Precision, High-Side
Current-Sense Amplifiers
Peak Sense Current
The MAX471’s maximum sense current is 3A
RMS
. For
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.
50
45
40
SENSE CURRENT (A)
35
30
25
20
15
10
5
0
10µ
100µ
1m
10m
PULSE WIDTH (sec)
DIP safe
operating region
Small Outline safe
operating region
T
A
= +25°C
Small
DIP
Outline fuse
fuse
time
time
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.
MAX471/MAX472
R
SENSE
Choose R
SENSE
based on the following criteria:
a)
Voltage Loss:
A high R
SENSE
value will cause the
power-source voltage to degrade through IR loss.
For least voltage loss, use the lowest R
SENSE
value.
b)
Accuracy:
A high R
SENSE
value allows lower
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
levels, the I
2
R losses in R
SENSE
may be significant.
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-
ponent to I
SENSE
, you will want to keep inductance
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 1Ω.
e)
Cost:
If the cost of R
SENSE
becomes an issue, you
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.
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.
MAX472
R
SENSE
, RG1, and RG2 are externally connected on
the MAX472. V
CC
can be connected to either the
load/charge or power-source/battery side of the sense
resistor. Connect V
CC
to the load/charge side of
R
SENSE
if you want to include the MAX472 current drain
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. Suggested Component Values for the MAX472
FULL-SCALE
LOAD
CURRENT,
I
SENSE
(A)
0.1
1
5
10
CURRENT-
SENSE
RESISTOR,
R
SENSE
(mΩ)
500
50
10
5
GAIN-SETTING
RESISTORS,
RG1 = RG2
(Ω)
200
200
100
50
OUTPUT
RESISTOR,
R
OUT
(kΩ)
10
10
5
2
FULL-SCALE
OUTPUT
VOLTAGE,
V
OUT
(V)
2.5
2.5
2.5
2
SCALE
FACTOR,
V
OUT
/I
SENSE
(V/A)
25
2.5
0.5
0.2
TYPICAL ERROR AT X%
OF FULL LOAD (%)
1%
14
14
13
12
10%
2.5
2.5
2.0
2.0
100%
0.9
0.9
1.1
1.6
9
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
