LM3812,LM3813 LM3812/LM3813 Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM Output Literature Number: SNOS028C LM3812/LM3813 Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM Output 2% accuracy at room temperature (includes accuracy of General Description The LM3812/LM3813 Current Gauges provide easy to use precision current measurement with virtually zero insertion loss (typically 0.004). The LM3812 is used for high-side sensing and the LM3813 is used for low-side sensing. A Delta Sigma analog to digital converter is incorporated to precisely measure the current and to provide a current averaging function. Current is averaged over 50 msec time periods in order to provide immunity to current spikes. The ICs have a pulse-width modulated (PWM) output which indicates the current magnitude and direction. The shutdown pin can be used to inhibit false triggering during start-up, or to enter a low quiescent current mode. The LM3812 and LM3813 are factory-set in two different current options. The sense range is -1A to +1A or -7A to +7A. The sampling interval for these parts is 50ms. If faster sampling is desired, please refer to the data sheets for the part numbers LM3814 and LM3815. Key Specifications Ultra low insertion loss (typically 0.004) 2V to 5.25V supply range the internal sense element) (LM3812-1.0, LM3813-1.0) Low quiescent current in shutdown mode (typically 2.5 A) 50 msec sampling interval Features No external sense element required PWM output indicates the current magnitude and direction PWM output can be interfaced with microprocessors Precision current-sense technique Low temperature sensitivity Internal filtering rejects false trips Internal Power-On-Reset (POR) Applications Battery charge/discharge gauge Motion control diagnostics Power supply load monitoring and management Resettable smart fuse for High-Side Sensing Connection Diagrams 10012203 10012201 Top View LM3812 (c) 2011 National Semiconductor Corporation Top View LM3813 for Low-Side Sensing 100122 100122 Version 13 Revision 3 www.national.com Print Date/Time: 2011/09/22 15:32:17 LM3812/LM3813 Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM Output OBSOLETE September 22, 2011 LM3812/LM3813 Ordering Information Sense Range Sampling Interval* Sensing Method NS Package Number Package Type LM3812M-1.0 1A 50 ms High-side M08A SO-8 95 units in Rails LM3812MX-1.0 1A 50 ms High-side M08A SO-8 2.5k units on Tape and Reel LM3812M-7.0 7A 50 ms High-side M08A SO-8 95 units in Rails LM3812MX-7.0 7A 50 ms High-side M08A SO-8 2.5k units on Tape and Reel LM3813M-1.0 1A 50 ms Low-side M08A SO-8 95 units in Rails LM3813MX-1.0 1A 50 ms Low-side M08A SO-8 2.5k units on Tape and Reel LM3813M-7.0 7A 50 ms Low-side M08A SO-8 95 units in Rails LM3813MX-7.0 7A 50 ms Low-side M08A SO-8 2.5k units on Tape and Reel Order No.# Supplied As: Suffix M indicates that the part is available in Surface Mount package. Suffix X indicates that the part is available in 2.5k units on Tape and Reel. * Current is sampled over a fixed interval. The average current during this interval is indicated by the duty cycle of the PWM output during next interval. The Package code M08A is internal to National Semiconductor and indicates an 8-lead surface mount package, SO-8. # Pin Descriptions (High-Side, LM3812) Pin Name Function 1 SENSE+, VDD High side of internal current sense, also supply voltage. 2 SENSE- Low side of internal current sense. 3 FLTR+ Filter input -- provides anti-aliasing for delta sigma modulator. 4 FLTR- Filter input. 5 SD Shutdown pin. Connected to VDD through a pull up resistor for normal operation. When low, the IC goes into a low current mode (typically 3 A). 6 PWM PWM output indicates the current magnitude and direction. 7 GND Ground 8 GND Ground Pin Descriptions (Low-Side, LM3813) Pin Name Function 1 SENSE+, GND High side of internal current sense, also ground. 