Datasheet

Sense Resistor, R
SENSE
Choose R
SENSE
based on the following criteria:
● Voltage Loss: A high R
SENSE
value causes the
power-source voltage to degrade through IR loss. For
minimal voltage loss, use the lowest R
SENSE
value.
● 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. 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 resistors
are also available. Instead of being spiral wrapped
around a core, as in metal-film or wirewound 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 resistance temperature
coefficient is fairly high (approximately 0.4%/°C).
In Figure 2, assume that the load current to be measured
is 10A, and that you have determined a 0.3-inchwide,
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
impedance 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 paralleled
to further increase the current-sense limit. As an alternative,
PCB traces can be adjusted over a wide range.
Figure 2. MAX4172 Connections Showing Use of PC Board
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
FULL-SCALE
LOAD CURRENT
(A)
CURRENT-SENSE
RESISTOR,
RSENSE (mΩ)
OUTPUT
RESISTOR, ROUT
(kΩ)
FULL-SCALE
OUTPUT
VOLTAGE, V
OUT
(V)
SCALE FACTOR,
V
OUT
/ISENSE (V/A)
0.1 1000 3.48 3.48 34.8
1 100 3.48 3.48 3.48
5 20 3.48 3.48 0.696
10 10 3.48 3.48 .348
MAX4172 Low-Cost, Precision, High-Side
Current-Sense Amplier
www.maximintegrated.com
Maxim Integrated │ 7