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19-0335; Rev 2; 12/96
Precision, High-Side Current-Sense Amplifiers
_______________General Description
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 35m 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 100A over temperature, and include a 18A max shutdown mode.
____________________________Features
o Complete High-Side Current Sensing o Precision Internal Sense Resistor (MAX471) o 2% Accuracy Over Temperature o Monitors Both Charge and Discharge o 3A Sense Capability with Internal Sense Resistor (MAX471) o Higher Current-Sense Capability with External Sense Resistor (MAX472) o 100A Max Supply Current o 18A Max Shutdown Mode o 3V to 36V Supply Operation o 8-Pin DIP/SO Packages
MAX471/MAX472
______________Ordering Information
PART MAX471CPA MAX471CSA MAX471EPA MAX471ESA MAX472CPA MAX472CSA MAX472EPA MAX472ESA TEMP. RANGE 0C to +70C 0C to +70C -40C to +85C -40C to +85C 0C to +70C 0C to +70C -40C to +85C -40C to +85C PIN-PACKAGE 8 Plastic DIP 8 SO 8 Plastic DIP 8 SO 8 Plastic DIP 8 SO 8 Plastic DIP 8 SO
________________________Applications
Portable PCs: Notebooks/Subnotebooks/Palmtops Smart Battery Packs Cellular Phones Portable Phones Portable Test/Measurement Systems Battery-Operated Systems Energy Management Systems
__________Typical Operating Circuit
RS+ RS+ RSRSILOAD TO LOAD or CHARGER LOGIC SUPPLY 100k 3V TO 36V SHDN
_________________Pin Configurations
TOP VIEW
SHDN
1
8 7
OUT RSRSSIGN
MAX471
GND
RS+ 2 DISCHARGE/CHARGE VOUT (1V/A) RS+ 3 GND 4
SIGN OUT ILOAD 2000 2k
MAX471
6 5
DIP/SO
MAX472 Pin Configuration continued on last page.
________________________________________________________________ Maxim Integrated Products 1
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Precision, High-Side Current-Sense Amplifiers MAX471/MAX472
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 = +70C) MAX471 (Note 1): Plastic DIP (derate 17.5mW/C above +70C) ..................1.4W SO (derate 9.9mW/C above +70C) .............................791mW MAX472 : Plastic DIP (derate 9.09mW/C above +70C) ..............727mW SO (derate 5.88mW/C above +70C) ...........................471mW Operating Temperature Ranges MAX47_C_A ........................................................0C to +70C MAX47_E_A .....................................................-40C to +85C Junction Temperature Range ............................-60C to +150C Storage Temperature Range .............................-60C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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 = +25C.) PARAMETER Supply Voltage Supply Current Sense Current Sense Resistor Current-Sense Ratio No-Load OUT Error Low-Level OUT Error Power-Supply Rejection Ratio SIGN Threshold (ILOAD required to switch SIGN) SIGN Output Leakage Current SIGN 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 OUT Rise, Fall Time OUT Settling Time to 1% of Final Value 2 IOL VIL IIL VIH IIH VOUT ROUT tR, tF ts ILOAD = 3.0A, VOUT = 0V to (VRS+ - 1.5V) ILOAD = 50mA to 3.0A, ROUT = 2k, COUT = 50pF, 10% to 90% ILOAD = 100mA to 3.0A, ROUT = 2k, COUT = 50pF VSHDN = 2.4V 0 1 3 4 15 VSHDN = 0V 2.4 1.0 VRS+ - 1.5 PSRR SYMBOL VRS+ IRS+ ILOAD RSENSE IOUT/ ILOAD MAX471C ILOAD = 1A, RS+ = 10V MAX471E MAX471C ILOAD = 0A, RS+ = 10V MAX471E MAX471C ILOAD = 30mA, RS+ = 10V MAX471E 3V RS+ 36V, ILOAD = 1A MAX471C MAX471E VSIGN = 36V VSIGN = 0.3V 0.1 1.5 18.0 0.3 1.0 IRS+(SHDN) VSHDN = 2.4V; VCC = 3V to 20V 0.490 0.4875 35 0.500 0.500 ILOAD = 0A, excludes ISIGN CONDITIONS MIN 3 50 TYP MAX 36 113 3 70 0.510 0.5125 2.5 3.0 2.5 3.0 0.1 6.0 7.0 1.0 UNITS V A ARMS m mA/A A A %/V mA A mA A V A V A V M s s