2 SENSE- Low side of internal current sense. 3 FLTR+ Filter input - provides anti-aliasing for delta sigma modulator. 4 FLTR- Filter input. 5 SD Shutdown pin. Connected to VDD through a pull up resistor for normal operation. When low, the IC goes into a low current mode (typically 3 A). 6 PWM PWM output indicates the current magnitude and direction. 7 GND Ground 8 VDD VDD (supply) www.national.com 2 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Absolute Maximum Supply Voltage Power Dissipation ESD Susceptibility (Note 3) Sense Current (peak, for 200 msec) (Note 4) Sink Current for PWM pin Voltage on Pin 5 Operating Ratings 5.5V (Note 2) 1.5 kV 10A 1mA 5.25V 150C -65C to +150C 260C (Note 1) Input Voltage Sense Current (continuous) (Note 4) Junction Temperature Range 2.0V to 5.25V 7A -40C to +125C Electrical Characteristics LM3812-1.0, LM3813-1.0 VDD = 5.0V for the following specifications. Supply bypass capacitor is 1 F and filter capacitor is 0.1 F. Symbol IACC Parameter Conditions Average Current Accuracy (Note 7) at 0.9A current Typ (Note 5) Limit (Note 6) 0.9 A 0.882 / 0.864 0.918 / 0.936 en Effective Output Noise (rms) Units 2 A (min) A (max) mA LM3812-7.0, LM3813-7.0 VDD = 5.0V for the following specifications. Supply bypass capacitor is 1 F and filter capacitor is 0.1 F. Symbol IACC en Parameter Conditions Average Current Accuracy (Note 7) at 2.5A current (Note 8) Typ (Note 5) Limit (Note 6) 2.5 2.400 / 2.350 2.600 / 2.650 Effective Output Noise (rms) 20 Units A A (min) A (max) mA Common Device Parameters Unless otherwise specified, VDD = 5.0V for the following specifications. Supply bypass capacitor is 1 F and filter capacitor is 0.1 F. Symbol IQ1 IQ2 Parameter Conditions Quiescent Current Normal Mode, SD = high Quiescent Current Shutdown Mode, SD = low Typ (Note 5) Limit (Note 6) Units 160 A (max) 10 A A (max) 100 A 2.5 DRES PWM Resolution 0.1 % tS Sampling Time 52 ms fP VTH VTL Frequency of PWM Waveform ms (min) ms (max) 12.5 25 Hz Hz (min) Hz (max) 1.8 V V (min) 0.7 V V (max) 20 Threshold High Level for SD 1.2 Threshold Low Level for SD 1.3 3 100122 Version 13 Revision 3 40 80 Print Date/Time: 2011/09/22 15:32:17 www.national.com LM3812/LM3813 Maximum Junction Temperature Storage Temperature Lead Temperature (Soldering, 10 sec) Absolute Maximum Ratings (Note 1) LM3812/LM3813 Symbol VOH VOL PI Parameter Conditions Logic High Level for PWM Logic Low Level for PWM Insertion Loss Typ (Note 5) Limit (Note 6) Load current = 1 mA, 2V VDD 5.25V VDD - 0.05 Sink current = 1 mA, 2V VDD 5.25V ISENSE = 1A (Note 9) Units VDD - 0.2 V V (min) 0.2 V V (max) 0.04 0.004 Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test conditions. Note 2: At elevated temperatures, devices must be derated based on package thermal resistance. The device in the surface-mount package must be derated at JA= 150C/W (typically), junction-to-ambient. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 k resistor into each pin. Note 4: The absolute maximum peak and continuous currents specified are not tested. These specifications are dependent on the JA, which is 150C/W for the S08 package. Note 5: Typical numbers are at 25C and represent the most likely parametric norm. Specifications in standard type face are for TJ= 25C and those with boldface type apply over full operating temperature ranges. Note 6: Limits are 100% production tested at 25C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control (SQC) methods. The limits are used to calculate National's Averaging Outgoing Quality Level (AOQL). Note 7: There is a variation in accuracy over time due to thermal effects. Please refer to the "PWM Output and Current Accuracy" section for more information. Note 8: The PWM accuracy for LM3812-7.0 and LM3813-7.0 depends on the amount of copper area under pins 1 and 2, and the layout. Please refer to the "PWM Output and Current Accuracy" section for more