MAXIM MAX471ESA

19-0335; Rev 2; 12/96
Precision, High-Side
Current-Sense Amplifiers
The MAX471/MAX472 are complete, bidirectional, highside current-sense amplifiers for portable PCs, telephones, and other systems where battery/DC
power-line monitoring is critical. High-side power-line
monitoring is especially useful in battery-powered systems, since it does not interfere with the ground paths
of the battery chargers or monitors often found in
“smart” batteries.
The MAX471 has an internal 35mΩ current-sense resistor and measures battery currents up to ±3A. For applications requiring higher current or increased flexibility,
the MAX472 functions with external sense and gain-setting 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 voltages and currents.
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 temperature, and include a 18µA max shutdown mode.
________________________Applications
Portable PCs:
Notebooks/Subnotebooks/Palmtops
Smart Battery Packs
Cellular Phones
Portable Phones
Portable Test/Measurement Systems
____________________________Features
♦ Complete High-Side Current Sensing
♦ Precision Internal Sense Resistor (MAX471)
♦ 2% Accuracy Over Temperature
♦ Monitors Both Charge and Discharge
♦ 3A Sense Capability with Internal Sense Resistor
(MAX471)
♦ 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
______________Ordering Information
TEMP. RANGE
PART
PIN-PACKAGE
MAX471CPA
0°C to +70°C
8 Plastic DIP
MAX471CSA
MAX471EPA
MAX471ESA
MAX472CPA
MAX472CSA
MAX472EPA
MAX472ESA
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
8 SO
8 Plastic DIP
8 SO
8 Plastic DIP
8 SO
8 Plastic DIP
8 SO
Battery-Operated Systems
Energy Management Systems
__________Typical Operating Circuit
RS+
RS-
RS+
RS-
ILOAD TO
LOAD or CHARGER
LOGIC
SUPPLY
100k
3V
TO
36V
MAX471
SHDN
GND
_________________Pin Configurations
TOP VIEW
SHDN
1
RS+ 2
SIGN
DISCHARGE/CHARGE
RS+ 3
OUT
VOUT (1V/A)
GND 4
ILOAD
2000
2k
MAX471
8
OUT
7
RS-
6
RS-
5
SIGN
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
MAX471/MAX472
_______________General Description
MAX471/MAX472
Precision, High-Side
Current-Sense Amplifiers
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, RS+, RS-, VCC to GND....................-0.3V, +40V
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 only ...........................................-0.3V to (RS+ - 0.3V)
MAX472 only ..........................................-0.3V to (VCC + 0.3V)
Voltage at SIGN......................................................-0.3V to +40V
Current into SHDN, GND, OUT, RG1, RG2, VCC ................±50mA
Current into SIGN.................................................+10mA, -50mA
Continuous Power Dissipation (TA = +70°C)
MAX471 (Note 1):
Plastic DIP (derate 17.5mW/°C above +70°C) ..................1.4W
SO (derate 9.9mW/°C above +70°C) .............................791mW
MAX472 :
Plastic DIP (derate 9.09mW/°C above +70°C) ..............727mW
SO (derate 5.88mW/°C above +70°C) ...........................471mW
Operating Temperature Ranges
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
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.
