Specifications

with a current which is proportional to the input voltage. In
other words, the circuit is operating as a voltage to current
converter. This is ideal for driving remote signal sensors and
moving coil galvanometers.
Figure 6(f) shows how an active
integrator can be used to provide an output which falls with
increasing speed.
These are the basic configurations obtainable with the op
amp/comparator. Further combinations can be seen in the
applications shown in Part II of this application note.
Transient Protection
Many application areas use unregulated power supplies
which tend to expose the electronics to potentially damag-
ing transients on the power supply line. This is particularly
true in the case of automotive applications where two such
transients are common.
1
First is the load dump transient.
This occurs when a dead battery is being charged at a high
current and the battery cable comes loose, so that the cur-
rent in the alternator inductance produces a positive tran-
sient on the line in the order of 60V to 120V. The second
transient is called field decay. This occurs when the ignition
is turned ‘‘OFF’’ and the energy stored in the field winding
of the alternator causes a negative 75V transient on the
ignition line.
Figure 7 illustrates methods for protecting against these and
other transients.
Figure 7(a) shows a typical situation in
which the power supply to the LM2907 can be provided
through a dropping resistor and regulated by an external
zener diode Z1, but the output drive is required to operate
from the full available supply voltage. In this case, a sepa-
rate protection zener Z2 must be provided if the voltage on
the power line is expected to exceed the maximum rated
voltage of the LM2907.
In
Figure 7(b) and Figure 7(c), the output transistor is re-
quired only to drive a simple resistive load and no second-
ary protection circuits are required. (Note that the dropping
resistor to the zener also has to supply current to the output
circuit). With the foregoing circuits, reverse supply protec-
tion is supplied by the forward biased zener diode. This de-
vice should be a low forward resistance unit in order to limit
the maximum reverse voltage applied to the integrated cir-
cuit. Excessive reverse voltage on the IC can cause high
currents to be conducted by the substrate diodes with con-
sequent danger of permanent damage. Up to 1V negative
can generally be tolerated. Versions with internal zeners
may be self-protecting depending on the size of dropping
resistor used. In applications where large negative voltage
TL/H/7451–19
TL/H/7451–20
(a) (b)
TL/H/7451–21
TL/H/7451–22
(c) (d)
FIGURE 7. Transient Protection Schemes
7