MAX472ESA+ [MAXIM]
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| 型号： | MAX472ESA+ |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
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19-0335; Rev 2; 12/96
P re c is io n , Hig h -S id e
Cu rre n t -S e n s e Am p lifie rs
1/MAX472
_______________Ge n e ra l De s c rip t io n
____________________________Fe a t u re s
♦ Complete High-Side Current Sensing
♦ Precision Internal Sense Resistor (MAX471)
♦ 2% Accuracy Over Temperature
The MAX471/MAX472 are complete, bidirectional, high-
side current-sense amplifiers for portable PCs, tele-
p hone s , a nd othe r s ys te ms whe re b a tte ry/DC
power-line monitoring is critical. High-side power-line
monitoring is especially useful in battery-powered sys-
tems, since it does not interfere with the ground paths
of the b a tte ry c ha rg e rs or monitors ofte n found in
“smart” batteries.
♦ Monitors Both Charge and Discharge
♦ 3A Sense Capability with Internal Sense Resistor
(MAX471)
The MAX471 has an internal 35mΩ current-sense resis-
tor and measures battery currents up to ±3A. For appli-
cations requiring higher current or increased flexibility,
the MAX472 functions with external sense and gain-set-
ting resistors. Both devices have a current output that
can be converted to a ground-referred voltage with a
single resistor, allowing a wide range of battery volt-
ages and currents.
♦ Higher Current-Sense Capability with External
Sense Resistor (MAX472)
♦ 100µA Max Supply Current
♦ 18µA Max Shutdown Mode
♦ 3V to 36V Supply Operation
♦ 8-Pin DIP/SO Packages
An open-collector SIGN output indicates current-flow
direction, so the user can monitor whether a battery is
being charged or discharged. Both devices operate
from 3V to 36V, draw less than 100µA over tempera-
ture, and include a 18µA max shutdown mode.
______________Ord e rin g In fo rm a t io n
PART
TEMP. RANGE
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
8 Plastic DIP
8 SO
MAX471CPA
MAX471CSA
MAX471EPA
MAX471ESA
MAX472CPA
MAX472CSA
MAX472EPA
MAX472ESA
________________________Ap p lic a t io n s
Portable PCs:
8 Plastic DIP
8 SO
Notebooks/Subnotebooks/Palmtops
Smart Battery Packs
8 Plastic DIP
8 SO
Cellular Phones
8 Plastic DIP
8 SO
Portable Phones
Portable Test/Measurement Systems
Battery-Operated Systems
Energy Management Systems
__________Typ ic a l Op e ra t in g Circ u it
_________________P in Co n fig u ra t io n s
I
TO
RS+
RS+
RS-
RS-
TOP VIEW
LOAD
LOAD or CHARGER
LOGIC
SUPPLY
SHDN
RS+
1
2
3
4
8
7
6
5
OUT
RS-
100k
2k
3V
TO
36V
MAX471
MAX471
DISCHARGE/CHARGE
SIGN
OUT
RS+
RS-
V
OUT
(1V/A)
GND
SIGN
SHDN GND
I
LOAD
2000
DIP/SO
MAX472 Pin Configuration continued on last page.