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, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
Supply Voltage
SYMBOL
Supply Current
IRS+
Sense Current
ILOAD
Sense Resistor
RSENSE
Current-Sense Ratio
IOUT/
ILOAD
No-Load OUT Error
Low-Level OUT Error
Power-Supply Rejection Ratio
PSRR
VIL
IIL
SHDN Input High Voltage
VIH
IIH
OUT Output Voltage Range
VOUT
OUT Output Resistance
ROUT
OUT Rise, Fall Time
tR, tF
2
MAX471C
ILOAD = 1A,
RS+ = 10V
MAX471E
MAX471C
ILOAD = 0A,
RS+ = 10V
MAX471E
MAX471C
ILOAD = 30mA,
RS+ = 10V
MAX471E
3V ≤ RS+ ≤ 36V, ILOAD = 1A
50
0.490
0.4875
35
0.500
0.500
±4.0
VSIGN = 0.3V
ts
MAX
UNITS
36
V
113
µA
±3
ARMS
70
0.510
0.5125
2.5
3.0
±2.5
±3.0
0.1
mΩ
±6.0
±7.0
1.0
0.1
IRS+(SHDN) VSHDN = 2.4V; VCC = 3V to 20V
SHDN Input Low Current
OUT Settling Time to 1%
of Final Value
ILOAD = 0A, excludes ISIGN
VSIGN = 36V
IOL
SHDN Input Low Voltage
SHDN Input High Current
TYP
3
MAX471E
SIGN Output Leakage Current
Shutdown Supply Current
MIN
MAX471C
SIGN Threshold (ILOAD required
to switch SIGN)
SIGN Sink Current
CONDITIONS
VRS+
1.5
18.0
%/V
mA
µA
µA
V
1.0
µA
V
VSHDN = 2.4V
1.0
0
ILOAD = 50mA to 3.0A, ROUT = 2kΩ,
COUT = 50pF, 10% to 90%
ILOAD = 100mA to 3.0A, ROUT = 2kΩ,
COUT = 50pF
µA
0.3
2.4
1
µA
mA
VSHDN = 0V
ILOAD = 3.0A, VOUT = 0V to (VRS+ - 1.5V)
mA/A
VRS+ - 1.5
µA
V
3
MΩ
4
µs
15
µs
_______________________________________________________________________________________
Precision, High-Side
Current-Sense Amplifiers
(VCC = +3V to +36V, RG1 = RG2 = 200Ω, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
Supply Voltage
VCC
Supply Current
ICC
Input Offset Voltage
(Note 2)
VOS
Input Bias Current
CONDITIONS
MIN
3
ILOAD = 0A, excludes ISIGN; VCC = 3V to 20V
20
IOS
MAX
V
48
µA
120
MAX472E
140
IRG1 - IGR2
20
35
µA
±3.0
µA
MAX472C
±2
MAX472E
±2.5
No-Load OUT Error
VCC = 10V,
VSENSE = 0V
MAX472C
2.5
MAX472E
3
Low-Level OUT Error
VCC = 10V,
VSENSE = 3mV
MAX472C
±2.5
MAX472E
±3.0
Power-Supply Rejection Ratio
IRG/IOUT
PSRR
3V ≤ VCC ≤ 36V, VSENSE = 100mV
SIGN Threshold (VSENSE
required to switch SIGN)
VCC = 10V
SIGN Output Leakage Current
VSIGN = 36V
SIGN Output Sink Current
VSIGN = 0.3V
Shutdown Supply Current
ICC(SHDN)
SHDN Input Low Voltage
VIL
SHDN Input Low Current
IIL
SHDN Input High Voltage
VIH
SHDN Input High Current
IIH
0.1
MAX472C
60
120
MAX472E
60
140
1.0
0.1
VSHDN = 2.4V; VCC = 3V to 20V
µV
±0.4
VSENSE = 100mV,
VCC = 10V (Note 3)
OUT Current Accuracy
UNITS
36
MAX472C
IRG1, IRG2
Input Bias-Current Matching
TYP
%
µA
µA
%/V
µV
µA
mA
1.5
VSHDN = 0V
18.0
µA
0.3
V
1.0
µA
1.0
µA
2.4
V
VSHDN = 2.4V
OUT Output Voltage Range
VOUT
OUT Output Resistance
ROUT
IOUT = 1.5mA
3
MΩ
tR, tF
VSENSE = 5mV to 150mV, ROUT = 2kΩ,
COUT = 50pF, 10% to 90%
4
µs
OUT Settling Time to 1%
of Final Value
ts
VSENSE = 5mV to 150mV, ROUT = 2kΩ,
COUT = 50pF
15
µs
Maximum Output Current
IOUT
OUT Rise, Fall Time
0
1
1.5
VCC - 1.5
V
mA
Note 2: VOS is defined as the input voltage (VSENSE) required to give minimum IOUT.
Note 3: VSENSE is the voltage across the sense resistor.
_______________________________________________________________________________________
3
MAX471/MAX472
ELECTRICAL CHARACTERISTICS—MAX472
__________________________________________Typical Operating Characteristics
(Typical Operating Circuit (MAX471) or circuit of Figure 4, RG1 = RG2 = 200Ω, ROUT = 2kΩ (MAX472), TA = +25°C, unless
otherwise noted.)
