Certain current-sense amplifiers
have to contend with
frequent overvoltages. For
example, a current-sense
amplifier that monitors batterydischarge
currents in an automobile
must withstand high-voltage “load
dump” pulses produced when loads
are disconnected from the battery.
This causes inductive spikes and
overvoltages at the output of the
alternator. If these pulses exceed the
amplifier’s common-mode voltage,
the amplifier requires external protection
circuitry.
Such a circuit needs only a pair
of Zener diodes, a pair of resistors,
and another diode (Fig. 1). The
common-mode voltage range of the
example amplifier (MAX4372) is 0
to 28 V. That’s more than sufficient
for measuring automotive battery
voltages, which vary from 6 to
18 V. Load-dump voltages, however,
can reach 35 V and persist for
0.5 seconds, well over the amplifier’s
30-V absolute maximum rating
for input voltage. Thus, the amplifier
needs external protection.
You can avoid additional errors in
the input-offset voltage by using different
values for input-protection
resistors R1 and R2 (2 kΩ and 1
kΩ, respectively), thereby balancing
the effect of the amplifier’s unequal
bias currents. For details on selecting
these resistor values, see application
note APP3888 (www.maxim-ic.com/appnotes.cfm/
an_pk/3888). Zeners Z1 and Z2
have 24-V breakdown voltages, plus
sufficient power-dissipation capability to withstand the approximately
11-mA sink currents that flow during
a 35-V peak load-dump condition.
(The 35-V load-dump voltage minus
a 24-V clamp voltage appears
across the 1-kΩ series resistor, R2.)
Figure 2 depicts the amplifier output
in the presence of 35-V loaddump
pulses without D1. With normal
battery voltages applied, the 1-
V output value is as expected (input
V
SENSE = 50 mV and gain = 20).
When a load-dump voltage
appears, the Zeners clamp the input
common-mode voltage to 24 V, and
the amplifier output makes a few
transient excursions before settling
down to 0 V. Because the two Zeners generally
have slightly different breakdown
voltages, due to part-to-part variations,
and different operating currents
(Z1 operates at 5.5 mA and
Z2 operates at 11 mA), the quantity
(V
Z1 − V
Z2) appears as a changing
differential sense voltage, which
causes the unwanted output transients.
You can eliminate these transients
by adding D1 in series with
either Z1 or Z2. The diode forces
V
Z1 − V
Z2 to be positive or negative
during a load-dump, which in
turn forces the amplifier output to
one of the supply rails (V
CC or
GND), thereby preventing output
spikes during an input transient.
Connecting the diode in series with
Z1 forces the amplifier output to the
positive rail (
Fig. 3). Connecting the
diode in series with Z2 forces the
amplifier output to the negative rail.