Detailed Description
The MAX4080/MAX4081 unidirectional and bidirectional
high-side, current-sense amplifiers feature a 4.5V to
76V input common-mode range that is independent of
supply voltage. This feature allows the monitoring of
current out of a battery as low as 4.5V and also enables
high-side current sensing at voltages greater than the
supply voltage (VCC). The MAX4080/MAX4081 monitors
current through a current-sense resistor and amplifies
the voltage across the resistor. The MAX4080 senses
current unidirectionally, while the MAX4081 senses cur-
rent bidirectionally.
The 76V input voltage range of the MAX4080/MAX4081
applies independently to both supply voltage (VCC)
and common-mode, input-sense voltage (VRS+). High-
side current monitoring does not interfere with the
ground path of the load being measured, making the
MAX4080/MAX4081 particularly useful in a wide range
of high-voltage systems.
Battery-powered systems require a precise bidirectional
current-sense amplifier to accurately monitor the bat-
tery’s charge and discharge. The MAX4081 charging
current is represented by an output voltage from VREF
to VCC, while discharge current is given from VREF to
GND. Measurements of OUT with respect to VREF yield
a positive and negative voltage during charge and dis-
charge, as illustrated in Figure 1 for the MAX4081T.
Current Monitoring
The MAX4080 operates as follows: current from the
source flows through RSENSE to the load (Figure 2), cre-
ating a sense voltage, VSENSE. 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 ampli-
fier’s open-loop gain forces its noninverting input to the
same voltage as the inverting input. Therefore, the drop
across RG1 equals VSENSE. The internal current mirror
multiplies IRG1 by a current gain factor,
β, to give IA2 =
β IRG1. Amplifier A2 is used to convert the output
current to a voltage and then sent through amplifier A3.
Total gain = 5V/V for MAX4080F, 20V/V for the
MAX4080T, and 60V/V for the MAX4080S.
The MAX4081 input stage differs slightly from the
MAX4080 (Figure 3). Its topology allows for monitoring
of bidirectional currents through the sense resistor.
When current flows from RS+ to RS-, the MAX4081
matches the voltage drop across the external sense
resistor, RSENSE, by increasing the current through the
Q1 and RG1. In this way, the voltages at the input ter-
minals of the internal amplifier A1 are kept constant and
an accurate measurement of the sense voltage is
achieved. In the following amplifier stages of the
MAX4081, the output signal of amplifier A2 is level-
shifted to the reference voltage (VREF = VREF1A =
VREF1B), resulting in a voltage at the output pin (OUT)
76V, High-Side, Current-Sense Amplifiers with
Voltage Output
Pin Description
PIN
MAX4080
MAX4081
NAME
FUNCTION
1
RS+
Power connection to the external-sense resistor.
22
VCC
Supply Voltage Input. Decouple VCC to GND with at least a 0.1F capacitor to
bypass line transients.
3, 6, 7
3
N.C.
No Connection. No internal connection. Leave open or connect to ground.
4
GND
Ground
5
OUT
Voltage Output. For the unidirectional MAX4080, VOUT is proportional to
VSENSE. For the bidirectional MAX4081, the difference voltage (VOUT - VREF) is
proportional to VSENSE and indicates the correct polarity.
8
RS-
Load connection to the external sense resistor.
—
6
REF1B
Reference Voltage Input: Connect REF1B to REF1A or to GND (see the External
Reference section).
—
7
REF1A
Reference Voltage Input: Connect REF1A and REF1B to a fixed reference
voltage (VREF). VOUT is equal to VREF when VSENSE is zero (see the External
Reference section).
MAX4080/MAX4081
Maxim Integrated
9