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
P re c is io n , Hig h -S id e
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ABSOLUTE MAXIMUM RATINGS
Supply Voltage, RS+, RS-, V to GND....................-0.3V, +40V
CC
Continuous Power Dissipation (T = +70°C)
A
RMS Current, RS+ to RS- (MAX471 only)..........................±3.3A
Peak Current, (RS+ to RS-) ......................................see Figure 5
Differential Input Voltage, RG1 to RG2 (MAX472 only) .....±0.3V
Voltage at Any Pin Except SIGN
MAX471 (Note 1):
Plastic DIP (derate 17.5mW/°C above +70°C) ..................1.4W
SO (derate 9.9mW/°C above +70°C).............................791mW
MAX472 :
MAX471 only ...........................................-0.3V to (RS+ - 0.3V)
Plastic DIP (derate 9.09mW/°C above +70°C) ..............727mW
SO (derate 5.88mW/°C above +70°C)...........................471mW
Operating Temperature Ranges
MAX472 only ..........................................-0.3V to (V + 0.3V)
CC
Voltage at SIGN......................................................-0.3V to +40V
Current into SHDN, GND, OUT, RG1, RG2, V ................±50mA
Current into SIGN.................................................+10mA, -50mA
MAX47_C_A........................................................0°C to +70°C
MAX47_E_A .....................................................-40°C to +85°C
Junction Temperature Range ............................-60°C to +150°C
Storage Temperature Range .............................-60°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
CC
Note 1: Due to special packaging considerations, MAX471 (DIP, SO) has a higher power dissipation rating than the MAX472. RS+
and RS- must be soldered to large copper traces to achieve this dissipation rating.
1/MAX472
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS—MAX471
(RS+ = +3V to +36V, T = T
A
to T
, unless otherwise noted. Typical values are at T = +25°C.)
A
MAX
MIN
PARAMETER
Supply Voltage
SYMBOL
CONDITIONS
MIN
TYP
MAX
36
UNITS
V
V
RS+
3
Supply Current
Sense Current
I
I
= 0A, excludes I
SIGN
50
113
±3
µA
RS+
LOAD
I
A
RMS
LOAD
Sense Resistor
R
35
70
0.510
0.5125
2.5
mΩ
SENSE
MAX471C
0.490
0.4875
0.500
0.500
I
/
I
= 1A,
OUT
LOAD
Current-Sense Ratio
No-Load OUT Error
Low-Level OUT Error
mA/A
I
RS+ = 10V
MAX471E
MAX471C
MAX471E
MAX471C
MAX471E
= 1A
LOAD
I
= 0A,
LOAD
µA
RS+ = 10V
3.0
±2.5
±3.0
0.1
I
= 30mA,
LOAD
µA
RS+ = 10V
3V ≤ RS+ ≤ 36V, I
MAX471C
Power-Supply Rejection Ratio
PSRR
%/V
LOAD
±4.0
1.5
±6.0
±7.0
1.0
SIGN Threshold (I
to switch SIGN)
required
LOAD
mA
MAX471E
SIGN Output Leakage Current
SIGN Sink Current
V
36V
µA
mA
µA
V
SIGN =
I
OL
V
= 0.3V
0.1
2.4
SIGN
Shutdown Supply Current
SHDN Input Low Voltage
SHDN Input Low Current
SHDN Input High Voltage
SHDN Input High Current
OUT Output Voltage Range
OUT Output Resistance
I
V
= 2.4V; V = 3V to 20V
CC
18.0
0.3
RS+(SHDN)
SHDN
V
IL
I
IL
V
= 0V
1.0
µA
V
SHDN
V
IH
I
IH
V
= 2.4V
1.0
µA
V
SHDN
V
0
1
V
- 1.5
OUT
OUT
RS+
R
I
= 3.0A, V
= 0V to (V - 1.5V)
RS+
3
4
MΩ
LOAD
OUT
I
= 50mA to 3.0A, R
= 2kΩ,
LOAD
OUT
OUT Rise, Fall Time
t , t
µs
µs
R
F
C
= 50pF, 10% to 90%
OUT
OUT Settling Time to 1%
of Final Value
I
= 100mA to 3.0A, R = 2kΩ,
OUT
LOAD
t
s
15
C
= 50pF
OUT
2
_______________________________________________________________________________________
P re c is io n , Hig h -S id e
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1/MAX472
ELECTRICAL CHARACTERISTICS—MAX472
(V = +3V to +36V, RG1 = RG2 = 200Ω, T = T
to T
, unless otherwise noted. Typical values are at T = +25°C.)
