Precision, High-Side
Current-Sense Amplifiers
MAX471/MAX472
In Figure 6, assume the load current to be measured 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 copper is 30mΩ/ft (see Note 4). For
10A you may want R
SENSE
= 5mΩ for a 50mV drop at
full scale. This resistor will require about 2 inches of 0.1
inch wide copper trace.
R
SENSE
0.3" COPPER
0.1" COPPER
TO LOAD/CHARGER
0.3" COPPER
RG1 and RG2
Once R
SENSE
is chosen, RG1 and RG2 can be chosen
to define the current-gain ratio (R
SENSE
/RG). Choose
RG = RG1 = RG2 based on the following criteria:
a)
1Ω Input Resistance.
The minimum RG value is lim-
ited by the 1Ω input resistance, and also by the out-
put current limitation (see below). As RG is reduced,
the input resistance becomes a larger portion of the
total gain-setting resistance. With RG = 50Ω, the
input resistance produces a 2% difference between
the expected and actual current-gain ratio. This is a
gain error that does not affect linearity and can be
removed by adjusting RG or R
OUT
.
b)
Efficiency.
As RG is reduced, I
OUT
gets larger for a
given load current. Power dissipated in R
OUT
is not
going to the load, and therefore reduces overall effi-
ciency. This is significant only when the sense cur-
rent is small.
c)
Maximum Output Current Limitation.
I
OUT
is limit-
ed to 1.5mA, requiring RG
≥
V
SENSE
/ 1.5mA. For
V
SENSE
= 60mV, RG must be
≥
40Ω.
d)
Headroom.
The MAX472 requires a minimum of
1.5V between the lower of the voltage at RG1 or
RG2 (V
RG_
) and V
OUT
. As RG becomes larger, the
voltage drop across RG also becomes larger for a
given I
OUT
. This voltage drop further limits the maxi-
mum full-scale V
OUT
. Assuming the drop across
R
SENSE
is small and V
CC
is connected to either side
of R
SENSE
, V
OUT
(max) = V
CC
- (1.5V + I
OUT
(max) x
RG).
e)
Output Offset Error at Low Load Currents.
Large
RG values reduce I
OUT
for a given load current. As
I
OUT
gets smaller, the 2.5µA max output offset-error
current becomes a larger part of the overall output
current. Keeping the gain high by choosing a low
value for RG minimizes this offset error.
f)
Input Bias Current and Input Bias Current
Mismatching.
The size of RG also affects the errors
introduced by the input bias and input bias mis-
matching currents. After selecting the ratio, check to
POWER
SOURCE
OR
BATTERY
3V
TO
36V
RG1
RG2
1
2
3
4
SHDN
N.C.
RG1
GND
MAX472
OUT
V
CC
8
7
1.5k
RG2 6
SIGN 5
1k
Figure 6. MAX472 Connections Showing Use of PC Board
Trace
make sure RG is small enough that I
B
and I
OS
do
not add any appreciable errors. The full-scale error
is given by:
% Error = (RG1 - RG2) x I
B
+ I
OS
x RG x 100
I
FS
x R
SENSE
where RG1 and RG2 are the gain resistors, I
B
is the
bias current, I
OS
is the bias-current mismatch, I
FS
is the
full-scale current, and R
SENSE
is the sense resistor.
Assuming a 5A load current, 10mΩ R
SENSE
, and 100Ω
RG, the current-gain ratio is 100µA/A, yielding a full-
scale I
OUT
of 500µA. Using the maximum values for I
B
(20µA) and I
OS
(2µA), and 1% resistors for RG1 and
RG2 (RG1 - RG2 = 2Ω), the worst-case error at full
scale calculates to:
2Ω x 20µA + 100Ω x 2µA
= 0.48%
5mΩ x 5A
The error may be reduced by: a) better matching of
RG1 and RG2, b) increasing R
SENSE
, or c) decreasing
RG.
Current-Sense Adjustment
(Resistor Range, Output Adjust)
Choose R
OUT
after selecting R
SENSE
, RG1, and RG2.
Choose R
OUT
to obtain the full-scale voltage you
Note 4:
Printed Circuit Design, by Gerald L. Ginsberg; McGraw-Hill, Inc.; page 185.
10
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