4.0
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Precision, High-Side Current-Sense Amplifiers
ELECTRICAL CHARACTERISTICS--MAX472
(VCC = +3V to +36V, RG1 = RG2 = 200, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Supply Voltage Supply Current Input Offset Voltage (Note 2) Input Bias Current Input Bias-Current Matching OUT Current Accuracy No-Load OUT Error Low-Level OUT Error Power-Supply Rejection Ratio SIGN Threshold (VSENSE 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 OUT Rise, Fall Time OUT Settling Time to 1% of Final Value Maximum Output Current ICC(SHDN) VIL IIL VIH IIH VOUT ROUT tR, tF ts IOUT IOUT = 1.5mA VSENSE = 5mV to 150mV, ROUT = 2k, COUT = 50pF, 10% to 90% VSENSE = 5mV to 150mV, ROUT = 2k, COUT = 50pF 1.5 VSHDN = 2.4V 0 1 3 4 15 VSHDN = 0V 2.4 1.0 VCC - 1.5 PSRR SYMBOL VCC ICC VOS IRG1, IRG2 IOS IRG/IOUT IRG1 - IGR2 VSENSE = 100mV, VCC = 10V (Note 3) VCC = 10V, VSENSE = 0V VCC = 10V, VSENSE = 3mV MAX472C MAX472E MAX472C MAX472E MAX472C MAX472E MAX472C MAX472E 0.1 1.5 18.0 0.3 1.0 60 60 ILOAD = 0A, excludes ISIGN; VCC = 3V to 20V MAX472C MAX472E 20 0.4 CONDITIONS MIN 3 20 TYP MAX 36 48 120 140 35 3.0 2 2.5 2.5 3 2.5 3.0 0.1 120 140 1.0 UNITS V A V A A % A A %/V V A mA A V A V A V M s s mA
MAX471/MAX472
3V VCC 36V, VSENSE = 100mV VCC = 10V VSIGN = 36V VSIGN = 0.3V VSHDN = 2.4V; VCC = 3V to 20V
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.
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3
Precision, High-Side Current-Sense Amplifiers MAX471/MAX472
__________________________________________Typical Operating Characteristics
(Typical Operating Circuit (MAX471) or circuit of Figure 4, RG1 = RG2 = 200, ROUT = 2k (MAX472), TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX1471-01
SHUTDOWN CURRENT vs. SUPPLY VOLTAGE
MAX1471-02
SIGN THRESHOLD vs. SUPPLY VOLTAGE
TA = -40C 3 SIGN THRESHOLD (mA) 2 1 TA = +25C 0 -1 -2 TA = +85C
MAX1471-03
65 60 SUPPLY CURRENT (A) 55 TA = +85C
2.5 TA = -40C
4
2.0 ISHDN (A)
50 45
TA = +25C
1.5 TA = +25C 1.0 TA = +85C
TA = -40C 40 35 3 6 9 12 15 18 21 24 27 30 33 36 VRS+ (V)
0.5
0 3 6 9 12 15 18 21 24 27 30 33 36 VRS+(V)
3
6
9 12 15 18 21 24 27 30 33 36 VRS+ (V)
MAX471 NO-LOAD OFFSET CURRENT vs. SUPPLY VOLTAGE
MAX1471-04
MAX471 ERROR vs. LOAD CURRENT
MAX471-05
MAX471 POWER-SUPPLY REJECTION RATIO vs. FREQUENCY
ILOAD = 1A 35
RS+ RS- MAX471-06
2.4 VS+ = VS2.2 TA = -40C OFFSET CURRENT (A) 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 3 6 TA = +25C TA = +85C
15 12 9 6 ERROR (%) ILOAD FROM RS- TO RS+
40
V = 0V TO 0.5V
A
30 PSRR (%) 25 20 15
5V
OUT GND 1F
3 0 -3 -6 -9 -12 -15 ILOAD FROM RS+ TO RS-
V
5
V = 0V TO 1V
V = 0mV TO 50mV 10 5 0 0.01 0.10 ILOAD (A) 1 10 0.01 0.10 1 10 100 1000 POWER-SUPPLY FREQUENCY (kHz)
9 12 15 18 21 24 27 30 33 36 VRS+ (V)
MAX471 RS+ TO RS- RESISTANCE vs. TEMPERATURE
MAX1471-07
MAX472 NO-LOAD OUTPUT ERROR vs. SUPPLY VOLTAGE
MAX1471-08
MAX472 ERROR vs. SUPPLY VOLTAGE
VRG1-VRG2 = 60mV, RG1 = RG2 = 200 1.00 ERROR (%) TA = +85C 0.90 TA = +25C
MAX1471-09
40 38 RESISTANCE (m) 36
