Precision, High-Side
Current-Sense Amplifiers
require, given the full-scale I
OUT
determined by
R
SENSE
, RG1, and RG2. The high compliance of OUT
permits using R
OUT
values up to 10kΩ with minimal
error. Values above 10kΩ are not usually recommend-
ed. The impedance of OUT’s load (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 the measure-
ment accuracy.
The MAX471/MAX472 require no special bypassing,
and respond quickly to transient changes in line cur-
rent. If the noise at OUT caused by these transients is a
problem, you may want to place a 1µF capacitor at the
OUT pin to ground. You can also place a large capaci-
tor at the RS- terminal (or “load” side of the MAX472) to
decouple the load and, thereby, reduce the current
transients. These capacitors are not required for
MAX471/MAX472 operation or stability, and their use
will not degrade performance.
For the MAX472, the RG1 and RG2 inputs can be fil-
tered by placing a capacitor (e.g., 1µF) between them
to average the sensed current.
MAX471/MAX472
High-Current Measurement
The MAX472 can achieve higher current measurements
than the MAX471 can. Low-value sense resistors may
be paralleled to obtain even lower values, or the PC
board trace may be adjusted for any value.
An alternative method is to connect several MAX471s in
parallel and connect the high-impedance current-
source OUT pins together to indicate the total system
current (Figure 3). Pay attention to layout to ensure
equal IR drops in the paralleled connection. This is
necessary to achieve equal current sharing.
MAX471 Layout
The MAX471 must be soldered in place, since sockets
can cause uneven current sharing between the RS+
pins (pins 2 and 3) and the RS- pins (pins 6 and 7),
resulting in typical errors of 0.5%.
In order to dissipate sense-resistor heat from large
sense currents, solder the RS+ pins and the RS- pins to
large copper traces. Keep the part away from other
heat-generating devices. This procedure will ensure
continuous power dissipation rating.
Power-Supply Bypassing and Grounding
The MAX471 has been designed as a “high side” (posi-
tive terminal) current monitor to ease the task of
grounding any battery charger, thermistor, etc. that
may be a part of the battery pack. Grounding the
MAX471 requires no special precautions; follow the
same cautionary steps that apply to the system as a
whole. High-current systems can experience large volt-
age drops across a ground plane, and this drop may
add to or subtract from V
OUT
. For highest current-mea-
surement accuracy, use a single-point “star” ground.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
