M
Single/Dual/Quad High-Side Current-Sense
Amplifiers with Internal Gain
_______________________________________________________________________________________
7
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
Pin Description
PIN
MAX4376
MAX4376
MAX4377
μMAX-8/
SO-8
MAX4378
SO-14/
TSSOP-14
SOT23-5
SO-8
NAME
FUNCTION
1
4
1, 7
1, 7, 8, 14
OUT, OUT_
Output Voltage. V
OUT_
is proportional to the magnitude
of the sense voltage (V
RS+
- V
RS-
). V
OUT_
is
approximately zero when V
RS
-
> V
RS
-
+
(no phase
reversal).
Ground
Supply Voltage
Power connection to the external sense resistor
Load-side connection to the external sense resistor
No Connection. Not internally connected.
2
3
4
5
—
3
1
8
6
4
8
11
4
GND
V
CC
3, 5
2, 6
—
3, 5, 10, 12
2, 6, 9, 13
—
RS+, RS_+
RS-, RS_-
N.C.
2, 5, 7