3.0 2.5 2.0 IOUT (A) 1.5 TA = +25C 1.0 0.5 TA = -40C RG1 = RG2 = 0 TA = +85C
1.10
34 32 30 28 -40 -20 0 20 40 60 80 TEMPERATURE (C)
0.80 TA = -40C
0 3 6 9 12 15 18 21 24 27 30 33 36 VCC (V)
0.70 3 6 9 12 15 18 21 24 27 30 33 36 VCC (V)
4
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Precision, High-Side Current-Sense Amplifiers
____________________________Typical Operating Characteristics (continued)
(Typical Operating Circuit (MAX471) or circuit of Figure 4, RG1 = RG2 = 200, ROUT = 2k (MAX472), TA = +25C, unless otherwise noted.)
MAX472 ERROR vs. SENSE VOLTAGE
MAX471-10
MAX471/MAX472
MAX471 NOISE vs. LOAD CURRENT
MAX471-15
25
0.5 0.4 IOUT NOISE (ARMS)
15 VRG1-VRG2 ERROR (%) 5 0 -5 VRG2-VRG1
0.3
0.2
-15
0.1
-25 0.1 1 10 VSENSE (mV) 100 1000
0 1mA 10mA 100mA ISENSE 1A
MAX471 0mA to 100mA TRANSIENT RESPONSE
0A LOAD CURRENT 50mA/div
MAX471 -100mA to +100mA TRANSIENT RESPONSE
LOAD CURRENT 100mA/div
0A
VOUT 50mV/div VOUT 50mV/div 50mA/div 100s/div VCC = 10V, ROUT = 2k 1%, SIGN PULL-UP = 50k 1% SIGN 50mV/div
100s/div VCC = 10V, ROUT = 2k 1%, SIGN PULL-UP = 50k 1%
MAX471 START-UP DELAY
MAX471 0A TO 3A TRANSIENT RESPONSE
ILOAD 1A/div
VOUT 500mV/div VSHDN 5V/div VOUT 10mV/div
10s/div ILOAD = 1A, ROUT = 2k 1% ROUT = 2k 1%
10s/div
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5
Precision, High-Side Current-Sense Amplifiers MAX471/MAX472
______________________________________________________________Pin Description
PIN NAME MAX471 1 2, 3 -- -- 4 5 6, 7 -- -- 8 MAX472 1 -- 2 3 4 5 -- 6 7 8 SHDN RS+ N.C. RG1 GND SIGN RSRG2 VCC OUT Shutdown. Connect to ground for normal operation. When high, supply current is less than 5A. 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. 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 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. 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. Gain Resistor. Connect to load side of current-sense resistor through the gain resistor. Power input for MAX472. Connect to sense resistor (RSENSE) junction with RG1. Current output that is proportional to the magnitude of the sensed current flowing through RSENSE. A 2k resistor from this pin to ground will result in a voltage equal to 1V/Amp of sensed current in the MAX471. FUNCTION
_______________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 500A/A so that an output resistor (ROUT) of 2k 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 500A/A) OUT is a high-impedance current-source output that can be connected to other MAX471/MAX472 OUT pins
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Precision, High-Side Current-Sense Amplifiers MAX471/MAX472
RSENSE RS+ 2, 3 6, 7 RS-
RG1
RG2
A1
A2
Q1
Q2
8
OUT
MAX471
COMP
5
SIGN
Figure 1. MAX471 Functional Diagram
RSENSE POWER SOURCE OR BATTERY TO LOAD/CHARGER VSENSE RG1 RG2
3
6
A1
A2 7
Q1
Q2 8
VCC
OUT
COMP
5 SIGN
MAX472
Figure 2. MAX472 Functional Diagram
_______________________________________________________________________________________ 7
Precision, High-Side Current-Sense Amplifiers MAX471/MAX472
RS+ RS+ RSRSPOWER SOURCE OR BATTERY TO LOAD/ CHARGER 1 2 3 4 SHDN MAX472 N.C. RG1 GND 100k OUT VCC 8 7 ROUT RSENSE
TO LOAD/CHARGER
MAX471 SIGN
GND OUT
LOGIC SUPPLY
3V TO 36V
RG1
RG2 LOGIC SUPPLY
100k 3V TO 36V
RS+ RS+
RSRS-
MAX471 SIGN
OUT VOUT 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 connecting OUT to a capacitive load.
