Datasheet

MAX4172
Low-Cost, Precision, High-Side
Current-Sense Amplifier
7
Maxim Integrated
• Accuracy: A high R
SENSE
value allows lower cur-
rents to be measured more accurately. This is
because offsets become less significant when the
sense voltage is larger. For best performance,
select R
SENSE
to provide approximately 100mV of
sense voltage for the full-scale current in each
application.
• Efficiency and Power Dissipation: At high current
levels, the I
2
R losses in R
SENSE
can be significant.
Take this into consideration when choosing the
resistor value and its power dissipation (wattage)
rating. Also, the sense resistor’s value might drift if it
is allowed to heat up excessively.
• Inductance: Keep inductance low if I
SENSE
has a
large high-frequency component. Wire-wound resis-
tors have the highest inductance, while metal film is
somewhat better. Low-inductance metal-film resis-
tors are also available. Instead of being spiral
wrapped around a core, as in metal-film or wire-
wound resistors, they are a straight band of metal
and are available in values under 1Ω.
• Cost: If the cost of R
SENSE
is an issue, you might
want to use an alternative solution, as shown in
Figure 2. This solution uses the PCB traces to create
a sense resistor. Because of the inaccuracies of the
copper resistor, the full-scale current value must be
adjusted with a potentiometer. Also, copper’s resis-
tance temperature coefficient is fairly high (approxi-
mately 0.4%/°C).
In Figure 2, assume that the load current to be mea-
sured is 10A, and that you have determined a 0.3-inch-
wide, 2-ounce copper to be appropriate. The resistivity
of 0.1-inch-wide, 2-ounce (70µm thickness) copper is
30mΩ/ft. For 10A, you might want R
SENSE
= 5mΩ for a
50mV drop at full scale. This resistor requires about 2
inches of 0.1-inch-wide copper trace.
Current-Sense Adjustment
(Resistor Range, Output Adjust)
Choose R
OUT
after selecting R
SENSE
. Choose R
OUT
to
obtain the full-scale voltage you require, given the full-
scale I
OUT
determined by R
SENSE
. OUT’s high imped-
ance permits using R
OUT
values up to 200kΩ with
minimal error. OUT’s load impedance (e.g., the input of
an op amp or ADC) must be much greater than R
OUT
(e.g., 100 x R
OUT
) to avoid degrading measurement
accuracy.
High-Current Measurement
The MAX4172 can achieve high-current measurements
by using low-value sense resistors, which can be paral-
leled to further increase the current-sense limit. As an
alternative, PCB traces can be adjusted over a wide
range.
3.48 3.48 3.481001
3.48
3.48
OUTPUT
RESISTOR, R
OUT
(kΩ)
3.48 0.3481010
3.48 3.48
3.48
FULL-SCALE
OUTPUT
VOLTAGE, V
OUT
(V)
0.696205
34.8
FULL-SCALE
LOAD CURRENT
(A)
SCALE FACTOR,
V
OUT
/I
SENSE
(V/A)
10000.1
CURRENT-SENSE
RESISTOR,
R
SENSE
(mΩ)
Table 1. Suggested Component Values
OUT
GND
1
O.1 in. COPPER
V
SENSE
R
SENSE
INPUT
LOAD/BATTERY
V
SUPPLY
R
OUT
O.3 in. COPPER O.3 in. COPPER
2
8
RS+
RS-
V+
3V TO 32V
MAX4172
6
5
Figure 2. MAX4172 Connections Showing Use of PC Board