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> Amplifier
and Comparator Circuits > APP 4837
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> Circuit
Protection > APP 4837

Feb 09, 2011

Keywords: current-sense amplifiers, negative
fault voltage, ESD-protection diodes
Related
parts

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APPLICATION NOTE 4837

Protect current-sense amplifiers from negative
overvoltage

By:

Arpit
Mehta

Prashanth
Holenarsipur

Abstract: Current-sense
amplifiers normally operate well above ground, but a fault
condition can drive the sense inputs below ground, damaging
the device by drawing excessive current through the
ESD-protection diodes. This circuit protects a particular
high-side current-sense amplifier (MAX4080) by connecting PMOS
blocking transistors in series with each input.

A
similar version of this article appeared in the October 8,
2009 issue of Electronic Design magazine.

A
high-side current-sense amplifier typically amplifies the differential
voltage across a sense
resistor, and provides an output voltage proportional to
the current in that resistor. The sense-voltage rides on a
common-mode voltage that is rejected by the current-sense
amplifier. Such devices, therefore, can be used to detect
over-current faults in the load or to make system
power-management tradeoffs.

Most high-side
current-sense amplifiers are well suited for situations in
which the range of common-mode voltage extends from ~2V above
ground to more than 30V. The sense amplifier for some
industrial and automotive applications requires protection
against reversed-battery connections, and for some loads it
also requires protection against inductive kickbacks and other
negative-voltage transients. Since the common-mode voltage can
go negative (below ground) during these events, it is possible
to damage a sense amplifier by allowing excessive current flow
through the internal ESD
diodes.

For example, the data sheet for a particular
high-side current-sense amplifier (MAX4080) specifies
the absolute maximum voltage between ground and the RS+ or RS-
pin as -0.3V to 80V (Figure 1). A negative voltage much
larger than 0.3V below ground will draw a large current by
turning on one of the internal ESD diodes D1 or
D2.

Figure
1. The two p-channel MOSFETs shown protect this high-side
current-sense amplifier against negative common-mode
voltages.

One method of protecting the
current-sense amplifier is to connect external series diodes
from the sense resistor to the RS+ and RS- pins. During normal
operating conditions, however, any mismatch in the forward
voltage drops of these diodes can seriously degrade the
current-sense amplifier's precision input
characteristics.

A better solution, then, is to connect
PMOS transistors in the RS+ and RS- paths as shown in Figure
1. The PMOS switches are ON in the presence of positive
common-mode voltages, allowing the IC to operate normally.
When the common-mode voltage goes negative, the FETs instantly
turn OFF (become open), inserting a reverse diode
between the sense resistor and input pins, and thereby
protecting the IC by preventing the internal ESD diodes from
turning ON.

The PMOS switches have very low ON
resistance: RDSON is usually a few milliohms.
Because MAX4080 bias currents are also low (12µA max),
RDSON causes a negligible voltage drop in its path,
and therefore has a negligible effect on the IC's input offset
voltage. The waveforms of Figure 2 illustrate operation
for the gain-of-20 version of this IC (other versions provide
gains of 5V/V and 60V/V). The test signal applied between RS+
and RS- is differential, consisting of a 100mVP-P
sinewave riding on a 200mV DC offset, which in turn rides on a
common-mode voltage that varies between -20V and +20V. When
the input common mode is 4.5V or higher, the output is 100mV ×
20 = 2VDC with a 100mV × 20 = 2VP-P sinewave riding
on it, as expected for normal operation.

Figure
2. For the Figure 1 circuit with RS+/RS- inputs driven as
described in the text, these waveforms show that the IC works
normally for positive common-mode voltages up to 20V, and
protects itself by shutting OFF (VRS- = 0V) during
negative common-mode voltages down to -20V.

When
the input common-mode voltage becomes -20V (goes below
ground), the PMOS switches turn OFF to protect the part, and
the output sits at 0V. When the common mode recovers (i.e.
above 4.5V), the IC again behaves normally. This scheme works
equally well for reversed-battery protection, even if
VCC = 0 (as is often the case when one imposes a
reverse-battery condition).

Similar protection against
negative overvoltage can be implemented for other sense
amplifiers, such as the MAX9938, MAX9928, and MAX4173. (The
protection scheme is not required for current-sense amplifiers
such as the MAX9937 and MAX9918, which are
specifically designed to withstand the application conditions
mentioned.)

Related parts

MAX4080

76V, High-Side,
Current-Sense Amplifiers with Voltage Output

Free
samples

MAX9918

-20V to +75V Input
Range, Precision Uni-/Bidirectional, Current-Sense
Amplifiers

Free
samples

MAX9928F

-0.1V to +28V Input
Range, Micropower, Uni-/Bidirectional,
Current-Sense Amplifiers

Free
samples

MAX9937

Automotive
Current-Sense Amplifier with Reverse-Battery
Protection

Free
samples

MAX9938

1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier

Free
samples

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Download,
PDF format (63kB)
APP 4837: Feb 09, 2011

AN4837, AN 4837, APP4837,
Appnote4837, Appnote 4837, DI #817

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