MAX471CSA. I LOAD TO LOAD or CHARGER LOGIC SUPPLY DISCHARGE/CHARGE

Transcription

1 19-; Rev 2; 12/96 Precision, High-Side General Description The / 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 has an internal mω current-sense resistor and measures battery currents up to ±A. For applications requiring higher current or increased flexibility, the 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 output indicates current-flow direction, so the user can monitor whether a battery is being charged or discharged. Both devices operate from to 6, draw less than 1 over temperature, and include a 18 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 Typical Operating Circuit Features Complete High-Side Current Sensing Precision Internal Sense Resistor () 2% Accuracy Over Temperature Monitors Both Charge and Discharge A Sense Capability with Internal Sense Resistor () Higher Current-Sense Capability with External Sense Resistor () 1 Max Supply Current 18 Max Shutdown Mode to 6 Supply Operation 8-Pin DIP/SO Packages Ordering Information PART TEMP. RANGE PIN-PACKAGE CPA CSA EPA C to +7 C C to +7 C -4 C to +8 C 8 Plastic DIP 8 SO 8 Plastic DIP ESA -4 C to +8 C 8 SO CPA C to +7 C 8 Plastic DIP CSA C to +7 C 8 SO EPA -4 C to +8 C 8 Plastic DIP ESA -4 C to +8 C 8 SO Pin Configurations / TO 6 SHDN I LOAD 2 1k 2k I LOAD TO LOAD or CHARGER LOGIC SUPPLY DISCHARGE/CHARGE (1/A) TOP IEW SHDN DIP/SO Pin Configuration continued on last page. Maxim Integrated Products 1 For free samples & the latest literature: or phone

2 / ABSOLUTE MAXIMUM RATINGS Supply oltage,,, CC to...-., +4 RMS Current, to ( only)...±.a Peak Current, ( to )...see Figure Differential Input oltage, to ( only)...±. oltage at Any Pin Except only...-. to ( -.) only...-. to ( CC +.) oltage at...-. to +4 Current into SHDN,,,,, CC...±mA Current into...+1ma, -ma 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 ( = + to +6, T A = T MIN to T MAX, unless otherwise noted. Typical values are at T A = +2 C.) Continuous Power Dissipation (T A = +7 C) (Note 1): Plastic DIP (derate 17.mW/ C above +7 C)...1.4W SO (derate 9.9mW/ C above +7 C)...791mW : Plastic DIP (derate 9.9mW/ C above +7 C)...727mW SO (derate.88mw/ C above +7 C)...471mW Operating Temperature Ranges MAX47_C_A... C to +7 C MAX47_E_A...-4 C to +8 C Junction Temperature Range...-6 C to +1 C Storage Temperature Range...-6 C to +16 C Lead Temperature (soldering, 1sec)...+ C Note 1: Due to special packaging considerations, (DIP, SO) has a higher power dissipation rating than the. and must be soldered to large copper traces to achieve this dissipation rating. PARAMETER Supply oltage Supply Current Sense Current Sense Resistor Current-Sense Ratio No-Load Error Low-Level Error Power-Supply Rejection Ratio Threshold (I LOAD required to switch ) Output Leakage Current Sink Current Shutdown Supply Current SHDN Input Low oltage SHDN Input Low Current SHDN Input High oltage SHDN Input High Current SYMBOL CONDITIONS MIN TYP MAX 6 I I LOAD = A, excludes I 11 I LOAD ± R SENSE 7 I / I LOAD = 1A, C I LOAD = 1 E I LOAD = A, C 2. = 1 E. I LOAD = ma, C ±2. = 1 E ±. PSRR 6, I LOAD = 1A.1 C ±4. ±6. E ±7. = 6 I OL =..1 I (SHDN) SHDN = 2.4; CC = to IL I IL IH I IH SHDN = SHDN = 2.4 Output oltage Range - 1. Output Resistance R I LOAD =.A, = to ( - 1.) 1 MΩ Rise, Fall Time t R, t F I LOAD = ma to.a, R = 2kΩ, C = pf, 1% to 9% Settling Time to 1% of Final alue t s I LOAD = 1mA to.a, R = 2kΩ, C = pf UNITS A RMS mω ma/a %/ ma ma 4 µs 1 µs

