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19-1184; Rev 0; 12/96
Low-Cost, Precision, High-Side Current-Sense Amplifier
_______________General Description
The MAX4172 is a low-cost, precision, high-side currentsense amplifier 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 battery charger's ground path. Wide bandwidth and ground-sensing capability make the MAX4172 suitable for closed-loop battery-charger and generalpurpose current-source applications. The 0V to 32V input common-mode range is independent of the supply voltage, which ensures that current-sense feedback remains viable, even when connected to a battery in deep discharge. To provide a high level of flexibility, the MAX4172 functions with an external sense resistor to set the range of load current to be monitored. It has a current output that can be converted to a ground-referred voltage with a single resistor, accommodating a wide range of battery voltages and currents. An open-collector power-good output (PG) indicates when the supply voltage reaches an adequate level to guarantee proper operation of the current-sense amplifier. The MAX4172 operates with a 3.0V to 32V supply voltage, and is available in a space-saving, 8-pin MAX or SO package.
____________________________Features
o Low-Cost, High-Side Current-Sense Amplifier o 0.5% Typical Full-Scale Accuracy Over Temperature o 3V to 32V Supply Operation o 0V to 32V Input Range--Independent of Supply Voltage o 800kHz Bandwidth [VSENSE = 100mV (1C)] 200kHz Bandwidth [VSENSE = 6.25mV (C/16)] o Available in Space-Saving MAX and SO Packages
MAX4172
______________Ordering Information
PART MAX4172ESA MAX4172EUA TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 8 SO 8 MAX*
*Contact factory for availability.
________________________Applications
Portable PCs: Notebooks/Subnotebooks/Palmtops Battery-Powered/Portable Equipment Closed-Loop Battery Chargers/Current Sources Smart-Battery Packs Portable/Cellular Phones Portable Test/Measurement Systems Energy Management Systems
__________Typical Operating Circuit
UNREGULATED DC SUPPLY 3V TO 32V RSENSE 50m VSENSE
LOW-COST SWITCHING REGULATOR
0V TO 32V ANALOG OR LOGIC SUPPLY 100k
2A
RS+ V+
RSPG
__________________Pin Configuration
MAX4172
POWER GOOD
LOAD/ BATTERY
TOP VIEW
RS+ 1 RS- 2 N.C. 3 N.C. 4 8 7 V+ PG OUT GND
GND FEEDBACK LOOP VOUT = 500mV/A
OUT
IOUT = VSENSE / 100
MAX4172
6 5
ROUT 1k
MAX/SO
LOW-COST BATTERY CHARGER/CURRENT SOURCE
________________________________________________________________ Maxim Integrated Products
1
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Low-Cost, Precision, High-Side Current-Sense Amplifier MAX4172
ABSOLUTE MAXIMUM RATINGS
V+, RS+, RS-, PG ...................................................-0.3V to +36V OUT ..............................................................-0.3V to (V+ + 0.3V) Differential Input Voltage, VRS+ - VRS- ............................700mV Current into Any Pin..........................................................50mA Continuous Power Dissipation (TA = +70C) SO (derate 5.88mW/C above +70C) ..........................471mW MAX (derate 4.10mW/C above +70C) .....................330mW Operating Temperature Range MAX4172E_A ....................................................-40C to +85C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) .............................+300C
