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LM3815M-7.0 Folha de dados(PDF) 11 Page - National Semiconductor (TI) |
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LM3815M-7.0 Folha de dados(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page PWM Output and Current Accuracy Offset The PWM output is quantized to 128 levels. Therefore, the duty cycle can change only in increments of 1/128. There is a one-half (0.5) quantization cycle delay in the out- put of the PWM circuitry. That is to say that instead of a duty cycle of N/128, the duty cycle actually is (N+1⁄2)/128. The quantization error can be corrected for if a more precise result is desired. To correct for this error, simply subtract 1/256 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. An offset of 1⁄2 bit is 8 times larger than for precision mode parts, and results in approximately 8.8 mA for a 1 Amp part is 88 mA for a 7 Amp part. Jitter In addition to quantization, the duty cycle will contain some jitter. The jitter is quite small (for example, the standard de- viation of jitter is only 0.1% for the LM3814/15-1.0). Statisti- cally the jitter can cause an error in a current sample. Be- cause 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 maxi- mum error caused by jitter. The crest factor defines a limit within which 99.7% of the samples fall. The crest factor is de- fined as ±0.3% error in the duty cycle. Since the jitter is a random variable, averaging multiple out- puts will reduce the effective jitter. Obeying statistical laws, the jitter is reduced by the square root of the number of read- ings 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. 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 cur- rent 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 ele- ment and the die gets larger, and an error develops. Eventu- ally the temperature difference reaches steady state, which accounts for the under-damped exponential response. 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 measure- ment (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 av- eraging several outputs. Although there is variation in suc- cessive readings, a very accurate measurement can be ob- tained by averaging the readings. For example, on averaging the readings shown in this example, the average current measurement is 497.5 mA ( Figure 8). This value is very close to the actual value of 500 mA. Moreover, the ac- curacy depends on the number of readings that are averaged. DS101013-23 FIGURE 7. Transient Response to 7 Amp Step Current DS101013-25 FIGURE 8. Typical Response of LM3814-1.0/LM3815-1.0 www.national.com 11 |
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