MAX471CSA-T中文资料
Not Recommended for New Designs
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For further information, contact Maxim’s Applications Tech Support.
_______________General Description
The MAX471/MAX472 are complete, bidirectional, high-side current-sense amplifiers for portable PCs, tele-phones, and other systems where battery/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 battery chargers or monitors often found in “smart” batteries.
The MAX471 has an internal 35m ?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.
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.
________________________Applications
Portable PCs:
Notebooks/Subnotebooks/Palmtops Smart Battery Packs Cellular Phones Portable Phones
Portable Test/Measurement Systems Battery-Operated Systems Energy Management Systems
____________________________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 100μA Max Supply Current o 18μA Max Shutdown Mode o 3V to 36V Supply Operation o 8-Pin DIP/SO Packages
______________Ordering Information
MAX471/MAX472
Precision, High-Side Current-Sense Amplifiers
________________________________________________________________Maxim Integrated Products
1
_________________Pin Configurations
__________Typical Operating Circuit
19-0335; Rev 2; 12/96
For free samples & the latest literature: https://www.wendangku.net/doc/888122698.html,, or phone 1-800-998-8800
M A X 471/M A X 472
Precision, High-Side
Current-Sense Amplifiers
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS—MAX471
(RS+ = +3V to +36V, T A = T MIN to T MAX , unless otherwise noted. Typical values are at T A = +25°C.)
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.
Supply Voltage, RS+, RS-, V CC to GND....................-0.3V, +40V RMS Current, RS+ to RS- (MAX471 only)..........................±3.3A Peak Current, (RS+ to RS-)......................................see Figure 5Differential 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 (V CC + 0.3V)Voltage at SIGN......................................................-0.3V to +40V Current into SHDN, GND, OUT, RG1, RG2, V CC ................±50mA Current into SIGN.................................................+10mA, -50mA
Continuous Power Dissipation (T A = +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.
MAX471/MAX472
Precision, High-Side Current-Sense Amplifiers
_______________________________________________________________________________________3
ELECTRICAL CHARACTERISTICS—MAX472
(V CC = +3V to +36V, RG1 = RG2 = 200?, T A = T MIN to T MAX , unless otherwise noted. Typical values are at T A = +25°C.)
Note 2:V OS is defined as the input voltage (V SENSE ) required to give minimum I OUT .Note 3:V SENSE is the voltage across the sense resistor.
M A X 471/M A X 472
Precision, High-Side
Current-Sense Amplifiers 4_______________________________________________________________________________________
__________________________________________Typical Operating Characteristics
(Typical Operating Circuit (MAX471) or circuit of Figure 4, RG1 = RG2 = 200?, R OUT = 2k ?(MAX472), T A = +25°C, unless otherwise noted.)
6535
SUPPLY CURRENT vs. SUPPLY VOLTAGE
40
V RS+ (V)
S U P P L Y C U R R E N T (μA )
212415189123
6
27303336
45
5055
60
2.5
00.5
2.0I S H D N (μA )
1.5
1.0
SHUTDOWN CURRENT vs. SUPPLY VOLTAGE
V RS+(V)
212415189123
6
27303336
4
-2
SIGN THRESHOLD vs. SUPPLY VOLTAGE
-1S I G N T H R E S H O L D (m A )
212415189123627303336
01
23V RS+ (V)
0.6
O F F S E T C U R R E N T (μA )
MAX471
NO-LOAD OFFSET CURRENT vs.
SUPPLY VOLTAGE
V RS+ (V)
212415189123
6
27303336
0.81.01.21.41.61.82.02.22.428-40
80
TEMPERATURE (°C)
R E S I S T A N C E (m ?)
20
30-20
60
40
M A X 1471-07
MAX471
RS+ TO RS- RESISTANCE vs.
TEMPERATURE
323436
3840-120.01
0.10
MAX471
ERROR vs. LOAD CURRENT
I LOAD (A)
E R R O R (%)
1
10
-15
-6-9-30369121540
0.01
101000
MAX471
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
POWER-SUPPLY FREQUENCY (kHz)
P S R R (%)
110035
3025201510
50
0.103.00
MAX472
NO-LOAD OUTPUT ERROR vs.