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
(V+ = +3V to +32V; RS+, RS- = 0V to 32V; TA = TMIN to TMAX; unless otherwise noted. Typical values are at V+ = +12V, RS+ = 12V, TA = +25C.) PARAMETER Operating Voltage Range Input Voltage Range Supply Current Input Offset Voltage SYMBOL V+ VRSIV+ VOS IOUT = 0mA V+ = 12V, VRS+ = 12V VRS+ 2.0V Positive Input Bias Current Negative Input Bias Current Maximum VSENSE Voltage Low-Level Current Error VSENSE = 6.25mV, V+ = 12V, MAX4172ESA VRS+ = 12V (Note 1) MAX4172EUA MAX4172ESA, TA = -40C to 0C MAX4172EUA, TA = -40C to 0C MAX4172ESA, TA = 0C to +85C MAX4172EUA, TA = 0C to +85C TA = 0C to +85C OUT Power-Supply Rejection Ratio OUT Common-Mode Rejection Ratio IOUT / V+ 3V V+ 32V, VRS+ > 2.0V 0.2 0.03 IRS+ IRSVRS+ > 2.0V, IOUT = 0mA VRS+ 2.0V, IOUT = 0mA VRS+ > 2.0V VRS+ 2.0V 0 -325 0 -650 150 175 8.0 15 20 50 A 10 15 A/V A/V 50 MAX4172ESA MAX4172EUA CONDITIONS MIN 3 0 0.8 0.1 0.2 4 27 42.5 42.5 85 85 A A mV A TYP MAX 32 32 1.6 0.75 1.6 mV UNITS V V mA
Output Current Error
VSENSE = 100mV, V+ = 12V, VRS+ = 12V
IOUT / VRS+ 2.0V < VRS+ < 32V
2
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Low-Cost, Precision, High-Side Current-Sense Amplifier
ELECTRICAL CHARACTERISTICS (continued)
(V+ = +3V to +32V; RS+, RS- = 0V to 32V; TA = TMIN to TMAX; unless otherwise noted. Typical values are at V+ = +12V, RS+ = 12V, TA = +25C.) PARAMETER Maximum Output Voltage (OUT) Bandwidth Maximum Output Current Transconductance V+ Threshold for PG Output Low (Note 2) PG Output Low Voltage Leakage Current into PG Power-Off Input Leakage Current (RS+, RS-) OUT Rise Time OUT Fall Time OUT Settling Time to 1% OUT Output Resistance VOL IOUT Gm Gm = IOUT / (VRS+ - VRS-), VSENSE = 100mV, VRS+ > 2.0V V+ rising V+ falling ISINK = 1.2mA, V+ = 2.9V, TA = +25C V+ = 2.5V, TA = +25C V+ = 0V, VRS+ = VRS- = 32V VSENSE = 0mV to 100mV, 10% to 90% VSENSE = 100mV to 0mV, 90% to 10% VSENSE = 5mV to 100mV VSENSE = 150mV Rising Falling 0.1 400 800 1.3 6 20 TA = 0C to +85C TA = -40C to 0C SYMBOL IOUT 1.5mA VSENSE = 100mV VSENSE = 6.25mV (Note 1) 1.5 9.8 9.7 800 200 1.75 10 10 2.77 2.67 0.4 1 1 10.2 10.3 CONDITIONS MIN TYP MAX V+ - 1.2 UNITS V kHz mA mA/V V V A A ns ns s M
MAX4172
Note 1: 6.25mV = 1/16 of typical full-scale sense voltage (C/16). Note 2: Valid operation of the MAX4172 is guaranteed by design when PG is low.
__________________________________________Typical Operating Characteristics
(V+ = +12V, VRS+ = 12V, ROUT = 1k, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX4172-01
OUTPUT ERROR vs. SUPPLY VOLTAGE
MAX4172-03
C/16 LOAD OUTPUT ERROR vs. SUPPLY VOLTAGE
1.0 0.5 ERROR (%) 0 -0.5 -1.0 -1.5 TA = +25C TA = -40C VSENSE = 6.25mV
MAX4172-02
1000 950 SUPPLY CURRENT (A) 900 850 800 750 700 650 600 550 500 0 10 20 V+ (V) 30 IOUT = 0mA TA = -40C TA = +85C
0.5 0.4 0.3 ERROR (%) VSENSE = 100mV
1.5
TA = +25C
0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 40 0 10 TA = -40C 20 V+ (V) 30 40 TA = +25C TA = +85C
-2.0 -2.5 -3.0 0 10 20 V+ (V) 30 40 TA = +85C
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Low-Cost, Precision, High-Side Current-Sense Amplifier MAX4172
____________________________Typical Operating Characteristics (continued)
(V+ = +12V, VRS+ = 12V, ROUT = 1k, TA = +25C, unless otherwise noted.)
