Tag Archives: ATA6870

7/13-cell applications with Atmel’s ATA6870 (Part 2)

In part one of this series, Bits & Pieces took a closer look at several 7/13-cell application implementations with Atmel’s ATA6870. In part  two, we discuss a number of additional topics including ground offset, up-converting level shifter and down-converting level shifter.

Ground offset

Regardless of the MCU, additional circuitry must be inserted between the digital I/O pins connecting the MCU to the ATA6870 IC to compensate for the ground offset between these two devices. This offset/voltage difference occurs since the ATA6870 ground is no longer referenced to the same ground as the MCU. Meaning, the battery management MCU measures the voltage of the bottom-most cell in the battery stack, the VMCU.

The MCU also derives its supply voltage from VMCU, unlike the a standard measurement implementation where the MCU is supplied by the ATA68670 voltage regulator (VDDHVM). Due to this configuration, VMCU is now the ground reference for the bottom-most ATA6870 IC in the stack. As such, isolation between the two devices and level shifting of the digital I/O interconnections is required. A total of 6 digital I/O lines are affected by the ground offset and must be level shifted, including the SPI pins (MOSI, MISO, SCK and CS), the ADC reference clock (CLK) and the interrupt request line (IRQ).

Up-converting level shifter

Up-converting from the MCU to the ATA6870 is achieved through the use of the circuit shown in Figure 4.

batterymanagementfigure4

All ATA6870 SPI input signals (SCK, MOSI, CS_N) and the ADC reference clock (CLK) must use this circuit. The level shifter utilizes a high-speed switching NPN transistor and voltage divider to up-convert the low-level MCU output voltage to the regulated voltage supplied by the ATA6870 IC, VDDHVM (3.3V + VMCU). It should also be noted that resistor divider values are dependent upon the voltage offset between the MCU ground, GNDMCU, and the ATA6870 ground, GND.

Down-converting level shifter

Down-converting the voltage from the ATA6870 to the MCU is accomplished by the use of the circuit shown in Figure 5.

batterymanagementfigure5

All ATA6870 SPI output signals (MISO and IRQ) must use this circuit. The level shifter utilizes a P-channel MOSFET and a voltage divider to down-convert the ATA6870 IC’s output signal to the input voltage required by the MCU. As with the up-converted inputs, the same resistor divider rules apply.

“Since cost is the primary concern for any application, it is important to consider the type of architecture to be used. The 7/13-cell battery management application is a perfect example when it is useful to modify the system architecture in a way that completely eliminates one IC. In this case of a seven-cell application, the total IC count can be easily reduced from three ICs to two by replacing one ATA6870 circuit and the MCU with one Atmel battery measurement MCU,” Atmel engineering rep Darius Rydahl told Bits & Pieces.

“In making this architecture change to the system, the MCU and ATA6870 IC have different ground potentials, and the digital I/O pins between the two devices are offset by this difference in voltage. This offset can be easily eliminated by adding several low-cost transistors and resistors to the digital I/O lines that connect the two devices. The resulting cost-effective solution fulfills all end application requirements.”

7/13-cell applications with Atmel’s ATA6870 (Part I)

A standard (automotive) battery measurement system using Atmel’s ATA6870 is capable of measuring the voltage of up to 6 battery cells. Several of these ICs can be stacked in series to measure the voltage of up to 96 battery cells simultaneously. For the majority of applications, the “stacked” battery measurement IC approach is sufficient, as the number of cells measured in these applications is a multiple of three, four or six.

batterymanagementfigure1

“In some instances, such as an e-bike application, the cell count of the battery may be of an odd number: 7 or 13 cells,” Atmel engineering rep Darius Rydahl told Bits & Pieces. “With these applications, the use of multiple, stacked ATA6870 circuits combined with a standard microcontroller (MCU) may not be the most cost-effective solution for the end application.”

According to Rydahl, a more practical, lower cost implementation is to use one ATA6870 chip in conjunction with an Atmel battery management microcontroller.

“The standard implementation of an ATA6870 battery management system consists of at least one ATA6870 battery measurement IC (maximum sixteen, connected in series) plus a general-purpose MCU for control and data processing,” Rydahl continued. “As you can see in the image above (Figure 1), the MCU is powered by the lower ATA6870 IC’s on-board 3.3V voltage regulator (VDDHVM). Communication occurs via SPI where data is transferred serially between multiple ATA6870 circuits, one IC to the next, to/from the MCU.”

As shown in Figure 1, a common ground reference is shared between the bottom ATA6870 device and the MCU. In this instance, there is no voltage offset between the MCU and the ATA6870 circuit, neatly eliminating the need for additional interface circuitry between the CLK and SPI pins of the two ICs.

batterymanagementfigure2

In applications where the total cell count is a multiple of 7 or 13, the designer can simply add additional ATA6870 ICs to the battery stack as shown in Figure 2. However, the 7 battery cells must be split between the ICs to maintain the minimum operating voltage of 6.7V for each ATA6870 IC.

“Atmel offers two possible solutions for the seven-cell application using a battery measurement MCU as shown in Figure 3. In this example, the ATA68670 IC can be paired with either the  ATmega32HVE2, or ATmega32HVB MCU,” said Rydahl.

batterymanagementfigure3

“Both MCUs have battery voltage and current measurement capabilities. The feature sets and peripheral offerings (number of cell measurement inputs, LIN bus interface, etc) of two MCUs are slightly different, so the specific requirements of the end application must be taken into consideration before selecting the MCU.”

Interested in learning more about using 7/13-cell applications with Atmel’s ATA6870? Be sure to check back tomorrow for part two of this series.