SUPPLY VOLTAGE
0.5V CC (V)
I
O U T (μA )
212415189123
6
27303336
1.01.5
2.02.50.70
E R R O R (%)
MAX472
ERROR vs. SUPPLY VOLTAGE
V CC (V)
212415189123
6
27303336
0.80
0.90
1.001.10
MAX471/MAX472
Precision, High-Side Current-Sense Amplifiers
_______________________________________________________________________________________5
25-25
0.1
101000
MAX472
ERROR vs. SENSE VOLTAGE
V SENSE (mV)
E R R O R (%
)
1550-5
-15
1
100
1mA
10mA
100mA
1A
0.1
00.20.3
0.40.5MAX471 NOISE vs. LOAD C URRENT
I SENSE
I O U T N O I S E (μA R M S )M A X 471-15
____________________________Typical Operating Characteristics (continued)
(Typical Operating Circuit (MAX471) or circuit of Figure 4, RG1 = RG2 = 200?, R OUT = 2k ?(MAX472), T A = +25°C, unless otherwise noted.)
100μs/div
V CC = 10V, R OUT = 2k ? 1%, SIGN PULL-UP = 50k ? 1%
LOAD CURRENT 50mA/div V OUT 50mV/div
MAX471
0mA to 100mA TRANSIENT RESPONSE
0A
10μs/div
I LOAD = 1A, R OUT = 2k ? 1%
V OUT
500mV/div MAX471 START-UP DELAY V SHDN 5V/div
10μs/div
R OUT = 2k ? 1%
I LOAD 1A/div
MAX471
0A TO 3A TRANSIENT RESPONSE
V OUT 10mV/div
100μs/div
V CC = 10V, R OUT = 2k ? 1%, SIGN PULL-UP = 50k ? 1%
LOAD CURRENT 100mA/div
50mA/div V OUT 50mV/div
MAX471
-100mA to +100mA TRANSIENT RESPONSE
SIGN 50mV/div
0A
M A X 471/M A X 472
_______________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 R SENSE . Current flows through either 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 same time. The MAX472 is identical to the MAX471, except that R SENSE 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 I OUT flows from the emitter of Q1. Since no current flows through RG2 (Q2 is off), the negative input of A1 is equal to V SOURCE - (I LOAD x R SENSE ). 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 R SENSE . Then, since I OUT flows through Q1 and RG (ignoring the extremely low base currents), I OUT x RG1= I LOAD x R SENSE , or:
I OUT = (I LOAD x R SENSE )/ 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 (R OUT ) of 2k ?yields 1V/A for a full-scale value of +3V at ±3A. Other full-scale voltages can be set with differ-ent R OUT values, but the output voltage can be no greater than V RS+- 1.5V for the MAX471 or V RG_- 1.5V for the MAX472.
V OUT = (R SENSE x R OUT x I LOAD ) / RG
where V OUT = the desired full-scale output voltage,I LOAD = the full-scale current being sensed, R SENSE =the current-sense resistor, R OUT = the voltage-setting resistor, and RG = the gain-setting resistor (RG = RG1= RG2).
The above equation can be modified to determine the R OUT required for a particular full-scale range:
R OUT = (V OUT x RG) / (I LOAD x R SENSE )
For the MAX471, this reduces to:
R OUT = V OUT / (I LOAD 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 6
_______________________________________________________________________________________
______________________________________________________________Pin Description
MAX471/MAX472
Precision, High-Side Current-Sense Amplifiers
_______________________________________________________________________________________
7
Figure 1. MAX471 Functional Diagram
Figure 2. MAX472 Functional Diagram
M A X 471/M A X 472
for current summing. A single scaling resistor is required when summing OUT currents from multiple devices (Figure 3). Current can be integrated by con-necting OUT to a capacitive load.
SIGN Output
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-rent is such that I OUT is less than 3.5μA. The MAX471’s SIGN output accurately indicates the direction of cur-rent 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.
Shutdown
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.
__________Applications Information
MAX471
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 SENSE does not affect the MAX471’s perfor-mance.
Resistor Selection
Since OUT delivers a current, an external voltage gain-setting resistor (R OUT to ground) is required at the OUT pin in order to get a voltage. R SENSE is internal to the 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 R OUT for the desired full-scale output voltage up to RS-- 1.5V (see the Current Output section).
