Low-Cost, Precision, High-Side Current-Sense Amplifier MAX4172

Transcription

1 General Description The MAX472 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 MAX472 suitable for closed-loop battery-charger and general-purpose current-source applications. The 0 to 32 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 MAX472 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 () indicates when the supply voltage reaches an adequate level to guarantee proper operation of the current-sense amplifier. The MAX472 operates with a 3.0 to 32 supply voltage, and is available in a space-saving, 8-pin µmax or SO package. 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 Pin Configuration TOP IEW N.C. N.C MAX Benefits and Features Ideal for High-Side Monitoring 3 to 32 Supply Operation ±0.% Typical Full-Scale Accuracy Over Temperature High Accuracy +2 to +32 Common-Mode Range, Functional Down to 0, Independent of Supply oltage 800kHz Bandwidth [ SENSE = 00m (C)] 200kHz Bandwidth [ SENSE = 6.2m (C/6)] Minimizes Board Space Requirements µmax and SO Packages Ordering Information PART TEMP RANGE PIN-PACKAGE MAX472ESA+ MAX472EUA+ -40 C to +8 C -40 C to +8 C 8 SO 8 µmax +Denotes a lead(pb)-free/rohs-compliant package. Typical Operating Circuit UNREGULATED DC SUPPLY 3 TO 32 LOW-COST SWITCHING REGULATOR R SENSE 0mΩ SENSE FEEDBACK LOOP = 00m/A 0 TO 32 ANALOG OR LOGIC SUPPLY 00kΩ I = SENSE / 00Ω R kω 2A MAX472 POWER GOOD LOAD/ BATTERY μmax/so µmax is a registered trademark of Maxim Integrated Products, Inc. LOW-COST BATTERY CHARGER/CURRENT SOURCE 9-84; Rev 3; /

2 Absolute Maximum Ratings,,, to to ( + 0.3) Differential Input oltage, -...±700m Current into Any Pin...±0mA Continuous Power Dissipation (T A = +70 C) SO (derate.88mw/ C above +70 C)...47mW µmax (derate 4.0mW/ C above +70 C)...330mW Operating Temperature Range MAX472E_A C to +8 C Storage Temperature Range...-6 C to +0 C Lead Temperature (soldering, 0s) C Soldering Temperature (reflow) 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. Electrical Characteristics ( = +3 to +32;, = 0 to 32; T A = T MIN to T MAX ; unless otherwise noted. Typical values are at = +2, = 2, T A = +2 C.) PARAMETER Operating oltage Range Input oltage Range Supply Current Input Offset oltage Positive Input Bias Current Negative Input Bias Current Maximum SENSE oltage Low-Level Current Error SYMBOL I OS I I CONDITIONS MIN TYP MAX I = 0mA = 2, = 2 MAX472ESA ±0. ±0.7 MAX472EUA ±0.2 ± > 2.0, I = 0mA , I = 0mA > SENSE = 6.2m, = 2, MAX472ESA ±8.0 = 2 (Note ) MAX472EUA ± MAX472ESA, T A = -40 C to 0 C ±20 UNITS ma m m Output Current Error SENSE = 00m, = 2, = 2 MAX472EUA, T A = -40 C to 0 C MAX472ESA, T A = 0 C to +8 C ±0 ±0 MAX472EUA, T A = 0 C to to +8 C ± Power-Supply Rejection Ratio ΔI /Δ 3 32, > / Common-Mode Rejection Ratio ΔI /Δ 2.0 < < / Maxim Integrated 2

3 Electrical Characteristics (continued) ( = +3 to +32;, = 0 to 32; T A = T MIN to T MAX ; unless otherwise noted. Typical values are at = +2, = 2, T A = +2 C.) Maximum Output oltage () Bandwidth PARAMETER Maximum Output Current Transconductance Threshold for Output Low (Note 2) Output Low oltage Leakage Current into Power-Off Input Leakage Current (, ) SYMBOL I G m OL I.mA CONDITIONS MIN TYP MAX -.2 SENSE = 00m 800 SENSE = 6.2m (Note ) G m = I /( - ), T A = 0 C to +8 C SENSE = 00m, > 2.0 T A = -40 C to 0 C rising 2.77 falling 2.67 I SINK =.2mA, = 2.9, T A = +2 C 0.4 = 2., T A = +2 C UNITS = 0, = = khz ma ma/ Rise Time SENSE = 0 to 00m, 0% to 90% 400 ns Fall Time SENSE = 00m to 0m, 90% to 0% 800 ns Settling Time to % SENSE = m to 00m Rising Falling.3 6 µs Output Resistance SENSE = 0m 20 MΩ Note : 6.2m = /6 of typical full-scale sense voltage (C/6). Note 2: alid operation of the MAX472 is guaranteed by design when is low. Typical Operating Characteristics ( = +2, = 2, R = kω, T A = +2 C, unless otherwise noted.) SUPPLY CURRENT (μa) SUPPLY CURRENT vs. SUPPLY OLTAGE T A = +8 C T A = +2 C T A = -40 C MAX PUT ERROR vs. SUPPLY OLTAGE T A = +2 C SENSE = 00m T A = +8 C MAX C/6 LOAD PUT ERROR vs. SUPPLY OLTAGE SENSE = 6.2m T A = -40 C T A = +2 C MAX () I = 0mA T A = -40 C () T A = +8 C () 40 Maxim Integrated 3

