Datasheet

Detailed Description
The MAX9937 unidirectional high-side, current-sense
amplifier features a 4V to 28V input common-mode volt-
age range that is independent of supply voltage (V
CC
=
2.7V to 5.5V). The MAX9937 monitors the current through
a current-sense resistor by converting the sense voltage
to a current output (OUT). Gain is set by the ratio of an
output resistor (R
OUT
) and an input resistor (R
RSP
).
High-side current monitoring with the MAX9937 does not
interfere with the ground path of the load, making it useful
for a variety of battery/ECU monitoring.
Robust input ESD structure allows input common-mode
voltages to exceed the 28V maximum operating input
range for short durations, making the MAX9937 ideal
for applications that need to withstand short-duration
load-dump conditions. The MAX9937 is able to withstand
reverse-battery conditions by a suitable choice of input
resistors (R
RSN
, R
RSP
). See the Input Common-Mode
Voltages > 28V and < 0V section.
Current-Sense Amplier Operation
The MAX9937 current-sense amplifier operation is
best understood as a specialized op-amp circuit with a
p-channel FET in the feedback path. The op amp forces
a current through an external gain resistor at RSP (R
RSP
,
see the Typical Application Circuit) so that its voltage
drop equals the voltage drop across the external sense
resistor, R
SENSE
, making the voltage at RSP the same as
RSN. An external resistor at RSN (R
RSN
) has the same
value as R
RSP
to minimize input offset voltage due to
input bias currents.
The current through R
RSP
is now sourced by the
high-voltage p-channel FET into an external resistor
(R
OUT
) at OUT. This produces an output voltage whose
magnitude is given by the following equations:
SENSE LOAD SENSE
OUT
OUT SENSE
RSP
V IR
R
VV
R
= ×
= ×
The gain accuracy is primarily determined by the match-
ing of the two gain resistors, R
RSP
and R
OUT
. The volt-
age gain error of the MAX9937 is less than 1.5%.
Total gain = 20V/V with R
OUT
= 10kΩ
and R
RSP
= 500Ω.
Low temperature drift of input bias currents and input
offset currents minimizes their impact on total input offset
voltage of the current-sense amplifier.
Applications Information
Choosing R
SENSE
To measure lower currents more accurately, use a high
value for R
SENSE
. The high value develops a higher
sense voltage that reduces the effect of offset voltage
errors of the internal op amp. In applications monitoring
very high currents, however, R
SENSE
must be able to
dissipate the I
2
R losses. If the resistor’s rated power
dissipation is exceeded, its value may drift or it may fail
altogether, causing large differential voltages to develop
between RSP and RSN.
To minimize the effect of input offset voltage by produc-
tion calibration, see the Skewed Input Offset Voltage for
Production Calibration section. This can help reduce the
size of the sense resistor in high-current applications, as
well as measure wide-dynamic-range currents without
sacrificing accuracy.
If I
SENSE
has a large high-frequency component, mini-
mize the inductance of R
SENSE
and use input differential
filters (see the Flexible EMI Filtering section). Low-
inductance metal-film resistors are best suited for these
applications.
Calculation of Total Input Offset Voltage
Because of the use of op-amp style architecture, cal-
culation of total input offset voltage involves the same
methodology as is used for any standard op-amp cir-
cuit. Interaction of the input bias currents and tolerance
of the external resistors, combined with the core input
offset voltage of the op amp, are important to consider.
Finally, RSS (root-sum-of-squares) calculation for all
these uncorrelated sources of error gives the final input
offset voltage.
22 2 2
OS FINAL OS B RS B RS
(V ) (V) (I R) (IR)
−
= + ×∆ + ∆ ×
PIN NAME FUNCTION
1 V
CC
Power Supply. Bypass to GND with a
0.1µF capacitor.
2 GND Ground
3 OUT Current Output
4 RSN
Load-Side Connection Through
External R
RSN
Resistor
5 RSP
Supply-Side Connection Through
External R
RSP
Resistor
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Maxim Integrated │ 8
MAX9937 Current-Sense Amplier with
Reverse-Battery Protection
Pin Description