MAX4376/MAX4377/MAX4378
Single/Dual/Quad, High-Side Current-Sense
Amplifiers with Internal Gain
7
Maxim Integrated
Detailed Description
The MAX4376/MAX4377/MAX4378 high-side current-
sense amplifiers feature a 0 to +28V input common-mode
range that is independent of supply voltage. This feature
allows the monitoring of current out of a battery in deep
discharge and also enables high-side current sensing at
voltages greater than the supply voltage (V
CC
).
The MAX4376/MAX4377/MAX4378 operate as follows:
current from the source flows through R
SENSE
to the
load (Figure 1). Since the internal sense amplifier’s
inverting input has high impedance, negligible current
flows through RG2 (neglecting the input bias current).
Therefore, the sense amplifier’s inverting-input voltage
equals V
SOURCE
- (I
LOAD
)(R
SENSE
).
The amplifier’s open-loop gain forces its noninverting
input to the same voltage as the inverting input.
Therefore, the drop across RG1 equals (I
LOAD
)
(R
SENSE
). Since I
RG1
flows through RG1, I
RG1
=
(I
LOAD
)(R
SENSE
)/RG1. The internal current mirror multi-
plies I
RG1
by a current gain factor, β, to give I
RGD
= β x
I
RG1
. Solving I
RGD
= β x (I
LOAD
)(R
SENSE
)/RG1.
Therefore:
V
OUT
= β x (RGD/RG1)(R
SENSE
x I
LOAD
) x amp gain
where amp gain is 2, 5, or 10.
The part’s gain equals (β x RGD / RG1) x amp gain.
Therefore:
V
OUT
= (GAIN)(R
SENSE
)(I
LOAD
)
where GAIN = 20 for MAX437_T.
GAIN = 50 for MAX437_F.
GAIN = 100 for MAX437_H.
Set the full-scale output range by selecting R
SENSE
and
the appropriate gain version of the MAX4376/
MAX4377/MAX4378.
Applications Information
Recommended Component Values
The MAX4376/MAX4377/MAX4378 sense a wide variety
of currents with different sense resistor values. Table 1
lists common resistor values for typical operation of the
MAX4376/MAX4377/MAX4378.
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 offset voltage errors of the
internal op amp.
In applications monitoring very high currents, 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 a differential voltage
across the terminals in excess of the absolute maxi-
mum ratings.
If I
SENSE
has a large high-frequency component, mini-
mize the inductance of R
SENSE
. Wire-wound resistors
have the highest inductance, metal-film resistors are
somewhat better, and low-inductance metal-film resis-
tors are best suited for these applications.
Bidirectional Current-Sense Amplifier
Systems such as laptop computers and other devices
that have internal charge circuitry require a precise
bidirectional current-sense amplifier to monitor accu-
rately the battery’s current regardless of polarity. Figure
2 shows the MAX4377 used as a bidirectional current
monitor. This is useful for implementing either smart
battery packs or fuel gauges.
Current Source Circuit
Figure 3 shows a block diagram using the MAX4376
with a switching regulator to make a current source.