information. Note 9: The tolerance of the internal lead frame resistor is corrected internally. The temperature coefficient of this resistor is 2600 ppm/C. www.national.com 4 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 Supply bypass capacitor is 0.1 F and filter capacitor is 0.1 F. Measured Current vs Actual Current (LM3812-1.0 and LM3813-1.0) Measured Current vs Actual Current (LM3812-7.0 and LM3813-7.0) 10012215 10012224 PWM Frequency vs Supply Voltage PWM Frequency vs Temperature 10012216 10012213 Operating Current vs Supply Voltage Shutdown Current vs Supply Voltage 10012218 10012219 5 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 www.national.com LM3812/LM3813 Typical Performance Characteristics LM3812/LM3813 Operating Current vs Temperature Shutdown Current vs Temperature 10012220 10012221 Current vs Duty Cycle Accuracy vs Supply Voltage 10012222 10012228 Accuracy vs Temperature (LM3812-1.0 and LM3813-1.0) Accuracy vs Temperature (LM3812-7.0 and LM3813-7.0) 10012229 www.national.com 10012230 6 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 10012231 10012227 Note 10: These curves represent a statistical average such that the noise is insignificant. 7 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 www.national.com LM3812/LM3813 Error vs Current (LM3812-7.0 and LM3813-7.0) (Note 10) Error vs Current (LM3812-1.0 and LM3813-1.0) (Note 10) LM3812/LM3813 Typical Application Circuits In the application circuits, the 0.1 F ceramic capacitor between pins 1 and 8 is used for bypassing, and the 0.1 F ceramic capacitor between pins 3 and 4 is used for filtering. Shutdown (SD) is tied to VDD through a 10 k resistor. 10012205 FIGURE 1. High Side Sense 10012206 FIGURE 2. Low Side Sense www.national.com 8 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 LM3812/LM3813 10012207 FIGURE 3. Paralleling LM3812 for Higher Load Current ITOTAL = 2.2(D1-0.5)IMAX + 2.2(D2-0.5)IMAX where D1 is the duty cycle of PWM1 and D2 is the duty cycle of PWM2. Please refer to the Product Operation section for more information. 9 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 www.national.com LM3812/LM3813 10012208 FIGURE 4. High Voltage Operation -- VIN Greater Than 5.25V (High Side Sense) (PWM output is referred to Pin 7) 10012209 FIGURE 5. High Voltage Operation -- VIN Greater Than 5.25V (Low Side Sense) www.national.com 10 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 The current is sampled by the delta-sigma modulator, as illustrated in Figure 6. The pulse density output of the deltasigma modulator is digitally filtered. The digital output is then compared to the output of a digital ramp generator. This produces a PWM output. The duty cycle of the PWM output is proportional to the amount of current flowing. A duty cycle of 50% indicates zero current flow. If the current is flowing in positive direction, the duty cycle will be greater than 50%. Conversely, the duty cycle will be less than 50% for currents flowing in the negative direction. A duty cycle of 95.5% (4.5%) indicates the current is at IMAX (-IMAX). The IC can sense currents from -IMAX to +IMAX. Options for IMAX are 1A or 10A. The sense current is given by: ISENSE = 2.2 (D-0.5)(IMAX) where D is the duty cycle of the PWM waveform, and IMAX is the full scale current (1A or 10A). Similarly, the duty cycle is given by: D = [ISENSE/(2.2 IMAX)] + 0.5 10012210 FIGURE 6. Functional block diagram of LM3812 and LM3813 11 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 www.national.com LM3812/LM3813 For quick reference, see the Conversion Tables in Table 1 and Table 2. The user should note that, while the LM3812-7.0/ LM3813-7.0 will read 10A full scale, it is rated for 10A operation for a duration of no more than 200 msec, and 7A operation continuously. In this IC, the current is averaged over 50 msec time slots. Hence, momentary current surges of less than 50 msec