Operation
from a Single Supply.
The 3580 family. of integrated
circuit
high voltage
amplifiers
provides performance
which previously was
only available in bulky modular packages (see Figures I
and 2). In addition
to the smaller size and inherent
reliability,
the integrated circuit construction
offers other
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