TA = +25°C
1.0
45
TA = +85°C
TA = -40°C
0.5
35
6
VRS+(V)
MAX471
NO-LOAD OFFSET CURRENT vs.
SUPPLY VOLTAGE
MAX471
ERROR vs. LOAD CURRENT
TA = -40°C
12
3
9 12 15 18 21 24 27 30 33 36
MAX471
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
40
ILOAD FROM RS- TO RS+
ILOAD = 1A
35
9
2.0
6
VRS+ (V)
15
MAX1471-04
VS+ = VS2.2
TA = +85°C
9 12 15 18 21 24 27 30 33 36
VRS+ (V)
2.4
TA = +25°C
0
-2
3
9 12 15 18 21 24 27 30 33 36
MAX471-05
6
1
-1
0
3
2
OUT
TA = +85°C
1.4
TA = +25°C
1.2
3
0
-3
-9
0.8
-12
ILOAD FROM RS+ TO RS-
V = 0V TO 1V
0
0.01
9 12 15 18 21 24 27 30 33 36
5Ω
V
20
5
0.10
1
0.01
10
0.10
10
1
100
VRS+ (V)
ILOAD (A)
POWER-SUPPLY FREQUENCY (kHz)
MAX471
RS+ TO RS- RESISTANCE vs.
TEMPERATURE
MAX472
NO-LOAD OUTPUT ERROR vs.
SUPPLY VOLTAGE
MAX472
ERROR vs. SUPPLY VOLTAGE
3.0
MAX1471-07
40
38
RG1 = RG2 = 0Ω
2.5
36
IOUT (µA)
1.5
TA = +25°C
30
0.5
28
0
1000
1.00
TA = +85°C
0.90
TA = +25°C
1.0
32
VRG1-VRG2 = 60mV,
RG1 = RG2 = 200Ω
TA = +85°C
2.0
34
1.10
ERROR (%)
6
1µF
V = 0mV TO 50mV
MAX1471-08
3
5V
10
-15
0.6
25
15
-6
1.0
A
GND
PSRR (%)
ERROR (%)
1.6
V = 0V TO 0.5V
RS+ RS–
30
6
1.8
MAX471-06
50
1.5
MAX1471-09
TA = +25°C
TA = -40°C
3
SIGN THRESHOLD (mA)
55
40
OFFSET CURRENT (µA)
TA = -40°C
2.0
4
MAX1471-02
TA = +85°C
ISHDN (µA)
SUPPLY CURRENT (µA)
2.5
MAX1471-01
65
60
SIGN THRESHOLD vs.
SUPPLY VOLTAGE
SHUTDOWN CURRENT vs.
SUPPLY VOLTAGE
MAX1471-03
SUPPLY CURRENT vs.
SUPPLY VOLTAGE
RESISTANCE (mΩ)
MAX471/MAX472
Precision, High-Side
Current-Sense Amplifiers
TA = -40°C
0.80
TA = -40°C
-40
-20
0
20
40
TEMPERATURE (°C)
4
60
80
0.70
3
6
9 12 15 18 21 24 27 30 33 36
VCC (V)
3
6
9 12 15 18 21 24 27 30 33 36
VCC (V)
_______________________________________________________________________________________
Precision, High-Side
Current-Sense Amplifiers
MAX472
ERROR vs. SENSE VOLTAGE
MAX471
NOISE vs. LOAD CURRENT
0.4
15
IOUT NOISE (µARMS)
VRG1-VRG2
ERROR (%)
MAX471-15
0.5
MAX471-10
25
5
0
-5
VRG2-VRG1
-15
0.3
0.2
0.1
0
-25
0.1
10
1
100
1000
1mA
VSENSE (mV)
10mA
100mA
ISENSE
1A
MAX471
-100mA to +100mA TRANSIENT RESPONSE
MAX471
0mA to 100mA TRANSIENT RESPONSE
LOAD
CURRENT
100mA/div
0A
LOAD
CURRENT
50mA/div
0A
VOUT
50mV/div
VOUT
50mV/div
50mA/div
SIGN
50mV/div
100µs/div
100µs/div
VCC = 10V, ROUT = 2kΩ 1%, SIGN PULL-UP = 50kΩ 1%
VCC = 10V, ROUT = 2kΩ 1%, SIGN PULL-UP = 50kΩ 1%
MAX471
START-UP DELAY
MAX471
0A TO 3A TRANSIENT RESPONSE
ILOAD
1A/div
VOUT
500mV/div
VOUT
10mV/div
VSHDN
5V/div
10µs/div
ILOAD = 1A, ROUT = 2kΩ 1%
10µs/div
ROUT = 2kΩ 1%
_______________________________________________________________________________________
5
MAX471/MAX472
____________________________Typical Operating Characteristics (continued)
(Typical Operating Circuit (MAX471) or circuit of Figure 4, RG1 = RG2 = 200Ω, ROUT = 2kΩ (MAX472), TA = +25°C, unless
otherwise noted.)
