Detailed Description
The MAX4376/MAX4377/MAX4378 high-side cur-
rent-sense amplifiers feature a 0 to +28V input com-
mon-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 cur-
rent sensing at voltages greater than the supply voltage
(VCC).
The MAX4376/MAX4377/MAX4378 operate as follows:
current from the source flows through RSENSE 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 VSOURCE - (ILOAD)(RSENSE).
The amplifier’s open-loop gain forces its noninverting
input to the same voltage as the inverting input.
Therefore, the drop across RG1 equals (ILOAD)
(RSENSE). Since IRG1 flows through RG1, IRG1 =
(ILOAD)(RSENSE)/RG1. The internal current mirror multi-
plies IRG1 by a current gain factor, β, to give IRGD = β x
IRG1. Solving IRGD = β x (ILOAD)(RSENSE)/RG1.
Therefore:
VOUT = b x (RGD/RG1)(RSENSE x ILOAD) x amp gain
where amp gain is 2, 5, or 10.
The part’s gain equals (β x RGD / RG1) x amp gain.
Therefore:
VOUT = (GAIN)(RSENSE)(ILOAD)
where GAIN = 20 for MAX437_T.
GAIN = 50 for MAX437_F.
GAIN = 100 for MAX437_H.
Set the full-scale output range by selecting RSENSE 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 RSENSE
To measure lower currents more accurately, use a high
value for RSENSE. The high value develops a higher
sense voltage that reduces offset voltage errors of the
internal op amp.
In applications monitoring very high currents, RSENSE
must be able to dissipate the I2R 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 maximum ratings.
If ISENSE has a large high-frequency component, min-
imize the inductance of RSENSE. Wire-wound resistors
have the highest inductance, metal-film resistors are
somewhat better, and low-inductance metal-film resistors
are best suited for these applications.
Bidirectional Current-Sense Amplier
Systems such as laptop computers and other devices
that have internal charge circuitry require a precise bidi-
rectional current-sense amplifier to monitor accurately 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.
www.maximintegrated.com Maxim Integrated
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MAX4376/MAX4377/
MAX4378
Single/Dual/Quad, High-Side
Current-Sense Ampliers with Internal Gain