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LM3813M-1.0 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM3813M-1.0 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Product Operation The current is sampled by the delta-sigma modulator, as il- lustrated in Figure 6. The pulse density output of the delta-sigma 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 flow- ing 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 I MAX (−IMAX). The IC can sense currents from −I MAX to +IMAX. Options for IMAX are 1A or 10A. The sense current is given by: I SENSE = 2.2 (D−0.5)(IMAX) where D is the duty cycle of the PWM waveform, and I MAX is the full scale current (1A or 10A). Similarly, the duty cycle is given by: D = [I SENSE/(2.2 IMAX)] + 0.5 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 op- eration 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 Infor- mation table for details. DS100122-10 FIGURE 6. Functional block diagram of LM3812 and LM3813 www.national.com 10 |
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