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AD7280 Datasheet(PDF) 10 Page - Analog Devices |
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AD7280 Datasheet(HTML) 10 Page - Analog Devices |
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10 / 33 page  AD7280 Preliminary Technical Data Rev. PrD | Page 10 of 33 THEORY OF OPERATION CIRCUIT INFORMATION The AD7280 is a Lithium Ion battery monitoring chip with the ability to monitor the voltage and temperature of 6 series connected battery cells. The AD7280 also provides an interface which can be used to control transistors for cell balancing. The VDD and VSS supplies required by the AD7280 can be taken from the upper and lower voltages of the series connected battery cells. An internal VREG rail is generated from the supply voltage which provides power for the ADC and the internal interface circuitry. This VREG voltage is available on an output pin for use external to the AD7280. The AD7280 consists of a high voltage input multiplexer, a low voltage input multiplexer and a 12 bit ADC. The high voltage multiplexer allows up to 6 series connected Lithium Ion battery cells to be measured. The low voltage multiplexer allows the temperature of each cell to be measured. A single CNVST signal is required to initiate conversions on all 12 channels, that is 6 voltage and 6 temperature channels. Alternatively the conversion can be initiated through the rising edge of CS on the SPI interface. Each conversion result is stored in a results register (See Register section). On power-up, the CNVST signal is the default option, this can be changed by writing to the CONTROL register. The default sequence of conversions completed following the CNVST signal, or software convert start, is all 6 voltage channels followed by all 6 temperature channels. Two further conversion sequences may be selected by the user, 6 voltage channels followed by 3 temperature channels or just 6 voltage channels. The conversion sequence may be selected by writing to the CONTROL register. Each voltage and temperature measurement requires a minimum of 1us to acquire and complete a conversion. Depending on the external components connected to the analog inputs of the AD7280 additional acquisition time may be required. A higher acquisition time may be selected through the CONTROL register. The user may also select the averaging option through the CONTROL register. This option allows the user to complete 2, 4 or 8 averages on each cell voltage and cell temperature measurement. The averaged conversion results are stored in the results registers. On power-up the default combined acquisition and conversion time will be 1us, with the averaging register set to zero, that is a single conversion per channel. The results of the voltage and temperature conversions are read back via the 4 wire Serial Peripheral Interface. The SPI interface is also used to write to and read data from the internal registers. The AD7280 features an ALERT function which is triggered if the voltage conversion results or the temperature conversion results exceed the maximum and minimum voltage thresholds selected by the user. The threshold levels are selected by writing to the internal registers. The AD7280 provides 6 analog output voltages which can be used to control external transistors as part of a cell balancing circuit. Each Cell Balance output provides a 0V or 5V voltage, with respect to the potential on base of each individual cell, which can be applied to the gate of the external cell balancing transistors. The AD7280 features a daisy chain interface. Individual AD7280s can monitor the cell voltages and temperatures of 6 cells, a chain of AD7280s can be used to monitor the cell voltages and temperatures of a larger number of cells. The conversion data from each AD7280 in the chain passes to the system controller via a single standard serial interface. Control data can similarly be passed via the standard serial interface up the chain to each individual AD7280s The AD7280 includes an on-chip 2.5V reference. The reference voltage is available for use external to the AD7280. CONVERTER OPERATION The AD7280 consists of a high voltage input multiplexer, a low voltage input multiplexer and a 12 bit ADC. The high voltage multiplexer selects which pair of analog inputs, Vin0 to Vin6, are to be converted. The voltage of each individual cell is measured by converting the difference between adjacent analog inputs, that is, Vin1 – Vin0, Vin2 – Vin1, etc. This is illustrated in Figure 4 and Figure 5. The conversion results for each cell may be accessed after the programmed conversion sequence is complete. The second multiplexer selects which voltage temperature input, VT1 to VT6, is to be converted. The conversion results for each cell may be accessed after the programmed conversion sequence is complete. Vin0 Vin1 Vin2 Vin3 Vin4 Vin5 Vin6 ADC Vin+ ADC Vin- Figure 4. MUX Configuration During Vin1-Vin0 Sampling |
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