Precision, High-Side
Current-Sense Amplifiers 8
_______________________________________________________________________________________
Figure 3. Paralleling MAX471s to Sense Higher Load Current Figure 4. MAX472 Standard Application Circuit
Peak Sense Current
The MAX471’s maximum sense current is 3A RMS . For power-up, fault conditions, or other infrequent events,
MAX472
R SENSE , 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 R SENSE 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 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 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.R SENSE
Choose R SENSE based on the following criteria:
a)Voltage Loss: A high R SENSE value will cause the power-source voltage to degrade through IR loss.For least voltage loss, use the lowest R SENSE 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 I 2R losses in R SENSE may be significant.Take this into consideration when choosing the resistor value and power dissipation (wattage) rat-ing. Also, if the sense resistor is allowed to heat up excessively, its value may drift.
d)Inductance:If there is a large high-frequency com-ponent to I SENSE , you will want to keep inductance 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 1?.e)Cost:If the cost of R SENSE 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.
MAX471/MAX472
Precision, High-Side Current-Sense Amplifiers
Table 1. Suggested Component Values for the MAX472
M A X 471/M A X 472
In Figure 6, assume the load current to be measured is 10A and that you have determined a 0.3 inch wide, 2ounce copper to be appropriate. The resistivity of 0.1inch wide, 2 ounce copper is 30m ?/ft (see Note 4). For 10A you may want R SENSE = 5m ?for a 50mV drop at full scale. This resistor will require about 2 inches of 0.1inch wide copper trace.
RG1 and RG2
Once R SENSE is chosen, RG1 and RG2 can be chosen to define the current-gain ratio (R SENSE /RG). Choose RG = RG1 = RG2 based on the following criteria:
a)1?Input Resistance.The minimum RG value is lim-ited by the 1?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 = 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 R OUT .
b)Efficiency.As RG is reduced, I OUT gets larger for a given load current. Power dissipated in R OUT is not going to the load, and therefore reduces overall effi-ciency. This is significant only when the sense cur-rent is small.
c)Maximum Output Current Limitation.I OUT is limit-ed to 1.5mA, requiring RG ≥V SENSE / 1.5mA. For V SENSE = 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 (V RG_) and V OUT . As RG becomes larger, the voltage drop across RG also becomes larger for a given I OUT . This voltage drop further limits the maxi-mum full-scale V OUT . Assuming the drop across R SENSE is small and V CC is connected to either side of R SENSE , V OUT (max) = V CC - (1.5V + I OUT (max) x RG).
e)Output Offset Error at Low Load https://www.wendangku.net/doc/888122698.html,rge RG values reduce I OUT for a given load current. As I OUT 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.
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
make sure RG is small enough that I B and I OS do not add any appreciable errors. The full-scale error is given by:
% Error = (RG1 - RG2) x I B + I OS x RG x 100
I FS x R SENSE
where RG1 and RG2 are the gain resistors, I B is the bias current, I OS is the bias-current mismatch, I FS is the full-scale current, and R SENSE is the sense resistor.
Assuming a 5A load current, 10m ?R SENSE , and 100?RG, the current-gain ratio is 100μA/A, yielding a full-scale I OUT of 500μA. Using the maximum values for I B (20μA) and I OS (2μA), and 1% resistors for RG1 and RG2 (RG1 - RG2 = 2?), the worst-case error at full scale calculates to:
2?x 20μA + 100?x 2μA = 0.48%
5m ?x 5A
The error may be reduced by: a) better matching of RG1 and RG2, b) increasing R SENSE , or c) decreasing RG.
Current-Sense Adjustment (Resistor Range, Output Adjust)
Choose R OUT after selecting R SENSE , RG1, and RG2.Choose R OUT to obtain the full-scale voltage you
Precision, High-Side
Current-Sense Amplifiers 10
______________________________________________________________________________________
Note 4:Printed Circuit Design, by Gerald L. Ginsberg; McGraw-Hill, Inc.; page 185.Figure 6. MAX472 Connections Showing Use of PC Board Trace
require, given the full-scale I OUT determined by R SENSE, RG1, and RG2. The high compliance of OUT permits using R OUT values up to 10k?with minimal error. Values above 10k?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 OUT (e.g., 100 x R OUT) to avoid degrading the measure-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.
An alternative method is to connect several MAX471s in parallel and connect the high-impedance current-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.
Power-Supply Bypassing and Grounding The MAX471 has been designed as a “high side” (posi-tive 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 volt-age drops across a ground plane, and this drop may add to or subtract from V OUT. For highest current-mea-surement accuracy, use a single-point “star” ground.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-
tor 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 fil-
tered 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 and7), 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.
MAX471/MAX472 Precision, High-Side
Current-Sense Amplifiers ______________________________________________________________________________________11
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.
M A X 471/M A X 472
Precision, High-Side
Current-Sense Amplifiers ____Pin Configurations (continued)