3 ELECTRICAL CHARACTERISTICS ( CC = + to +6, = = 2Ω, T A = T MIN to T MAX, unless otherwise noted. Typical values are at T A = +2 C.) Supply oltage Supply Current PARAMETER Input Offset oltage (Note 2) Input Bias Current Input Bias-Current Matching Current Accuracy No-Load Error Low-Level Error Power-Supply Rejection Ratio Threshold ( SENSE required to switch ) Output Leakage Current Output Sink Current Shutdown Supply Current SHDN Input Low oltage SHDN Input Low Current SHDN Input High oltage SHDN Input High Current SYMBOL CONDITIONS MIN TYP MAX CC 6 I CC I LOAD = A, excludes I ; CC = to OS C 12 E 14 I, I 2 I OS I - I GR2 ±.4 ±. I RG /I SENSE = 1m, C ±2 CC = 1 (Note ) E ±2. CC = 1, C 2. SENSE = E CC = 1, C ±2. SENSE = m E ±. PSRR CC 6, SENSE = 1m.1 CC = 1 C 6 12 E 6 14 I CC(SHDN) IL I IL IH I IH = 6 =. SHDN = 2.4; CC = to 2 SHDN = SHDN = 2.4 Output oltage Range CC - 1. Output Resistance R I = 1.mA 1 MΩ Rise, Fall Time t R, t F SENSE = m to 1m, R = 2kΩ, C = pf, 1% to 9% Settling Time to 1% of Final alue t s SENSE = m to 1m, R = 2kΩ, C = pf UNITS µ %/ µ ma 4 µs 1 µs Maximum Output Current I 1. ma % / Note 2: OS is defined as the input voltage ( SENSE ) required to give minimum I. Note : SENSE is the voltage across the sense resistor.

4 / Typical Operating Characteristics (Typical Operating Circuit () or circuit of Figure 4, = = 2Ω, R = 2kΩ (), T A = +2 C, unless otherwise noted.) SUPPLY CURRENT () SUPPLY CURRENT vs. SUPPLY OLTAGE T A = +8 C T A = +2 C T A = -4 C MAX ISHDN () SHUTDOWN CURRENT vs. SUPPLY OLTAGE T A = -4 C T A = +2 C T A = +8 C MAX THRESHOLD (ma) THRESHOLD vs. SUPPLY OLTAGE T A = -4 C T A = +2 C T A = +8 C MAX1471- OFFSET CURRENT () RESISTANCE (mω) () NO-LOAD OFFSET CURRENT vs. SUPPLY OLTAGE T A = +2 C T A = -4 C T A = +8 C S+ = S () TO RESISTANCE vs. TEMPERATURE TEMPERATURE ( C) MAX MAX ERROR (%) I () () ERROR vs. LOAD CURRENT I LOAD FROM TO -9 I LOAD FROM TO I LOAD (A) NO-LOAD PUT ERROR vs. SUPPLY OLTAGE = = Ω T A = -4 C T A = +8 C T A = +2 C CC () - MAX PSRR (%) ERROR (%) () POWER-SUPPLY REJECTION RATIO vs. FREQUENCY I LOAD = 1A RS 1µF A Ω = TO. = TO 1 = m TO m POWER-SUPPLY FREQUENCY (khz) ERROR vs. SUPPLY OLTAGE T A = -4 C T A = +2 C - = 6m, = = 2Ω T A = +8 C CC () -6 MAX

5 Precision, High-Side Typical Operating Characteristics (continued) (Typical Operating Circuit () or circuit of Figure 4, = = 2Ω, R = 2kΩ (), T A = +2 C, unless otherwise noted.) ERROR (%) ERROR vs. SENSE OLTAGE I NOISE (RMS) NOISE vs. LOAD CURRENT -1 / SENSE (m) 1mA 1mA 1mA 1A ISENSE ma to 1mA TRANSIENT RESPONSE -1mA to +1mA TRANSIENT RESPONSE A LOAD CURRENT ma/div A LOAD CURRENT 1mA/div m/div 1µs/div CC = 1, R = 2kΩ 1%, PULL-UP = kω 1% m/div ma/div 1µs/div CC = 1, R = 2kΩ 1%, PULL-UP = kω 1% m/div START-UP DELAY A TO A TRANSIENT RESPONSE I LOAD 1A/div m/div SHDN /div 1m/div 1µs/div I LOAD = 1A, R = 2kΩ 1% R = 2kΩ 1% 1µs/div