are tolerated. This is a sampled data system which requires an anti-aliasing filter, provided by the filter capacitor. The delta-sigma modulator converts the sensed current to the digital domain. This allows digital filtering, and provides immunity to current and noise spikes. This type of filtering would be difficult or impossible to accomplish on an IC with analog components. When ordering, the user has to specify whether the part is being used for low-side or high-side sense. The user also needs to specify the full scale value. See the Ordering Information table for details. Product Operation LM3812/LM3813 the die gets larger, and an error develops. Eventually the temperature difference reaches steady state, which accounts for the under-damped exponential response. PWM Output and Current Accuracy OFFSET The PWM output is quantized to 1024 levels. Therefore, the duty cycle can change only in increments of 1/1024. There is a one-half (0.5) quantization cycle delay in the output of the PWM circuitry. That is to say that instead of a duty cycle of N/1024, the duty cycle actually is (N+1/2)/1024. The quantization error can be corrected for if a more precise result is desired. To correct for this error, simply subtract 1/2048 from the measured duty cycle. The extra half cycle delay will show up as a DC offset of 1/2 bit if it is not corrected for. This is approximately 1.1 mA for 1 Amp parts, and 11 mA for 7 Amp parts. JITTER In addition to quantization, the duty cycle will contain some jitter. The jitter is quite small (for example, the standard deviation of jitter is only 0.1% for the LM3812/13-1.0). Statistically the jitter can cause an error in a current sample. Because the jitter is a random variable, the mean and standard deviation are used. The mean, or average value, of the jitter is zero. The standard deviation (0.1%) can be used to define the peak error caused from jitter. The "crest factor" has often been used to define the maximum error caused by jitter. The crest factor defines a limit within which 99.7% of the samples fall. The crest factor is defined as 0.3% error in the duty cycle. Since the jitter is a random variable, averaging multiple outputs will reduce the effective jitter. Obeying statistical laws, the jitter is reduced by the square root of the number of readings that are averaged. For example, if four readings of the duty cycle are averaged, the resulting jitter (and crest factor) are reduced by a factor of two. 10012223 FIGURE 7. Transient Response to 7 Amp Step Current ACCURACY VERSUS NOISE The graph shown in Figure 8 illustrates the typical response of 1 Ampere current gauges. In this graph, the horizontal axis indicates time, and the vertical axis indicates measured current (the PWM duty cycle has been converted to current). The graph was generated for an actual current of 500 mA. The difference between successive readings manifests itself as jitter in the PWM output or noise in the current measurement (when duty cycle of the PWM output is converted to current). The accuracy of the measurement depends on the noise in the current waveform. The accuracy can be improved by averaging several outputs. Although there is variation in successive readings, a very accurate measurement can be obtained by averaging the readings. For example, on averaging the readings shown in this example, the average current measurement is 502.3 mA (Figure 8). This value is very close to the actual value of 500 mA. Moreover, the accuracy depends on the number of readings that are averaged. JITTER AND NOISE Jitter in the PWM output appears as noise in the current measurement. The Electrical Characteristics show noise measured in current RMS (root mean square). Arbitrarily one could specify PWM jitter, as opposed to noise. In either case the effect results in a random error in an individual current measurement. Noise, just like