4 Typical Operating Characteristics (continued) ( = +2, = 2, R = kω, T A = +2 C, unless otherwise noted.) ERROR vs. SENSE OLTAGE MAX POWER-SUPPLY REJECTION RATIO vs. FREQUENCY SENSE = 00m MAX P-P 0. P-P 0 m P-P - 0.m m 0m 00m SENSE () POWER-SUPPLY FREQUENCY (khz) PUT ERROR vs. COMMON-MODE OLTAGE T A = +2 C T A = +8 C SENSE = 00m T A = -40 C MAX TRIP THRESHOLD () THRESHOLD FOR PUT LOW vs. TEMPERATURE FALLING OLTAGE RISING OLTAGE MAX () TEMPERATURE ( C) 0 to 0m SENSE TRANSIENT RESPONSE MAX to 00m SENSE TRANSIENT RESPONSE MAX SENSE m/div SENSE 0m/div 0m/div 00m/div 0μs/div 0μs/div Maxim Integrated 4

5 Typical Operating Characteristics (continued) ( = +2, = 2, R = kω, T A = +2 C, unless otherwise noted.) STARTUP DELAY MAX472-0 to POWER-UP DELAY MAX472-00m/div 2/div 2/div 2/div SENSE = 00m μs/div 0μs/div 00kΩ PULLUP RESISTOR FROM TO +4 Pin Description PIN 2 3, NAME N.C. 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. Ground Current Output. is proportional to the magnitude of the sense voltage ( - ). A kω resistor from to ground will result in a voltage equal to 0/ of sense voltage. Power Good Open-Collector Logic Output. A low level indicates that is sufficient to power the MAX472, and adequate time has passed for power-on transients to settle out. Supply oltage Input for the MAX472 FUNCTION Detailed Description The MAX472 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 (). The MAX472 current-sense amplifier s unique topology simplifies current monitoring and control. The MAX472 s amplifier operates as shown in Figure. The battery/load current flows through the external sense resistor (RSENSE), from the node to the RSnode. Current flows through RG and Q, and into the current mirror, where it is multiplied by a factor of 0 before appearing at. To analyze the circuit of Figure, assume that current flows from to, and that is connected to through a resistor. Since A s inverting input is high impedance, no current flows though RG2 (neglecting the input bias current), so A s negative input is equal to SOURCE - (ILOAD x RSENSE). A s open-loop gain forces its positive input to essentially the same voltage level as the negative input. Therefore, the drop Maxim Integrated

