Precision, High-Side
Current-Sense Amplifiers
MAX471/MAX472
RS+
RS+
RS-
RS-
POWER
SOURCE
OR
BATTERY
TO LOAD/
CHARGER
1 SHDN
MAX472
2
3
4
N.C.
RG1
GND
100k
OUT
V
CC
8
7
R
OUT
R
SENSE
TO LOAD/CHARGER
MAX471
SIGN
GND
OUT
LOGIC
SUPPLY
3V
TO
36V
RG1
RG2
LOGIC
SUPPLY
100k
3V
TO
36V
RS+
RS+
RS-
RS-
MAX471
SIGN
OUT
V
OUT
1k
RG2 6
SIGN 5
GND
Figure 3. Paralleling MAX471s to Sense Higher Load Current
Figure 4. MAX472 Standard Application Circuit
for current summing. A single scaling resistor is
required when summing OUT currents from multiple
devices (Figure 3). Current can be integrated by con-
necting OUT to a capacitive load.
Shutdown
When SHDN is high, the MAX471/MAX472 are shut
down and consume less than 18µA. In shutdown mode,
SIGN is high impedance and OUT turns off.
SIGN Output
The current at OUT indicates magnitude. The SIGN out-
put indicates the current’s direction. Operation of the
SIGN comparator is straightforward. When Q1 (Figures
1 and 2) conducts, the output of A1 is high while A2’s
output is zero. Under this condition, a high SIGN output
indicates positive current flow (from RS+ to RS-). In bat-
tery-operated systems, this is useful for determining
whether the battery is charging or discharging. The
SIGN output may not correctly indicate if the load cur-
rent is such that I
OUT
is less than 3.5µA. The MAX471’s
SIGN output accurately indicates the direction of cur-
rent flow for load currents greater than 7mA.
SIGN is an open-collector output (sinks current only),
allowing easy interface with logic circuits powered from
any voltage. Connect a 100kΩ pull-up resistor from
SIGN to the logic supply. The convention chosen for
the polarity of the SIGN output ensures that it draws no
current when the battery is being discharged. If current
direction is not needed, float the SIGN pin.
8
__________Applications Information
MAX471
The MAX471 obtains its power from the RS- pin. This
includes MAX471 current consumption in the total sys-
tem current measured by the MAX471. The small drop
across R
SENSE
does not affect the MAX471’s perfor-
mance.
Resistor Selection
Since OUT delivers a current, an external voltage gain-
setting resistor (R
OUT
to ground) is required at the OUT
pin in order to get a voltage. R
SENSE
is internal to the
MAX471. RG1 and RG2 are factory trimmed for an out-
put current ratio (output current to load current) of
500µA/A. Since they are manufactured of the same
material and in very close proximity on the chip, they
provide a high degree of temperature stability. Choose
R
OUT
for the desired full-scale output voltage up to RS-
- 1.5V (see the
Current Output
section).
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