MAX4173FESA datasheet(7/10 Pages) MAXIM

7 / 10 page

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

greater than the supply voltage (VCC).

The MAX4173 operates 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

multiplies IRG1 by a current gain factor,

β, to give

IRGD =

β · IRG1. Solving IRGD = β · (ILOAD)(RSENSE) /

RG1. Assuming infinite output impedance, VOUT = (IRGD)

(RGD). Substituting in for IRGD and rearranging, VOUT =

β · (RGD / RG1)(RSENSE · ILOAD). The parts gain equals

β · RGD / RG1. Therefore, VOUT = (GAIN) (RSENSE)

(ILOAD), where GAIN = 20 for MAX4173T, GAIN = 50 for

MAX4173F, and GAIN = 100 for MAX4173H.

Set the full-scale output range by selecting RSENSE and

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.

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 maxi-

mum ratings.

If ISENSE has a large high-frequency component, mini-

mize the inductance of RSENSE. 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.

Using a PCB Trace as RSENSE

If the cost of RSENSE is an issue and accuracy is not

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-

rent of 10A and an RSENSE of 5m

Ω) creates a full-scale

VSENSE of 50mV that yields a maximum VOUT of 1V.

RSENSE in this case requires about 2 inches of 0.1 inch-

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

RSENSE

VSOURCE

0 TO +28V

+3V TO +28V

RGD = 12k

VOUT

IRG1

IRGD

ILOAD

RG1

RG2

RS-

RS+

OUT

GND

TO LOAD BATTERY

VCC

CURRENT

MIRROR

MAX4173

Figure 1. Functional Diagram

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