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MAX4081SASA

76V, High-Side, Current-Sense Amplifiers with Voltage Output

Maxim Integrated 

76V, High-Side, Current-Sense Amplifiers with

Voltage Output

PIN

MAX4080 MAX4081

1

1

2

2

3, 6, 7

3

4

4

5

5

8

8

—

6

—

7

NAME

RS+

VCC

N.C.

GND

OUT

RS-

REF1B

REF1A

Pin Description

FUNCTION

Power connection to the external-sense resistor.

Supply Voltage Input. Decouple VCC to GND with at least a 0.1µF capacitor to

bypass line transients.

No Connection. No internal connection. Leave open or connect to ground.

Ground

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.

Load connection to the external sense resistor.

Reference Voltage Input: Connect REF1B to REF1A or to GND (see the External

Reference section).

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).

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)

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