6 across R G equals I LOAD x R SENSE. Then, since I RG flows through R G, I RG x R G = I LOAD x R SENSE. The internal current mirror multiplies I RG by a factor of 0 to give I = 0 x I RG. Substituting I /0 for I RG, (I/0) x RG = ILOAD x RSENSE, or: I = 0 x I LOAD x (R SENSE /R G ) The internal current gain of 0 and the factory-trimmed resistor R G combine to result in the MAX472 transconductance (G m ) of 0mA/. G m is defined as being equal to I /( - ). Since ( - ) = ILOAD x RSENSE, the output current (I) can be calculated with the following formula: I = G m x ( - ) = (0mA/) x (I LOAD x R SENSE ) Current Output The output voltage equation for the MAX472 is given below: = (G m ) x (R SENSE x R x I LOAD ) where = the desired full-scale output voltage, I LOAD = the full-scale current being sensed, R SENSE = the current-sense resistor, R = the voltage-setting resistor, and Gm = MAX472 transconductance (0mA/). The full-scale output voltage range can be set by changing the R resistor value, but the output voltage must be no greater than -.2. The above equation can be modified to determine the R required for a particular full-scale range: R = ()/(ILOAD x RSENSE x Gm) is a high-impedance current source that can be integrated by connecting it to a capacitive load. Output The output is an open-collector logic output that indicates the status of the MAX472 s power supply. A logic low on the output indicates that is sufficient to power the MAX472. This level is temperature dependent (see Typical Operating Characteristics graphs), and is typically 2.7 at room temperature. The internal comparator has a 00m (typical) hysteresis to prevent possible oscillations caused by repeated toggling of the output, making the device ideal for power-management systems lacking soft-start capability. An internal delay (µs typical) in the comparator allows adequate time for power-on transients to settle out. The 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. INPUT I RG R G R SENSE Applications Information Q :0 CURRENT MIRROR Figure. Functional Diagram SENSE Suggested Component alues for arious Applications The Typical Operating Circuit is useful in a wide variety of applications. Table shows suggested component values and indicates the resulting scale factors for various applications required to sense currents from 00mA to 0A. Adjust the RSENSE value to monitor higher or lower current levels. Select R SENSE using the guidelines and formulas in the following section. Sense Resistor, RSENSE Choose RSENSE based on the following criteria: oltage Loss: A high RSENSE value causes the power-source voltage to degrade through IR loss. For minimal voltage loss, use the lowest RSENSE value. A TH R G2 I = 0 I RG I LOAD MAX472 TO LOAD/ BATTERY Maxim Integrated 6

7 Table. Suggested Component alues FULL-SCALE LOAD CURRENT (A) CURRENT-SENSE RESISTOR, R SENSE (mω) PUT RESISTOR, R (kω) FULL-SCALE PUT OLTAGE, () SCALE FACTOR, /I SENSE (/A) 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. For best performance, select RSENSE to provide approximately 00m of sense voltage for the full-scale current in each application. Efficiency and Power Dissipation: At high current levels, the I 2 R losses in R SENSE 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 Ω. Cost: If the cost of R SENSE is an issue, you might want to use an alternative solution, as shown in Figure 2. This solution uses the PCB 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 0A, and that you have determined a 0.3-inchwide, 2-ounce copper to be appropriate. The resistivity of 0.-inch-wide, 2-ounce (70µm thickness) copper is 30mΩ/ft. For 0A, you might want RSENSE = mω for a 0m drop at full scale. This resistor requires about 2 inches of 0.-inch-wide copper trace. INPUT R SENSE LOAD/BATTERY O.3 in. COPPER O. in. COPPER O.3 in. COPPER 2 SENSE MAX472 Current-Sense Adjustment (Resistor Range, Output Adjust) Choose R after selecting RSENSE. Choose R to obtain the full-scale voltage you require, given the fullscale I determined by R SENSE. s high impedance permits using R values up to 200kΩ with minimal error. s load impedance (e.g., the input of an op amp or ADC) must be much greater than R (e.g., 00 x R) to avoid degrading measurement accuracy. High-Current Measurement The MAX472 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, PCB traces can be adjusted over a wide range. 8 SUPPLY 3 TO 32 R Figure 2. MAX472 Connections Showing Use of PC Board 6 Maxim Integrated 7

8 Power-Supply Bypassing and Grounding In most applications, grounding the MAX472 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. Use a single-point star ground for the highest currentmeasurement accuracy. The MAX472 requires no special bypassing and responds quickly to transient changes in line current. If the noise at caused by these transients is a problem, you can place a µf capacitor at the pin to ground. You can also place a large capacitor at the RS terminal (or load side of the MAX472) to decouple the load, reducing the current transients. These capacitors are not required for MAX472 operation or stability. The and inputs can be filtered by placing a capacitor (e.g., µf) between them to average the sensed current. Chip Information SUBSTRATE CONNECTED TO Package Information For the latest package outline information and land patterns (footprints), go to /packages. Note that a +, #, or - in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE LINE NO. LAND PATTERN NO. SO S µmax U Maxim Integrated 8

9 Revision History REISION NUMBER REISION DATE DESCRIPTION PAGES CHANGED 0 2/96 Initial release 6/0 Clarified 0 to 2 is not a high-accuracy range for the device, removed future product reference, added lead-free options and soldering temperature 2 0/2 Revised the Package Information 8 3 / Revised Benefits and Features section, 2 For pricing, delivery, and ordering information, please contact Maxim Direct at , or visit Maxim Integrated s website at. Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance. Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc. 20 Maxim Integrated Products, Inc. 9