MAX471/MAX472
Precision, High-Side
Current-Sense Amplifiers
______________________________________________________________Pin Description
PIN
NAME
FUNCTION
MAX471
MAX472
1
1
SHDN
2, 3
—
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
N.C.
No Connect—no internal connection
—
3
RG1
Gain Resistor. Connect to battery side of current-sense resistor through the gain resistor.
4
4
GND
Ground or Battery Negative Terminal
5
5
SIGN
An open-collector logic output. For the MAX471, a low level indicates current is flowing from
RS- to RS+. For the MAX472, a low level indicates a negative VSENSE (see Figure 2). SIGN is
high impedance when SHDN is high. Leave open if SIGN is not needed.
6, 7
—
RS-
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
RG2
Gain Resistor. Connect to load side of current-sense resistor through the gain resistor.
—
7
VCC
Power input for MAX472. Connect to sense resistor (RSENSE) junction with RG1.
8
8
OUT
Current output that is proportional to the magnitude of the sensed current flowing through
RSENSE. A 2kΩ resistor from this pin to ground will result in a voltage equal to 1V/Amp of
sensed current in the MAX471.
Shutdown. Connect to ground for normal operation. When high, supply current is
less than 5µA.
_______________Detailed Description
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
versa) through RSENSE. Current flows through either
RG1 and Q1 or RG2 and Q2, depending on the senseresistor current direction. Internal circuitry, not shown in
Figures 1 and 2, prevents Q1 and Q2 from turning on at
the same time. The MAX472 is identical to the
MAX471, except that RSENSE and gain-setting resistors
RG1 and RG2 are external (Figure 2).
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
active and output current IOUT flows from the emitter of
Q1. Since no current flows through RG2 (Q2 is off), the
negative input of A1 is equal to VSOURCE - (ILOAD x
RSENSE). The open-loop gain of A1 forces its positive
input to essentially the same level as the negative input.
Therefore, the drop across RG1 equals I LOAD x
RSENSE. Then, since IOUT flows through Q1 and RG
(ignoring the extremely low base currents), IOUT x RG1
= ILOAD x RSENSE, or:
6
IOUT = (ILOAD x RSENSE) / RG1
Current Output
The output voltage equation for the MAX471/MAX472 is
given below. In the MAX471, the current-gain ratio has
been preset to 500µA/A so that an output resistor
(ROUT) of 2kΩ yields 1V/A for a full-scale value of +3V
at ±3A. Other full-scale voltages can be set with different R OUT values, but the output voltage can be no
greater than VRS+ - 1.5V for the MAX471 or VRG_ - 1.5V
for the MAX472.
VOUT = (RSENSE x ROUT x ILOAD) / RG
where VOUT = the desired full-scale output voltage,
ILOAD = the full-scale current being sensed, RSENSE =
the current-sense resistor, ROUT = the voltage-setting
resistor, and RG = the gain-setting resistor (RG = RG1
= RG2).
