Features
| • | No external sense element required |
| • | PWM output indicates the current magnitude and direction |
| • | PWM output can be interfaced with microprocessors |
| • | Precision [Delta][Sigma] current-sense technique |
| • | Low temperature sensitivity |
| • | Internal filtering rejects false trips |
| • | Internal Power-On-Reset (POR) |
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 Specification
| • | Ultra low insertion loss (typically 0.004) |
| • | 2V to 5.25V supply range |
| • | ±2% accuracy at room temperature (includes accuracy of the internal sense element) (LM3812-1.0, LM3813-1.0) |
| • | Low quiescent current in shutdown mode (typically 2.5 µA) |
| • | 50 msec sampling interval |
Applications
- Battery charge/discharge gauge
- Motion control diagnostics
- Power supply load monitoring and management
- Resettable smart fuse
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+½)/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 ½ bit if it is not corrected for. This is approximately 1.1 mA for 1 Amp parts, and 11 mA for 7 Amp parts.
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.
The graph of 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 the die gets larger, and an error develops. Eventually the temperature difference reaches steady state, which accounts for the under-damped exponential response.
The following tables show how to convert the duty cycle of the PWM output to a current value, and vice versa. The quantization error of ½ bit is not shown in these tables. Please see the "PWM Output and Current Accuracy" section for more details.
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 |
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 |
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