CC
A
MIN
MAX
A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
36
UNITS
V
Supply Voltage
Supply Current
V
CC
3
I
CC
I
= 0A, excludes I ; V = 3V to 20V
SIGN CC
20
48
µA
LOAD
MAX472C
MAX472E
120
140
35
Input Offset Voltage
(Note 2)
V
µV
OS
Input Bias Current
I
, I
20
µA
µA
RG1 RG2
Input Bias-Current Matching
I
OS
I
- I
±0.4
±3.0
±2
RG1 GR2
MAX472C
MAX472E
MAX472C
MAX472E
MAX472C
MAX472E
= 100mV
MAX472C
MAX472E
V
SENSE
= 100mV,
OUT Current Accuracy
No-Load OUT Error
I
/I
%
RG OUT
V
CC
= 10V (Note 3)
±2.5
2.5
V
CC
= 10V,
µA
V
SENSE
= 0V
3
±2.5
±3.0
0.1
V
CC
= 10V,
Low-Level OUT Error
µA
%/V
µV
V
SENSE
= 3mV
Power-Supply Rejection Ratio
PSRR
3V ≤ V ≤ 36V, V
CC SENSE
60
60
120
140
1.0
SIGN Threshold (V
SENSE
V
CC
= 10V
required to switch SIGN)
SIGN Output Leakage Current
SIGN Output Sink Current
Shutdown Supply Current
SHDN Input Low Voltage
SHDN Input Low Current
SHDN Input High Voltage
SHDN Input High Current
OUT Output Voltage Range
OUT Output Resistance
V
SIGN
= 36V
µA
mA
µA
V
V
SIGN
= 0.3V
0.1
2.4
I
V
SHDN
= 2.4V; V = 3V to 20V
CC
1.5
18.0
0.3
CC(SHDN)
V
IL
I
IL
V
SHDN
= 0V
1.0
µA
V
V
IH
I
IH
V
SHDN
= 2.4V
1.0
µA
V
V
0
1
V
CC
- 1.5
OUT
OUT
R
I
= 1.5mA
3
4
MΩ
OUT
V
C
= 5mV to 150mV, R
= 50pF, 10% to 90%
= 2kΩ,
= 2kΩ,
SENSE
OUT
OUT Rise, Fall Time
t , t
R
µs
F
OUT
OUT Settling Time to 1%
of Final Value
V
SENSE
= 5mV to 150mV, R
OUT
t
15
µs
s
C
= 50pF
OUT
Maximum Output Current
I
1.5
mA
OUT
Note 2: V is defined as the input voltage (V
) required to give minimum I
.
OS
SENSE
OUT
Note 3: V
is the voltage across the sense resistor.
SENSE
_______________________________________________________________________________________
3
P re c is io n , Hig h -S id e
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__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(Typical Operating Circuit (MAX471) or circuit of Figure 4, RG1 = RG2 = 200Ω, R
= 2kΩ (MAX472), T = +25°C, unless
OUT
A
otherwise noted.)
SUPPLY CURRENT vs.
SUPPLY VOLTAGE
SHUTDOWN CURRENT vs.
SUPPLY VOLTAGE
SIGN THRESHOLD vs.
SUPPLY VOLTAGE
65
60
2.5
4
T
= -40°C
A
T
= +85°C
A
T
= -40°C
A
3
2
2.0
1.5
1.0
0.5
0
55
50
45
40
35
T
= +25°C
A
T
A
= +25°C
= +85°C
1
T
A
= +25°C
= +85°C
0
T
A
T
= -40°C
A
T
A
-1
-2
1/MAX472
3
6
9
12 15 18 21 24 27 30 33 36
(V)
3
6
9
12 15 18 21 24 27 30 33 36
(V)
3
6
9
12 15 18 21 24 27 30 33 36
(V)
V
V
RS+
V
RS+
RS+
MAX471
MAX471
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
MAX471
ERROR vs. LOAD CURRENT
NO-LOAD OFFSET CURRENT vs.