The above equation can be modified to determine the
ROUT required for a particular full-scale range:
ROUT = (VOUT x RG) / (ILOAD x RSENSE)
For the MAX471, this reduces to:
ROUT = VOUT / (ILOAD x 500µA/A)
OUT is a high-impedance current-source output that
can be connected to other MAX471/MAX472 OUT pins
_______________________________________________________________________________________
Precision, High-Side
Current-Sense Amplifiers
MAX471/MAX472
RSENSE
RS+
6, 7 RS-
2, 3
RG1
RG2
A1
A2
Q2
Q1
COMP
MAX471
8
OUT
5
SIGN
Figure 1. MAX471 Functional Diagram
RSENSE
POWER SOURCE
OR
BATTERY
TO LOAD/CHARGER
VSENSE
RG1
RG2
3
6
A1
A2
7
Q2
Q1
8
COMP
MAX472
VCC
OUT
5
SIGN
Figure 2. MAX472 Functional Diagram
_______________________________________________________________________________________
7
MAX471/MAX472
Precision, High-Side
Current-Sense Amplifiers
RS+
RS-
RS+
RS-
RSENSE
MAX471 SIGN
GND
3V
TO
36V
RG2
RG1
LOGIC
SUPPLY
OUT
100k
3V
TO
36V
POWER
SOURCE
OR
BATTERY
LOGIC
SUPPLY
TO LOAD/CHARGER
TO LOAD/
CHARGER
100k
RS+
RS-
1
SHDN MAX472
OUT
8
RS+
RS-
2
N.C.
VCC
7
3
RG1
RG2 6
4
GND
SIGN 5
MAX471 SIGN
GND
ROUT
OUT
VOUT
1k
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 connecting OUT to a capacitive load.
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
__________Applications Information
The current at OUT indicates magnitude. The SIGN output 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 battery-operated systems, this is useful for determining
whether the battery is charging or discharging. The
SIGN output may not correctly indicate if the load current is such that IOUT is less than 3.5µA. The MAX471’s
SIGN output accurately indicates the direction of current 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
Shutdown
MAX471
The MAX471 obtains its power from the RS- pin. This
includes MAX471 current consumption in the total system current measured by the MAX471. The small drop
across RSENSE does not affect the MAX471’s performance.
Resistor Selection
Since OUT delivers a current, an external voltage gainsetting resistor (ROUT to ground) is required at the OUT
pin in order to get a voltage. RSENSE is internal to the
MAX471. RG1 and RG2 are factory trimmed for an output 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
ROUT for the desired full-scale output voltage up to RS- 1.5V (see the Current Output section).
_______________________________________________________________________________________
Precision, High-Side
Current-Sense Amplifiers
50
SENSE CURRENT (A)
40
RSENSE
Choose RSENSE based on the following criteria:
a) Voltage Loss: A high RSENSE value will cause the
power-source voltage to degrade through IR loss.
For least voltage loss, use the lowest RSENSE value.
b) Accuracy: A high R SENSE value allows lower
currents to be measured more accurately. This is
because offsets become less significant when the
sense voltage is larger.
Small
DIP
Outline fuse
fuse
time
time
45
TA = +25°C
35
30
25
20
15
10
c) Efficiency and Power Dissipation: At high current
levels, the I2R losses in RSENSE may be significant.
Take this into consideration when choosing the
resistor value and power dissipation (wattage) rating. Also, if the sense resistor is allowed to heat up
excessively, its value may drift.
5
0
10µ
100µ
1m
10m
PULSE WIDTH (sec)
DIP safe
operating region
Table 1 shows suggested component values and indicates 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.
Small Outline safe
operating region
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.
MAX472
RSENSE, RG1, and RG2 are externally connected on
the MAX472. V CC can be connected to either the
load/charge or power-source/battery side of the sense
resistor. Connect V CC to the load/charge side of
RSENSE if you want to include the MAX472 current drain
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 applications (greater than 3A), and also for those where the fullscale load current is less than the 3A of the MAX471.
d) Inductance: If there is a large high-frequency component to ISENSE, you will want to keep inductance
low. Wire-wound resistors have the highest inductance, while metal film is somewhat better. Lowinductance metal-film resistors are available. Instead
of being spiral wrapped around a core, as in metalfilm or wire-wound resistors, these are a straight
band of metal. They are made in values under 1Ω.
e) Cost: If the cost of RSENSE becomes an issue, you
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.
