Peak Sense Current

power-up, fault conditions, or other infrequent events,

larger peak currents are allowed, provided they are

short—that is, within a safe operating region, as shown

in Figure 5.

MAX472

load/charge or power-source/battery side of the sense

in the measured current.

Suggested Component Values

for Various Applications

The general circuit of Figure 4 is useful in a wide variety

of applications. It can be used for high-current applica-

tions (greater than 3A), and also for those where the full-

scale load current is less than the 3A of the MAX471.

Table 1 shows suggested component values and indi-

cates the resulting scale factors for various applications

required to sense currents from 100mA to 10A.

Higher or lower sense-current circuits can also be built.

Select components and calculate circuit errors using

the guidelines and formulas in the following section.

power-source voltage to degrade through IR loss.

currents to be measured more accurately. This is

because offsets become less significant when the

sense voltage is larger.

c) Efficiency and Power Dissipation: At high current

Take this into consideration when choosing the

resistor value and power dissipation (wattage) rat-

ing. Also, if the sense resistor is allowed to heat up

excessively, its value may drift.

d) Inductance: If there is a large high-frequency com-

low. Wire-wound resistors have the highest induc-

tance, while metal film is somewhat better. Low-

inductance metal-film resistors are available. Instead

of being spiral wrapped around a core, as in metal-

film or wire-wound resistors, these are a straight

band of metal. They are made in values under 1

Ω.

may want to use an alternative solution, as shown in

Figure 6. This solution uses the PC board traces to

create a sense resistor. Because of the inaccuracies

of the copper “resistor,” you will need to adjust the

full-scale current value with a potentiometer. Also,

the resistance temperature coefficient of copper is

fairly high (approximately 0.4%/°C), so systems that

experience a wide temperature variance should take

this into account.

Precision, High-Side

Current-Sense Amplifiers

_______________________________________________________________________________________

9

Table 1. Suggested Component Values for the MAX472

FULL-SCALE

LOAD

CURRENT,

CURRENT-

SENSE

RESISTOR,

(m

Ω)

GAIN-SETTING

RESISTORS,

RG1 = RG2

(

Ω)

OUTPUT

RESISTOR,

(k

Ω)

FULL-SCALE

OUTPUT

VOLTAGE,

SCALE

FACTOR,

(V/A)

0.1

500

200

10

2.5

25

1

50

200

10

2.5

2.5

5

10

100

5

2.5

0.5

10

5

50

2

2

0.2

0.9

0.9

1.1

1.6

14

2.5

14

2.5

13

2.0

12

2.0

TYPICAL ERROR AT X%

OF FULL LOAD (%)

1%

100%

10%

50

45

0

10

µ

5

40

PULSE WIDTH (sec)

DIP safe

operating region

Small Outline safe

operating region

100

µ

1m

10m

30

35

15

20

25

10

DIP

fuse

time

Small

Outline

fuse

time

Figure 5. MAX471 Pulse Current Safe Operation for 10,000

Pulses and Fuse Time for Continuous Current. Pulse tests done

with 250mW average power dissipation.