SUPPLY VOLTAGE
40
2.4
15
12
9
I
= 1A
VS+ = VS-
LOAD
2.2
2.0
1.8
35
30
25
20
15
10
5
I
FROM RS- TO RS+
LOAD
T
= -40°C
A
RS+
RS–
V = 0V TO 0.5V
V = 0V TO 1V
OUT
6
GND
A
5V
1µF
3
1.6
1.4
1.2
1.0
V
5Ω
T
= +85°C
0
A
-3
-6
-9
T
= +25°C
A
V = 0mV TO 50mV
I
FROM RS+ TO RS-
1
LOAD
0.8
0.6
-12
-15
0
0.01
0.10
10
0.01
0.10
1
10
100
1000
3
6
9
12 15 18 21 24 27 30 33 36
(V)
I
(A)
POWER-SUPPLY FREQUENCY (kHz)
V
RS+
LOAD
MAX471
MAX472
MAX472
ERROR vs. SUPPLY VOLTAGE
RS+ TO RS- RESISTANCE vs.
TEMPERATURE
NO-LOAD OUTPUT ERROR vs.
SUPPLY VOLTAGE
40
38
3.0
2.5
1.10
1.00
0.90
0.80
0.70
V
-V
= 60mV,
RG1 RG2
RG1 = RG2 = 0Ω
RG1 = RG2 = 200Ω
T
= +85°C
A
T
A
= +85°C
36
34
2.0
1.5
1.0
0.5
0
T
A
= +25°C
T
= +25°C
A
32
30
T
A
= -40°C
T
= -40°C
A
28
3
6
9
12 15 18 21 24 27 30 33 36
(V)
-40 -20
0
20
40
60
80
3
6
9
12 15 18 21 24 27 30 33 36
(V)
V
CC
TEMPERATURE (°C)
V
CC
4
_______________________________________________________________________________________
P re c is io n , Hig h -S id e
Cu rre n t -S e n s e Am p lifie rs
1/MAX472
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(Typical Operating Circuit (MAX471) or circuit of Figure 4, RG1 = RG2 = 200Ω, R
= 2kΩ (MAX472), T = +25°C, unless
OUT
A
otherwise noted.)
MAX472
ERROR vs. SENSE VOLTAGE
25
MAX471
NOISE vs. LOAD CURRENT
0.5
0.4
15
V
-V
RG1 RG2
0.3
5
0
0.2
0.1
-5
V
-V
RG2 RG1
-15
-25
0
1A
10mA
0.1
1
10
100
1000
1mA
100mA
SENSE
I
V
(mV)
SENSE
MAX471
MAX471
-100mA to +100mA TRANSIENT RESPONSE
0mA to 100mA TRANSIENT RESPONSE
LOAD
CURRENT
100mA/div
0A
LOAD
CURRENT
50mA/div
0A
V
OUT
50mV/div
V
OUT
50mV/div
SIGN
50mV/div
100µs/div
100µs/div
V
CC
= 10V, R = 2kΩ 1%, SIGN PULL-UP = 50kΩ 1%
OUT
V
CC
= 10V, R = 2kΩ 1%, SIGN PULL-UP = 50kΩ 1%
OUT
MAX471
MAX471
START-UP DELAY
0A TO 3A TRANSIENT RESPONSE
I
LOAD
1A/div
V
OUT
500mV/div
V
OUT
V
SHDN
10mV/div
5V/div
10µs/div
10µs/div
I
= 1A, R = 2kΩ 1%
R
OUT
= 2kΩ 1%
LOAD
OUT
_______________________________________________________________________________________
5
P re c is io n , Hig h -S id e
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______________________________________________________________P in De s c rip t io n
PIN
NAME
FUNCTION
MAX471
MAX472
Shutdown. Connect to ground for normal operation. When high, supply current is
less than 5µA.
1
1
SHDN
RS+
Battery (or power) side of the internal current-sense resistor. The “+” indicates direction of
flow for SIGN output only. Connect pins 2 and 3 together at the package.