jitter, can be reduced by averaging many readings. The RMS value of the noise corresponds to one standard deviation. The "crest factor" can be calculated in terms of current, and is equal to 3 sigma (RMS value of the noise). Noise will also be reduced by averaging multiple readings, and follows the statistical laws of a random variable. ACCURACY OF 7A VERSIONS The graph of Figure 7 shows two possible responses to a 7A current step. The flat response shows basically a 7A level with some noise. This is what is possible with a good thick trace and a good thermal connection to the IC on the sense pins. The second trace that asymptotically approaches a higher value shows what can happen under extremely poor thermal conditions. Here a very small wire connects the IC to the current source. The very small wire does not allow heat in the sense resistor to dissipate. Hence, as the sense resistor heats up, a temperature difference between the sense element and www.national.com 10012226 FIGURE 8. Typical Response of LM3812-1.0/LM3813-1.0 12 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 The following tables show how to convert the duty cycle of the PWM output to a current value, and vice versa. The quanti- TABLE 1. Current to Duty Cycle Conversion Table Sense Current (Amps)* Duty Cycle (%) Sense Current (Amps)* Duty Cycle (%) 1.00 95.5 -1.00 4.5 0.95 93.2 -0.95 6.8 0.90 90.9 -0.90 9.1 0.85 88.6 -0.85 11.4 0.80 86.4 -0.80 13.6 0.75 84.1 -0.75 15.9 0.70 81.8 -0.70 18.2 0.65 79.5 -0.65 20.5 0.60 77.3 -0.60 22.7 0.55 75.0 -0.55 25.0 0.50 72.7 -0.50 27.3 0.45 70.5 -0.45 29.5 0.40 68.2 -0.40 31.8 0.35 65.9 -0.35 34.1 0.30 63.6 -0.30 36.4 0.25 61.4 -0.25 38.6 0.20 59.1 -0.20 40.9 0.15 56.8 -0.15 43.2 0.10 54.5 -0.10 45.5 0.05 52.3 -0.05 47.7 0.00 50.0 -0.00 50.0 *Maximum Sense Current = 1.0 Amps for LM3812-1.0 and LM3813-1.0. The sense current should be multiplied by 10 for LM3812-7.0 and LM3813-7.0. 13 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 www.national.com LM3812/LM3813 zation error of 1/2 bit is not shown in these tables. Please see the "PWM Output and Current Accuracy" section for more details. Look-Up Tables LM3812/LM3813 TABLE 2. Duty Cycle to Current Conversion Table Duty Cycle (%) Sense Current (Amps) Duty Cycle (%) Sense Current (Amps) 95.5 0.990 50.0 -0.000 92.5 0.935 47.5 -0.055 90.0 0.880 45.0 -0.110 87.5 0.825 42.5 -0.165 85.0 0.770 40.0 -0.220 82.5 0.715 37.5 -0.275 80.0 0.660 35.0 -0.330 77.5 0.605 32.5 -0.385 75.0 0.550 30.0 -0.440 72.5 0.495 27.5 -0.495 70.0 0.440 25.0 -0.550 67.5 0.385 22.5 -0.605 65.0 0.330 20.0 -0.660 62.5 0.275 17.5 -0.715 60.0 0.220 15.0 -0.770 57.5 0.165 12.5 -0.825 55.0 0.110 10.0 -0.880 52.5 0.055 7.5 -0.935 50.0 0.000 5.0 -0.990 *Maximum Sense Current = 1.0 Amps for LM3812-1.0 and LM3813-1.0. The sense current should be multiplied by 10 for LM3812-7.0 and LM3813-7.0. Timing Diagram 10012211 Duty cycle of the PWM waveform during any sampling interval indicates the current magnitude (average) and direction during the previous sampling interval. FIGURE 9. Typical Timing Diagram for Mostly Positive Current www.national.com 14 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 LM3812/LM3813 Physical Dimensions inches (millimeters) unless otherwise noted 8-lead (0.150 Wide) Molded Small Outline Package See Ordering Information table for Order Numbers NS Package Number M08A 15 100122 Version 13 Revision 3 Print Date/Time: 2011/09/22 15:32:17 www.national.com LM3812/LM3813 Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM Output Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH(R) Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise(R) Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagicTM www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise(R) Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION ("NATIONAL") PRODUCTS. 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