Shutdown
When SHDN is high, the MAX471/MAX472 are shut down and consume less than 18A. In shutdown mode, SIGN is high impedance and OUT turns off.
SIGN Output
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 3.5A. 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 100k 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 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 500A/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).
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Precision, High-Side Current-Sense Amplifiers
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.
50 45 40 SENSE CURRENT (A) 35 30 25 20 15 10 5 0 10 100 1m 10m PULSE WIDTH (sec) DIP safe operating region Small Outline safe operating region TA = +25C Small DIP Outline fuse fuse time time
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.
MAX471/MAX472
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.
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. 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 1. 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.
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.
Table 1. Suggested Component Values for the MAX472
FULL-SCALE LOAD CURRENT, ISENSE (A) 0.1 1 5 10 CURRENTSENSE RESISTOR, RSENSE (m) 500 50 10 5 GAIN-SETTING RESISTORS, RG1 = RG2 () 200 200 100 50 OUTPUT RESISTOR, ROUT (k) 10 10 5 2 FULL-SCALE OUTPUT VOLTAGE, VOUT (V) 2.5 2.5 2.5 2 SCALE FACTOR, VOUT/ISENSE (V/A) 25 2.5 0.5 0.2 TYPICAL ERROR AT X% OF FULL LOAD (%) 1% 14 14 13 12 10% 2.5 2.5 2.0 2.0 100% 0.9 0.9 1.1 1.6 9
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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 30m/ft (see Note 4). For 10A you may want RSENSE = 5m for a 50mV drop at full scale. This resistor will require about 2 inches of 0.1 inch wide copper trace.
RSENSE 0.3" COPPER 0.1" COPPER TO LOAD/CHARGER 0.3" COPPER
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) 1 Input Resistance. The minimum RG value is limited by the 1 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 = 50, 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. c) Maximum Output Current Limitation. IOUT is limited to 1.5mA, requiring RG VSENSE / 1.5mA. For VSENSE = 60mV, RG must be 40. 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 2.5A 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 mismatching 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 VCC
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 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 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, 10m RSENSE, and 100 RG, the current-gain ratio is 100A/A, yielding a fullscale IOUT of 500A. Using the maximum values for IB (20A) and IOS (2A), and 1% resistors for RG1 and RG2 (RG1 - RG2 = 2), the worst-case error at full scale calculates to: 2 x 20A + 100 x 2A = 0.48% 5m x 5A The error may be reduced by: a) better matching of RG1 and RG2, b) increasing RSENSE, or c) decreasing RG.
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
require, given the full-scale I OUT determined by RSENSE, RG1, and RG2. The high compliance of OUT permits using ROUT values up to 10k with minimal error. Values above 10k 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. 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 1F 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., 1F) between them to average the sensed current.
MAX471/MAX472
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.
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.
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.
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11
Precision, High-Side Current-Sense Amplifiers MAX471/MAX472
____Pin Configurations (continued)
SHDN
1
8 7
OUT VCC RG2 SIGN
N.C. 2 RG1 3 GND 4
MAX472
6 5
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 (c) 1996 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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