2, 3
—
—
—
4
2
3
4
N.C.
RG1
GND
No Connect—no internal connection
Gain Resistor. Connect to battery side of current-sense resistor through the gain resistor.
Ground or Battery Negative Terminal
An open-collector logic output. For the MAX471, a low level indicates current is flowing from
5
5
SIGN
RS- to RS+. For the MAX472, a low level indicates a negative V
(see Figure 2). SIGN is
SENSE
1/MAX472
high impedance when SHDN is high. Leave open if SIGN is not needed.
Load side of the internal current-sense resistor. The “-” indicates direction of flow for SIGN
output only. Connect pins 6 and 7 together at the package.
6, 7
—
RS-
—
—
6
7
RG2
Gain Resistor. Connect to load side of current-sense resistor through the gain resistor.
V
CC
Power input for MAX472. Connect to sense resistor (R ) junction with RG1.
SENSE
Current output that is proportional to the magnitude of the sensed current flowing through
R . A 2kΩ resistor from this pin to ground will result in a voltage equal to 1V/Amp of
SENSE
8
8
OUT
sensed current in the MAX471.
I
= (I
x R
) / RG1
SENSE
OUT
LOAD
_______________De t a ile d De s c rip t io n
The MAX471 and MAX472 current-sense amplifier’s
unique topology allows a simple design to accurately
monitor current flow. The MAX471/MAX472 contain two
amplifiers operating as shown in Figures 1 and 2. The
battery/load current flows from RS+ to RS- (or vice
Cu rre n t Ou t p u t
The output voltage equation for the MAX471/MAX472 is
given below. In the MAX471, the current-gain ratio has
b e e n p re s e t to 500µA/A s o tha t a n outp ut re s is tor
) of 2kΩ yields 1V/A for a full-scale value of +3V
at ±3A. Other full-scale voltages can be set with differ-
ent R values, but the output voltage can be no
(R
OUT
versa) through R
. Current flows through either
SENSE
RG1 and Q1 or RG2 and Q2, depending on the sense-
resistor current direction. Internal circuitry, not shown in
Figures 1 and 2, prevents Q1 and Q2 from turning on at
the s a me time . The MAX472 is id e ntic a l to the
OUT
greater than V
for the MAX472.
- 1.5V for the MAX471 or V
- 1.5V
RG_
RS+
V
= (R
x R
x I
) / RG
OUT
SENSE
OUT LOAD
MAX471, except that R
and gain-setting resistors
SENSE
where V
= the desired full-scale output voltage,
OUT
RG1 and RG2 are external (Figure 2).
I
= the full-scale current being sensed, R
=
LOAD
SENSE
To analyze the circuit of Figure 1, assume that current
flows from RS+ to RS- and that OUT is connected to
GND through a resistor. In this case, amplifier A1 is
the current-sense resistor, R
= the voltage-setting
resistor, and RG = the gain-setting resistor (RG = RG1
= RG2).
OUT
active and output current I
flows from the emitter of
OUT
The above equation can be modified to determine the
Q1. Since no current flows through RG2 (Q2 is off), the
negative input of A1 is equal to V - (I
R
required for a particular full-scale range:
OUT
x
LOAD
SOURCE
R
). The open-loop gain of A1 forces its positive
SENSE
R
= (V
x RG) / (I
x R
)
SENSE
OUT
OUT
LOAD
input to essentially the same level as the negative input.