POWER-SUPPLY REJECTION RATIO vs. FREQUENCY
MAX4172-04
ERROR vs. SENSE VOLTAGE
40 35 30 ERROR (%) ERROR (%) 25 20 15 10 5 0 -5 0.1m 1m 10m VSENSE (V) 100m 1 10 5 0 0.01 25 20 15 35 30
VSENSE = 100mV
0.5Vp-p
1.0Vp-p 5mVp-p
0.1
1
10
100
1000
POWER-SUPPLY FREQUENCY (kHz)
OUTPUT ERROR vs. COMMON-MODE VOLTAGE
MAX4172-06
V+ THRESHOLD FOR PG OUTPUT LOW vs. TEMPERATURE
2.90 V+ TRIP THRESHOLD (V) 2.85 2.80 2.75 2.70 2.65 2.60 2.55 2.50 2.45 V+ FALLING VOLTAGE V+ RISING VOLTAGE
MAX4172-07
0.75 0.55 0.35 ERROR (%) 0.15 -0.05 -0.25 -0.45 -0.65 0 6 12 18 VRS- (V) 24 30 TA = +25C TA = +85C TA = -40C VSENSE = 100mV
2.95
40
-40
-15
10
35
60
85
TEMPERATURE (C)
0mV to 10mV VSENSE TRANSIENT RESPONSE
MAX4172-08
0mV to 100mV VSENSE TRANSIENT RESPONSE
MAX4172-09
VSENSE 5mV/div GND
VSENSE 50mV/div GND
VOUT 50mV/div GND
VOUT 500mV/div GND 10s/div
10s/div
4
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MAX4172-05
Low-Cost, Precision, High-Side Current-Sense Amplifier
____________________________Typical Operating Characteristics (continued)
(V+ = +12V, VRS+ = 12V, ROUT = 1k, TA = +25C, unless otherwise noted.)
START-UP DELAY
MAX4172-10
MAX4172
V+ to PG POWER-UP DELAY
MAX4172-11
VOUT 500mV/div GND
PG 2V/div GND
V+ 2V/div GND
V+ 2V/div GND
5s/div VSENSE = 100mV
10s/div 100k PULL-UP RESISTOR FROM PG TO +4V
______________________________________________________________Pin Description
PIN 1 2 3, 4 5 6 7 8 NAME RS+ RSN.C. GND OUT PG V+ FUNCTION Power connection to the external sense resistor. The "+" indicates the direction of current flow. Load-side connection for the external sense resistor. The "-" indicates the direction of current flow. No Connect. No internal connection. Leave open or connect to GND. Ground Current Output. OUT is proportional to the magnitude of the sense voltage (VRS+ - VRS-). A 1k resistor from OUT to ground will result in a voltage equal to 10V/V of sense voltage. Power Good Open-Collector Logic Output. A low level indicates that V+ is sufficient to power the MAX4172, and adequate time has passed for power-on transients to settle out. Supply Voltage Input for the MAX4172
_______________Detailed Description
The MAX4172 is a unidirectional, high-side current-sense amplifier with an input common-mode range that is independent of supply voltage. This feature not only allows the monitoring of current flow into a battery in deep discharge, but also enables high-side current sensing at voltages far in excess of the supply voltage (V+). The MAX4172 current-sense amplifier's unique topology simplifies current monitoring and control. The MAX4172's amplifier operates as shown in Figure 1. The battery/load current flows through the external sense resistor (RSENSE), from the RS+ node to the RS-
node. Current flows through RG1 and Q1, and into the current mirror, where it is multiplied by a factor of 50 before appearing at OUT. 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. Since A1's inverting input is high impedance, no current flows though RG2 (neglecting the input bias current), so A1's negative input is equal to VSOURCE - (ILOAD x RSENSE). A1's open-loop gain forces its positive input to essentially the same voltage level as the negative input. Therefore, the drop across RG1 equals ILOAD x RSENSE. Then, since IRG1
