Lithium-ion (Li-Ion) battery packs contain a large number of cells that must be properly monitored to improve battery efficiency, extend battery life and ensure safety. The 6-channel AD7280A device in the circuit shown in Figure 1 acts as the primary monitor, providing accurate voltage measurement data to the System Demonstration Platform (SDP-B) evaluation board, while the 6-channel AD8280 device serves as the secondary monitor and protection system. Both devices operate from a single supply with a wide operating voltage range of 8 V to 30 V and operate over the –40°C to +105°C industrial temperature range.

The AD7280A has a built-in ±3 ppm voltage reference that provides battery voltage measurement accuracy of ±1.6 mV. The ADC resolution is 12 bits, and it only takes 7 μs to convert 48 units.

The AD7280A has a cell balancing interface output that controls external FET transistors, allowing individual cells to discharge and forcing all cells in the stack to have the same voltage.

The AD8280 operates independently of the main monitor and provides an alarm function to indicate out-of-tolerance conditions. The device has its own built-in reference and LDO, both of which are powered entirely by the battery pack. The reference voltage source, along with an external resistor divider, is used to set the overvoltage/undervoltage trip points. Each battery channel contains programmable deglitch (D/G) circuitry to prevent transient input levels from triggering alarms.

The AD7280A and AD8280 are located on the high-voltage side of the battery management system (BMS) and feature a daisy-chain interface that allows up to eight AD7280A and eight AD8280s to be stacked together to monitor the voltage of 48 lithium-ion battery cells. Adjacent AD7280A and AD8280 in the stack can communicate directly, passing data up and down without isolation.

The master device at the bottom of the stack communicates with the SDP-B evaluation board using the SPI interface and GPIO. Only here is high voltage galvanic isolation required to protect the low voltage side of the SDP-B board. The digital isolators ADuM1400 , ADuM1401 and the isolator ADuM5404 with integrated DC-DC converter together provide the required 11 channels of isolation in a compact, cost-effective solution. The ADuM5404 also provides an isolated 5 V output for the VDRIVE input of the lower AD7280A and provides the VDD2 supply voltage for the ADuM1400 and ADuM1401 isolators.

The AD7280A is a complete data acquisition system that includes a high-voltage input multiplexer, a low-voltage input multiplexer, a 12-bit, 1 μs SAR ADC, and on-chip registers for channel timing control. The HV MUX is used to measure series connected lithium-ion battery cells as shown in Figure 1. The LV MUX provides a single-ended ADC input that can be used in conjunction with an external thermistor to measure the temperature of individual battery cells; if temperature measurement is not required, the auxiliary ADC input can be used to convert any other 0 V to 5 V input signal. A 2.5 V precision reference and on-chip voltage regulator are also available.

The AD8280 is a pure hardwired safety monitor for lithium-ion battery packs. When used with the AD7280A, it provides a low-cost, redundant, backup battery monitor with adjustable threshold detection and shared or individual alarm outputs. It has self-test capabilities, making it suitable for high-reliability applications such as hybrid electric vehicles or high-voltage industrial applications such as uninterruptible power supplies. Both the AD7280A and AD8280 obtain power from a monitored battery cell.

The ADuM5404 integrates a DC-DC converter to provide power to the high voltage side of the ADuM1400 and ADuM1401 isolators, as well as VDRIVE power to the AD7280A SPI interface. These 4-channel, magnetically isolated circuits are a safe, reliable, easy-to-use optocoupler replacement solution.

To optimize the communication performance of the daisy chain under high-noise conditions (such as when encountering battery interference), the daisy chain signals are shielded on an inner layer of the printed circuit board (PCB), with shielding provided above and below by the VSS power plane, which layer is connected to the VSS pin of the previous device in the daisy chain. Figure 2 shows the top layer of the EVAL-CN0235-SDP PCB, containing the upper shield of the AD7280A, and Figure 5 shows the bottom layer, containing the upper shield of the AD8280. Figure 3 shows the inner layer (Layer 2), which contains the shielded daisy chain signals, with the shielding below implemented on Layer 3 shown in Figure 4. Each daisy chain connection is equipped with 22 pF capacitors that are terminated at the VSS pin of the previous device or the VDD pin of the next device, depending on the direction of data flow in the daisy chain. The PD, CS, SCLK, SDI, and CNVST daisy chain connections pass data up the daisy chain, so the 22 pF capacitors on these pins are terminated to the VSS pin of the previous device.

The SDOlo and ALERTlo daisy chain connections pass data down the daisy chain, so the 22 pF capacitors on these pins are terminated to the VDD pin of the next device. Use a low-impedance trace to directly connect the VDD of the next device to the VSS of the previous device so that these two potentials are as close as possible in a high-noise environment.

The ground guardrail at the isolation barrier is used to surround the low-voltage end formed on the left side of the PCB. This guardrail consists of a guard ring tied together by vias, connected to the digital ground of all layers on the board. Noise reaching the power and ground planes at the edge of the board may be radiated, but with this shielding structure, the noise is reflected back.

Input-to-output dipole radiation may also occur when a current source is driven across the gap between ground planes. To minimize this effect, use a continuous shield across the isolation gap, thereby extending the ground plane to all layers of the PCB to utilize shield overlap for cross-isolation barrier coupling. Isolation gaps on the layers were kept to a minimum, with the test board using a 0.008-inch gap. For more recommendations on radiation control from isoPower® devices, such as the ADuM5404 used in this circuit, see application note AN-0971 .

Test Results

An important measure of the performance of this circuit is the amount of noise in the final output voltage measurement.

Figure 6 shows a histogram of 10,000 measured samples for the VIN3−VIN2 channel. This data was obtained using the CN0235 evaluation board connected to the EVAL-SDP-CB1Z System Demonstration Platform (SDP-B) evaluation board. See the "Circuit Evaluation and Testing" section of this circuit note for setup details.

Twelve lithium-ion batteries are connected to the input screw terminals. Note that only a small portion of the codewords are affected by noise and fall outside the main bin. Figures 6 and 7 show 3 LSB peak-to-peak noise corresponding to approximately 0.5 LSB rms.

For the complete design support package for this circuit note, see www.analog.com/CN0235-DesignSupport .