6 / Pin Description 1 2, 4 6, 7 PIN NAME SHDN N.C. Shutdown. Connect to ground for normal operation. When high, supply current is less than. Battery (or power) side of the internal current-sense resistor. The + indicates direction of flow for output only. Connect pins 2 and 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 FUNCTION An open-collector logic output. For the, a low level indicates current is flowing from to. For the, a low level indicates a negative SENSE (see Figure 2). is high impedance when SHDN is high. Leave open if is not needed. Load side of the internal current-sense resistor. The - indicates direction of flow for 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. 7 CC Power input for. Connect to sense resistor (R SENSE ) junction with. 8 8 Current output that is proportional to the magnitude of the sensed current flowing through R SENSE. A 2kΩ resistor from this pin to ground will result in a voltage equal to 1/Amp of sensed current in the. Detailed Description The and current-sense amplifier s unique topology allows a simple design to accurately monitor current flow. The / contain two amplifiers operating as shown in Figures 1 and 2. The battery/load current flows from to (or vice versa) through RSENSE. Current flows through either and Q1 or 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 is identical to the, except that RSENSE and gain-setting resistors and are external (Figure 2). To analyze the circuit of Figure 1, assume that current flows from to and that is connected to through a resistor. In this case, amplifier A1 is active and output current I flows from the emitter of Q1. Since no current flows through (Q2 is off), the negative input of A1 is equal to SOURCE - (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 equals ILOAD x RSENSE. Then, since I flows through Q1 and RG (ignoring the extremely low base currents), I x = ILOAD x RSENSE, or: I = (I LOAD x R SENSE ) / Current Output The output voltage equation for the / is given below. In the, the current-gain ratio has been preset to /A so that an output resistor (R) of 2kΩ yields 1/A for a full-scale value of + at ±A. Other full-scale voltages can be set with different R values, but the output voltage can be no greater than - 1. for the or RG_ - 1. for the. = (RSENSE x R x ILOAD) / RG where = the desired full-scale output voltage, ILOAD = the full-scale current being sensed, RSENSE = the current-sense resistor, R = the voltage-setting resistor, and RG = the gain-setting resistor (RG = = ). The above equation can be modified to determine the R required for a particular full-scale range: R = ( x RG) / (ILOAD x RSENSE) For the, this reduces to: R = / (ILOAD x /A) is a high-impedance current-source output that can be connected to other / pins 6

7 2, Q1 A1 R SENSE A2 Q2 6, 7 8 / COMP Figure 1. Functional Diagram R SENSE POWER SOURCE OR BATTERY SENSE TO LOAD/CHARGER 6 A1 A2 Q1 Q2 7 CC 8 COMP Figure 2. Functional Diagram 7

8 / TO 6 LOGIC SUPPLY 1k TO LOAD/ CHARGER POWER SOURCE OR BATTERY TO 6 1 SHDN 2 N.C. 4 R SENSE 8 CC 7 6 TO LOAD/CHARGER LOGIC SUPPLY 1k R 1k Figure. Paralleling s to Sense Higher Load Current Figure 4. Standard Application Circuit for current summing. A single scaling resistor is required when summing currents from multiple devices (Figure ). Current can be integrated by connecting to a capacitive load. Output The current at indicates magnitude. The output indicates the current s direction. Operation of the 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 output indicates positive current flow (from to ). In battery-operated systems, this is useful for determining whether the battery is charging or discharging. The output may not correctly indicate if the load current is such that I is less than.. The s output accurately indicates the direction of current flow for load currents greater than 7mA. is an open-collector output (sinks current only), allowing easy interface with logic circuits powered from any voltage. Connect a 1kΩ pull-up resistor from to the logic supply. The convention chosen for the polarity of the output ensures that it draws no current when the battery is being discharged. If current direction is not needed, float the pin. Shutdown When SHDN is high, the / are shut down and consume less than 18. In shutdown mode, is high impedance and turns off. Applications Information The obtains its power from the pin. This includes current consumption in the total system current measured by the. The small drop across R SENSE does not affect the s performance. Resistor Selection Since delivers a current, an external voltage gainsetting resistor (R to ground) is required at the pin in order to get a voltage. R SENSE is internal to the. and are factory trimmed for an output current ratio (output current to load current) of /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 for the desired full-scale output voltage up to - 1. (see the Current Output section). 8