The re fore , the d rop a c ros s RG1 e q ua ls I
R
For the MAX471, this reduces to:
= V / (I
x
LOAD
R
x 500µA/A)
LOAD
OUT
OUT
. Then, since I
flows through Q1 and RG
SENSE
OUT
(ignoring the extremely low base currents), I
x RG1
OUT
OUT is a high-impedance current-source output that
can be connected to other MAX471/MAX472 OUT pins
= I
x R
, or:
SENSE
LOAD
6
_______________________________________________________________________________________
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1/MAX472
R
SENSE
6, 7
2, 3
RS+
RS-
RG1
RG2
A1
A2
Q1
Q2
8
5
OUT
SIGN
COMP
MAX471
Figure 1. MAX471 Functional Diagram
R
SENSE
POWER SOURCE
OR
TO LOAD/CHARGER
V
SENSE
BATTERY
RG1
RG2
3
6
A1
A2
7
V
CC
Q1
Q2
8
5
OUT
COMP
SIGN
MAX472
Figure 2. MAX472 Functional Diagram
_______________________________________________________________________________________
7
P re c is io n , Hig h -S id e
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R
SENSE
RS+
RS+
RS-
RS-
TO LOAD/CHARGER
POWER
SOURCE
OR
3V
TO
36V
MAX471
GND
SIGN
OUT
LOGIC
SUPPLY
RG2
RG1
BATTERY
LOGIC
SUPPLY
TO LOAD/
CHARGER
100k
3V
TO
36V
100k
MAX472
OUT
1
8
7
6
5
SHDN
N.C.
RS+
RS+
RS-
RS-
V
CC
2
3
4
R
OUT
RG2
RG1
MAX471
GND
SIGN
OUT
SIGN
GND
1/MAX472
V
OUT
1k
Figure 3. Paralleling MAX471s to Sense Higher Load Current
Figure 4. MAX472 Standard Application Circuit
for c urre nt s umming . A s ing le s c a ling re s is tor is
required when summing OUT currents from multiple
devices (Figure 3). Current can be integrated by con-
necting OUT to a capacitive load.
S h u t d o w n
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.
S IGN Ou t p u t
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-
__________Ap p lic a t io n s In fo rm a t io n
MAX4 7 1
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
mance.
does not affect the MAX471’s perfor-
SENSE
Resistor Selection
Since OUT delivers a current, an external voltage gain-
rent is such that I
is less than 3.5µA. The MAX471’s
OUT
setting resistor (R to ground) is required at the OUT
OUT
SIGN output accurately indicates the direction of cur-
rent flow for load currents greater than 7mA.
pin in order to get a voltage. R
is internal to the
SENSE
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
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.
R
for the desired full-scale output voltage up to RS-
OUT
- 1.5V (see the Current Output section).
8
_______________________________________________________________________________________
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1/MAX472
Peak Sense Current
The MAX471’s maximum sense current is 3ARMS. For
power-up, fault conditions, or other infrequent events,
larger peak currents are allowed, provided they are
short—that is, within a safe operating region, as shown
in Figure 5.
Table 1 shows suggested component values and indi-
cates the resulting scale factors for various applications
required to sense currents from 100mA to 10A.
Higher or lower sense-current circuits can also be built.
Select components and calculate circuit errors using
the guidelines and formulas in the following section.
50
R
SENSE
Small
Outline
fuse
DIP
45
Choose R
based on the following criteria:
SENSE
fuse
time
T = +25°C
A
40
35
30
25
20
15
10
5
a) Voltage Loss: A high R
value will cause the
power-source voltage to degrade through IR loss.
SENSE
time
For least voltage loss, use the lowest R value.
SENSE
b) Accuracy: A hig h R
va lue a llows lowe r
SENSE
currents to be measured more accurately. This is
because offsets become less significant when the
sense voltage is larger.
c) Efficiency and Power Dissipation: At high current
levels, the I2R losses in R
may be significant.
SENSE
0
Ta ke this into c ons id e ra tion whe n c hoos ing the
resistor value and power dissipation (wattage) rat-
ing. Also, if the sense resistor is allowed to heat up
excessively, its value may drift.