Table 1. Suggested Component Values for the MAX472
0.1
CURRENTSENSE
RESISTOR,
RSENSE
(mΩ)
500
1
50
200
10
2.5
2.5
14
2.5
0.9
5
10
100
5
2.5
0.5
13
2.0
1.1
10
5
50
2
2
0.2
12
2.0
1.6
FULL-SCALE
LOAD
CURRENT,
ISENSE (A)
SCALE
FACTOR,
VOUT/ISENSE
(V/A)
GAIN-SETTING
RESISTORS,
RG1 = RG2
(Ω)
OUTPUT
RESISTOR,
ROUT
(kΩ)
FULL-SCALE
OUTPUT
VOLTAGE,
VOUT (V)
1%
10%
100%
200
10
2.5
25
14
2.5
0.9
TYPICAL ERROR AT X%
OF FULL LOAD (%)
_______________________________________________________________________________________
9
MAX471/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.
MAX471/MAX472
Precision, High-Side
Current-Sense Amplifiers
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 RSENSE = 5mΩ for a 50mV drop at
full scale. This resistor will require about 2 inches of 0.1
inch wide copper trace.
RG1 and RG2
Once RSENSE is chosen, RG1 and RG2 can be chosen
to define the current-gain ratio (RSENSE/RG). Choose
RG = RG1 = RG2 based on the following criteria:
a) 1Ω Input Resistance. The minimum RG value is limited by the 1Ω input resistance, and also by the output 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 ROUT.
b) Efficiency. As RG is reduced, IOUT gets larger for a
given load current. Power dissipated in ROUT is not
going to the load, and therefore reduces overall efficiency. This is significant only when the sense current is small.
TO LOAD/CHARGER
RSENSE
0.3" COPPER
POWER
SOURCE
OR
BATTERY
3V
TO
36V
0.1" COPPER
0.3" COPPER
RG1
RG2
MAX472
OUT
8
N.C.
VCC
7
3
RG1
RG2 6
4
GND
SIGN 5
1
SHDN
2
1.5k
1k
Figure 6. MAX472 Connections Showing Use of PC Board
Trace
make sure RG is small enough that IB and IOS do
not add any appreciable errors. The full-scale error
is given by:
% Error = (RG1 - RG2) x IB + IOS x RG x 100
IFS x RSENSE
c) Maximum Output Current Limitation. IOUT is limited to 1.5mA, requiring RG ≥ VSENSE / 1.5mA. For
VSENSE = 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 (VRG_) and VOUT. As RG becomes larger, the
voltage drop across RG also becomes larger for a
given IOUT. This voltage drop further limits the maximum full-scale V OUT. Assuming the drop across
RSENSE is small and VCC is connected to either side
of RSENSE, VOUT (max) = VCC - (1.5V + IOUT (max) x
RG).
e) Output Offset Error at Low Load Currents. Large
RG values reduce IOUT for a given load current. As
IOUT 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.
where RG1 and RG2 are the gain resistors, I B is the
bias current, IOS is the bias-current mismatch, IFS is the
full-scale current, and RSENSE is the sense resistor.
Assuming a 5A load current, 10mΩ RSENSE, and 100Ω
RG, the current-gain ratio is 100µA/A, yielding a fullscale IOUT of 500µA. Using the maximum values for IB
(20µA) and IOS (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 RSENSE, or c) decreasing
RG.
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 mismatching currents. After selecting the ratio, check to
Current-Sense Adjustment
(Resistor Range, Output Adjust)
Choose ROUT after selecting RSENSE, 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
______________________________________________________________________________________
Precision, High-Side
Current-Sense Amplifiers
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 currentsource 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.
Power-Supply Bypassing and Grounding
The MAX471 has been designed as a “high side” (positive 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 voltage drops across a ground plane, and this drop may
add to or subtract from VOUT. For highest current-measurement accuracy, use a single-point “star” ground.
The MAX471/MAX472 require no special bypassing,
and respond quickly to transient changes in line current. 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 capacitor 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 filtered by placing a capacitor (e.g., 1µF) between them
to average the sensed current.
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.
______________________________________________________________________________________
11
MAX471/MAX472
require, given the full-scale I OUT determined by
RSENSE, RG1, and RG2. The high compliance of OUT
permits using ROUT values up to 10kΩ with minimal
error. Values above 10kΩ are not usually recommended. 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 ROUT) to avoid degrading the measurement accuracy.
MAX471/MAX472
Precision, High-Side
Current-Sense Amplifiers
____Pin Configurations (continued)
SHDN
1
N.C. 2
RG1 3
MAX472
GND 4
8
OUT
7
VCC
6
RG2
5
SIGN
DIP/SO
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 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1996 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.