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Low-Cost, Precision, High-Side Current-Sense Amplifier
flows through RG1, IRG1 x RG1 = ILOAD x RSENSE. The internal current mirror multiplies IRG1 by a factor of 50 to give IOUT = 50 x IRG1. Substituting IOUT / 50 for IRG1, (IOUT / 50) x RG1 = ILOAD x RSENSE, or: IOUT = 50 x ILOAD x (RSENSE / RG1) The internal current gain of 50 and the factory-trimmed resistor R G1 combine to result in the MAX4172 transconductance (G m ) of 10mA/V. G m is defined as being equal to IOUT / (VRS+ - VRS-). Since (VRS+ - VRS-) = ILOAD x RSENSE, the output current (IOUT) can be calculated with the following formula: IOUT = Gm x (VRS+ - VRS-) = (10mA/V) x (ILOAD x RSENSE)
MAX4172
RSENSE INPUT RS+ VSENSE RSRG2
ILOAD TO LOAD/ BATTERY
RG1
IRG1 A1
Q1 V+ 1:50 CURRENT MIRROR V+
MAX4172
Current Output
The output voltage equation for the MAX4172 is given below: VOUT = (Gm) x (RSENSE x ROUT x ILOAD) 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 G m = MAX4172 transconductance (10mA/V). The full-scale output voltage range can be set by changing the ROUT resistor value, but the output voltage must be no greater than V+ - 1.2V. The above equation can be modified to determine the R OUT required for a particular full-scale range: ROUT = (VOUT ) / (ILOAD x RSENSE x Gm) OUT is a high-impedance current source that can be integrated by connecting it to a capacitive load.
IOUT = 50 IRG1
OUT
PG
VTH GND
Figure 1. Functional Diagram
PG Output
The PG output is an open-collector logic output that indicates the status of the MAX4172's V+ power supply. A logic low on the PG output indicates that V+ is sufficient to power the MAX4172. This level is temperature dependent (see Typical Operating Characteristics graphs), and is typically 2.7V at room temperature. The internal PG comparator has a 100mV (typical) hysteresis to prevent possible oscillations caused by repeated toggling of the PG output, making the device ideal for power-management systems lacking soft-start capability. An internal delay (15s typical) in the PG comparator allows adequate time for power-on transients to settle out. The PG status indicator greatly simplifies the design of closed-loop systems by ensuring that the components in the control loop have sufficient voltage to operate correctly.
__________Applications Information
Suggested Component Values for Various Applications
The Typical Operating Circuit is useful in a wide variety of applications. Table 1 shows suggested component values and indicates the resulting scale factors for various applications required to sense currents from 100mA to 10A. Adjust the RSENSE value to monitor higher or lower current levels. Select RSENSE using the guidelines and formulas in the following section.
Sense Resistor, RSENSE
Choose RSENSE based on the following criteria: * Voltage Loss: A high R SENSE value causes the power-source voltage to degrade through IR loss. For minimal voltage loss, use the lowest R SENSE value.
