Low-Cost, SOT23, Voltage-Output,

High-Side Current-Sense Amplifier

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Detailed Description

The MAX4173 high-side current-sense amplifier fea-

tures 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

The MAX4173 operates as follows: Current from the

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

gain forces its noninverting input to the same voltage as

the inverting input. Therefore, the drop across RG1

β, to give

MAX4173F, and GAIN = 100 for MAX4173H.

the appropriate gain version of the MAX4173.

Applications Information

Recommended Component Values

The MAX4173 senses a wide variety of currents with

different sense resistor values. Table 1 lists common

resistor values for typical operation of the MAX4173.

To measure lower currents more accurately, use a high

sense voltage that reduces offset voltage errors of the

internal op amp.

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.

have the highest inductance, metal-film resistors are

somewhat better, and low-inductance metal-film resis-

tors are best suited for these applications.

critical, use the alternative solution shown in Figure 2.

This solution uses copper PC board traces to create a

sense resistor. The resistivity of a 0.1-inch-wide trace of

2-ounce copper is approximately 30m

Ω/ft. The resis-

tance-temperature coefficient of copper is fairly high

(approximately 0.4%/°C), so systems that experience a

wide temperature variance must compensate for this

effect. In addition, do not exceed the maximum power

dissipation of the copper trace.

For example, the MAX4173T (with a maximum load cur-

Ω) creates a full-scale

wide copper trace.

Output Impedance

The output of the MAX4173 is a current source driving a

12k

Ω resistance. Resistive loading added to OUT

reduces the output gain of the MAX4173. To minimize

output errors for most applications, connect OUT to a

high-impedance input stage. When output buffering is

required, choose an op amp with a common-mode

input range and an output voltage swing that includes

ground when operating with a single supply. The op

0 TO +28V

+3V TO +28V

RGD = 12k

RS-

RS+

OUT

GND

TO LOAD BATTERY

CURRENT

MIRROR

A1

MAX4173

Figure 1. Functional Diagram