10µ
100µ
PULSE WIDTH (sec)
DIP safe
Small Outline safe
1m
10m
operating region
operating region
d) Inductance: If there is a large high-frequency com-
Figure 5. MAX471 Pulse Current Safe Operation for 10,000
Pulses and Fuse Time for Continuous Current. Pulse tests done
with 250mW average power dissipation.
ponent to I , you will want to keep inductance
SENSE
low. Wire-wound resistors have the highest induc-
tance, while metal film is somewhat better. Low-
inductance metal-film resistors are available. Instead
of being spiral wrapped around a core, as in metal-
film or wire-wound resistors, these are a straight
band of metal. They are made in values under 1Ω.
MAX4 7 2
R
, RG1, and RG2 are externally connected on
SENSE
the MAX472. V
c a n b e c onne c te d to e ithe r the
CC
load/charge or power-source/battery side of the sense
re s is tor. Conne c t V to the loa d /c ha rg e s id e of
e) Cost: If the cost of R
becomes an issue, you
SENSE
CC
may want to use an alternative solution, as shown in
Figure 6. This solution uses the PC board traces to
create a sense resistor. Because of the inaccuracies
of the copper “resistor,” you will need to adjust the
full-scale current value with a potentiometer. Also,
the resistance temperature coefficient of copper is
fairly high (approximately 0.4%/°C), so systems that
experience a wide temperature variance should take
this into account.
R
if you want to include the MAX472 current drain
SENSE
in the measured current.
Suggested Component Values
for Various Applications
The general circuit of Figure 4 is useful in a wide variety
of applications. It can be used for high-current applica-
tions (greater than 3A), and also for those where the full-
scale load current is less than the 3A of the MAX471.
Table 1. Suggested Component Values for the MAX472
CURRENT-
SENSE
RESISTOR,
SCALE
FACTOR,
FULL-SCALE
LOAD
CURRENT,
GAIN-SETTING
RESISTORS,
RG1 = RG2
(Ω)
OUTPUT
RESISTOR,
FULL-SCALE
OUTPUT
VOLTAGE,
TYPICAL ERROR AT X%
OF FULL LOAD (%)
V
/I
OUT SENSE
R
OUT
R
(V/A)
SENSE
I
(A)
(kΩ)
V
OUT
(V)
SENSE
1%
14
14
13
12
10%
2.5
100%
0.9
(mΩ)
500
50
0.1
1
200
200
100
50
10
10
5
2.5
2.5
2.5
2
25
2.5
0.5
0.2
2.5
0.9
5
10
2.0
1.1
10
5
2
2.0
1.6
_______________________________________________________________________________________
9
P re c is io n , Hig h -S id e
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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
TO LOAD/CHARGER
0.3" COPPER
R
SENSE
0.3" COPPER
3V
0.1" COPPER
10A you may want R
= 5mΩ for a 50mV drop at
SENSE
full scale. This resistor will require about 2 inches of 0.1
inch wide copper trace.
POWER
SOURCE
OR
RG1
RG2
TO
36V
BATTERY
RG1 and RG2
Once R
is chosen, RG1 and RG2 can be chosen
SENSE
to define the current-gain ratio (R
/RG). Choose
SENSE
RG = RG1 = RG2 based on the following criteria:
MAX472
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
OUT
1
2
3
4
8
7
6
5
SHDN
N.C.
V
CC
1.5k
1k
RG2
RG1
1/MAX472
SIGN
GND
removed by adjusting RG or R
.
OUT
Figure 6. MAX472 Connections Showing Use of PC Board
Trace
b) Efficiency. As RG is reduced, I
gets larger for a
OUT
given load current. Power dissipated in R
is not
OUT
going to the load, and therefore reduces overall effi-
ciency. This is significant only when the sense cur-
rent is small.
make sure RG is small enough that I and I
not add any appreciable errors. The full-scale error
is given by:
do
B
OS
c) Maximum Output Current Limitation. I
is limit-
OUT
(RG1 - RG2) x I + I x RG
B
OS
x R
SENSE
% Error =
x 100
ed to 1.5mA, requiring RG ≥ V
/ 1.5mA. For
SENSE
I
FS
V
SENSE
= 60mV, RG must be ≥ 40Ω.
where RG1 and RG2 are the gain resistors, I is the
B
d) Headroom. The MAX472 requires a minimum of
bias current, I is the bias-current mismatch, I is the
OS
FS
1.5V between the lower of the voltage at RG1 or
full-scale current, and R
is the sense resistor.