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Low-Cost, Precision, High-Side Current-Sense Amplifier MAX4172
Table 1. Suggested Component Values
FULL-SCALE LOAD CURRENT (A) 0.1 1 5 10 CURRENT-SENSE RESISTOR, RSENSE (m) 1000 100 20 10 OUTPUT RESISTOR, ROUT (k) 3.48 3.48 3.48 3.48 FULL-SCALE OUTPUT VOLTAGE, VOUT (V) 3.48 3.48 3.48 3.48 SCALE FACTOR, VOUT/ISENSE (V/A) 34.8 3.48 0.696 0.348
Accuracy: A high RSENSE value allows lower currents to be measured more accurately. This is because offsets become less significant when the sense voltage is larger. For best performance, select RSENSE to provide approximately 100mV of sense voltage for the full-scale current in each application. * Efficiency and Power Dissipation: At high current levels, the I2R losses in RSENSE can be significant. Take this into consideration when choosing the resistor value and its power dissipation (wattage) rating. Also, the sense resistor's value might drift if it is allowed to heat up excessively. * Inductance: Keep inductance low if ISENSE has a large high-frequency component. Wire-wound resistors have the highest inductance, while metal film is somewhat better. Low-inductance metal-film resistors are also available. Instead of being spiral wrapped around a core, as in metal-film or wirewound resistors, they are a straight band of metal and are available in values under 1. * Cost: If the cost of RSENSE is an issue, you might want to use an alternative solution, as shown in Figure 2. This solution uses the PC board traces to create a sense resistor. Because of the inaccuracies of the copper resistor, the full-scale current value must be adjusted with a potentiometer. Also, copper's resistance temperature coefficient is fairly high (approximately 0.4%/C). In Figure 2, assume that the load current to be measured is 10A, and that you have determined a 0.3-inchwide, 2-ounce copper to be appropriate. The resistivity of 0.1-inch-wide, 2-ounce (70m thickness) copper is 30m/ft. For 10A, you might want RSENSE = 5m for a 50mV drop at full scale. This resistor requires about 2 inches of 0.1-inch-wide copper trace.
*
INPUT O.3 in. COPPER
RSENSE O.1 in. COPPER
LOAD/BATTERY O.3 in. COPPER
VSENSE 1 2 VSUPPLY 3V TO 32V
RS+ MAX4172 V+ RSOUT GND
8
6 5 ROUT
Figure 2. MAX4172 Connections Showing Use of PC Board
Current-Sense Adjustment (Resistor Range, Output Adjust)
Choose ROUT after selecting RSENSE. Choose ROUT to obtain the full-scale voltage you require, given the fullscale IOUT determined by RSENSE. OUT's high impedance permits using R OUT values up to 200k with minimal error. OUT's load impedance (e.g., the input of an op amp or ADC) must be much greater than ROUT (e.g., 100 x ROUT) to avoid degrading measurement accuracy.
High-Current Measurement
The MAX4172 can achieve high-current measurements by using low-value sense resistors, which can be paralleled to further increase the current-sense limit. As an alternative, PC board traces can be adjusted over a wide range.
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Low-Cost, Precision, High-Side Current-Sense Amplifier MAX4172
Power-Supply Bypassing and Grounding
In most applications, grounding the MAX4172 requires no special precautions. However, in high-current systems, large voltage drops can develop across the ground plane, which can add to or subtract from VOUT. Use a single-point star ground for the highest currentmeasurement accuracy. The MAX4172 requires no special bypassing and responds quickly to transient changes in line current. If the noise at OUT caused by these transients is a problem, you can 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 MAX4172) to decouple the load, reducing the current transients. These capacitors are not required for MAX4172 operation or stability. The RS+ and RS- inputs can be filtered by placing a capacitor (e.g., 1F) between them to average the sensed current.
___________________Chip Information
TRANSISTOR COUNT: 177 SUBSTRATE CONNECTED TO GND
________________________________________________________Package Information
DIM
C A 0.101mm 0.004 in B A1 L
e
A A1 B C D E e H L
INCHES MAX MIN 0.044 0.036 0.008 0.004 0.014 0.010 0.007 0.005 0.120 0.116 0.120 0.116 0.0256 0.198 0.188 0.026 0.016 6 0
MILLIMETERS MIN MAX 0.91 1.11 0.10 0.20 0.25 0.36 0.13 0.18 2.95 3.05 2.95 3.05 0.65 4.78 5.03 0.41 0.66 0 6
21-0036D
E
H
8-PIN MAX MICROMAX SMALL-OUTLINE PACKAGE
D
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.
8 ___________________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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