The individual string currents are monitored by the ADuC7039. This device is a highly sophisticated system-on-a-chip with the capability to measure low-side current, stack voltage and single point temperature. This ARM-7 based device is also used to communicate with its associated series-connected string of Li-ion cell monitors, running cell balancing algorithms for that particular string and communicating with the master controller.
Control / Processing
In terms of control, ADI's latest low power fixed point Blackfin® DSP controller family is suitable for the control and communications functions required in this application. The BF50x family delivers 400MHz of processing performance in a dual multiply and accumulate architecture equivalent to 800 MegaMACS of performance.
The BF50x communicates with the various system blocks via ADI's proprietary iCoupler® isolators to allow the entire front end system to float and to electrically isolate the back-end from high voltages. When communicating at high data rates or over relatively long distances found within a typical battery stack, differential data transmission offers superior performance to single-ended transmission. Popular protocols for this communication task are RS-485 and CAN. Analog Devices offers a wide range of iCoupler®-based isolated RS-485/RS-422 transceivers to suit this application with the ADM248xE and ADM258xE families which include integrated isolated DC/DC converters. More recently, Analog Devices has released the first of a family of isolated CAN transceivers, the ADM3052/ADM3053, with and without an integrated isolated DC/DC converter. For proprietary protocols there are also numerous standard iCouplers available such as the ADuM140x family, which are quad channel isolators.
Measurement / Sensing
Typical dc current sensors used in battery stack applications include Hall Effect (HE) sensors and shunt resistors. Devices like the AD8212 are high side current monitors producing an analog output proportional to the current flow through a shunt while a device like the AD7400 is a sigma-delta modulator producing an output 1's density stream proportional to current flow, also through a shunt. This latter device incorporates its own isolation barrier inside the package to provide galvanic isolation between the sensed current and the digital output. Each sensor type has its own advantages and disadvantages but all require precision, low noise, low power op amps like ADI's OPX177 family to complete the signal conditioning demanded by the different current sensor signals.