SENSE
RG2 (V
) and V . As RG becomes larger, the
OUT
RG_
voltage drop across RG also becomes larger for a
given I . This voltage drop further limits the maxi-
Assuming a 5A load current, 10mΩ R
, and 100Ω
SENSE
RG, the current-gain ratio is 100µA/A, yielding a full-
scale I of 500µA. Using the maximum values for I
OUT
mum full-scale V . Assuming the drop across
OUT
OUT
B
R
is small and V is connected to either side
(20µA) and I
(2µA), and 1% resistors for RG1 and
SENSE
CC
OS
of R
, V
(max) = V - (1.5V + I
(max) x
RG2 (RG1 - RG2 = 2Ω), the worst-case error at full
SENSE OUT
CC
OUT
RG).
e) Output Offset Error at Low Load Currents. Large
RG values reduce I for a given load current. As
scale calculates to:
2Ω x 20µA + 100Ω x 2µA
= 0.48%
OUT
5mΩ x 5A
I
gets smaller, the 2.5µA max output offset-error
OUT
The error may be reduced by: a) better matching of
RG1 and RG2, b) increasing R
RG.
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.
, or c) decreasing
SENSE
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
Current-Sense Adjustment
(Resistor Range, Output Adjust)
Choose R
Choos e R
after selecting R
, RG1, and RG2.
OUT
OUT
SENSE
to ob ta in the full-s c a le volta g e you
Note 4: Printed Circuit Design, by Gerald L. Ginsberg; McGraw-Hill, Inc.; page 185.
10 ______________________________________________________________________________________
P re c is io n , Hig h -S id e
Cu rre n t -S e n s e Am p lifie rs
1/MAX472
re q uire , g ive n the full-s c a le I
d e te rmine d b y
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-
OUT
R
, RG1, and RG2. The high compliance of OUT
SENSE
permits using R
values up to 10kΩ with minimal
OUT
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
tor at the RS- terminal (or “load” side of the MAX472) to
decouple the load and, thereby, reduce the current
tra ns ie nts . The s e c a p a c itors a re not re q uire d for
MAX471/MAX472 operation or stability, and their use
will not degrade performance.
OUT
(e.g., 100 x R
) to avoid degrading the measure-
OUT
ment accuracy.
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.
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.
An alternative method is to connect several MAX471s in
p a ra lle l a nd c onne c t the hig h-imp e d a nc e c urre nt-
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.
MAX4 7 1 La yo u t
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.
P o w e r-S u p p ly Byp a s s in g a n d Gro u n d in g
The MAX471 has been designed as a “high side” (posi-
tive te rmina l) c urre nt monitor to e a s e the ta s k of
grounding any battery charger, thermistor, etc. that
ma y b e a p a rt of the b a tte ry p a c k. Ground ing 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 . For highest current-mea-
OUT
surement accuracy, use a single-point “star” ground.
______________________________________________________________________________________ 11
P re c is io n , Hig h -S id e
Cu rre n t -S e n s e Am p lifie rs
____P in Co n fig u ra t io n s (c o n t in u e d )
SHDN
N.C.
1
2
3
4
8
7
6
5
OUT
V
CC
MAX472
RG1
RG2
GND
SIGN
DIP/SO
1/MAX472
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 __________________Ma x im In t e g ra t e d P ro d u c t s , 1 2 0 S a n Ga b rie l Drive , S u n n yva le , CA 9 4 0 8 6 (4 0 8 ) 7 3 7 -7 6 0 0
© 1996 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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