MAX471CSA+T Maxim Integrated, MAX471CSA+T Datasheet - Page 11

• Manufacturer Part Number: MAX471CSA+T
• Description: Special Purpose Amplifiers
• Manufacturer: Maxim Integrated
• Series: MAX471, MAX472r

Specifications of MAX471CSA+T

• Rohs: yes
• Number Of Channels: Single
• Operating Supply Voltage: 5 V, 9 V, 12 V, 15 V
• Supply Current: 0.05 mA
• Maximum Power Dissipation: 791 mW
• Maximum Operating Temperature: + 70 C
• Minimum Operating Temperature: 0 C
• Supply Voltage - Max: 36 V
• Supply Voltage - Min: 3 V

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