9 Peak Sense Current The s maximum sense current is A RMS. 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. SENSE CURRENT (A) R SENSE,, and are externally connected on the. CC can be connected to either the load/charge or power-source/battery side of the sense resistor. Connect CC to the load/charge side of R SENSE if you want to include the current drain in the measured current. Suggested Component alues for arious 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 A), and also for those where the fullscale load current is less than the A of the. Table 1. Suggested Component alues for the FULL-SCALE LOAD CURRENT, I SENSE (A) T A = +2 C 1µ CURRENT- SENSE RESISTOR, R SENSE (mω) PULSE WIDTH (sec) DIP safe operating region Small DIP Outline fuse fuse time time 1µ 1m 1m Small Outline safe operating region Figure. Pulse Current Safe Operation for 1, Pulses and Fuse Time for Continuous Current. Pulse tests done with 2mW average power dissipation. GAIN-SETTING RESISTORS, = (Ω) PUT RESISTOR, R (kω) Table 1 shows suggested component values and indicates the resulting scale factors for various applications required to sense currents from 1mA to 1A. 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. RSENSE Choose RSENSE based on the following criteria: a) oltage 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 RSENSE 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 2 R 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 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.4%/ C), so systems that experience a wide temperature variance should take this into account. FULL-SCALE PUT OLTAGE, () SCALE FACTOR, /I SENSE (/A) TYPICAL ERROR AT X% OF FULL LOAD (%) 1% 1% 1% / 9

10 / In Figure 6, assume the load current to be measured is 1A and that you have determined a. inch wide, 2 ounce copper to be appropriate. The resistivity of.1 inch wide, 2 ounce copper is mω/ft (see Note 4). For 1A you may want RSENSE = mω for a m drop at full scale. This resistor will require about 2 inches of.1 inch wide copper trace. and Once RSENSE is chosen, and can be chosen to define the current-gain ratio (RSENSE/RG). Choose RG = = 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 = Ω, 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. b) Efficiency. As RG is reduced, I gets larger for a given load current. Power dissipated in R 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. I is limited to 1.mA, requiring RG SENSE / 1.mA. For SENSE = 6m, RG must be 4Ω. d) Headroom. The requires a minimum of 1. between the lower of the voltage at or (RG_) and. As RG becomes larger, the voltage drop across RG also becomes larger for a given I. This voltage drop further limits the maximum full-scale. Assuming the drop across is small and CC is connected to either side of RSENSE, (max) = CC - (1. + I (max) x RG). e) Output Offset Error at Low Load Currents. Large RG values reduce I for a given load current. As I gets smaller, the 2. 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 TO LOAD/CHARGER R SENSE." COPPER.1" COPPER." COPPER TO SHDN N.C. make sure RG is small enough that IB and IOS do not add any appreciable errors. The full-scale error is given by: % Error = ( - ) x I B + IOS x RG x 1 IFS x RSENSE where and are the gain resistors, IB is the bias current, IOS is the bias-current mismatch, IFS is the full-scale current, and RSENSE is the sense resistor. Assuming a A load current, 1mΩ RSENSE, and 1Ω RG, the current-gain ratio is 1/A, yielding a fullscale I of. Using the maximum values for IB (2) and IOS (2), and 1% resistors for and ( - = 2Ω), the worst-case error at full scale calculates to: 2Ω x 2 + 1Ω x 2 =.48% mω x A The error may be reduced by: a) better matching of and, b) increasing RSENSE, or c) decreasing RG. Current-Sense Adjustment (Resistor Range, Output Adjust) Choose R after selecting RSENSE,, and. Choose R to obtain the full-scale voltage you Figure 6. Connections Showing Use of PC Board Trace CC k 1k Note 4: Printed Circuit Design, by Gerald L. Ginsberg; McGraw-Hill, Inc.; page 18. 1

11 require, given the full-scale I determined by RSENSE,, and. The high compliance of permits using R values up to 1kΩ with minimal error. alues above 1kΩ are not usually recommended. The impedance of s load (e.g., the input of an op amp or ADC) must be much greater than R (e.g., 1 x R) to avoid degrading the measurement accuracy. High-Current Measurement The can achieve higher current measurements than the 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 s in parallel and connect the high-impedance currentsource pins together to indicate the total system current (Figure ). 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 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 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. For highest current-measurement accuracy, use a single-point star ground. The / require no special bypassing, and respond quickly to transient changes in line current. If the noise at caused by these transients is a problem, you may want to place a 1µF capacitor at the pin to ground. You can also place a large capacitor at the terminal (or load side of the ) to decouple the load and, thereby, reduce the current transients. These capacitors are not required for / operation or stability, and their use will not degrade performance. For the, the and inputs can be filtered by placing a capacitor (e.g., 1µF) between them to average the sensed current. Layout The must be soldered in place, since sockets can cause uneven current sharing between the pins (pins 2 and ) and the pins (pins 6 and 7), resulting in typical errors of.%. In order to dissipate sense-resistor heat from large sense currents, solder the pins and the pins to large copper traces. Keep the part away from other heat-generating devices. This procedure will ensure continuous power dissipation rating. / 11

12 / Pin Configurations (continued) SHDN 1 8 N.C CC 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, 12 San Gabriel Drive, Sunnyvale, CA 9486 (48) Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.