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 load-dump pulses
without D1. With normal battery voltages
applied, the 1-V output value is as expected
(input VSENSE = 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 (VZ1-VZ2)
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 VZ1-VZ2 to be positive
or negative during a load-dump, which
in turn forces the amplifier output to one of
the supply rails (VCC 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.
