ATA6870-DK11

APPLICATION NOTE User Guide for Atmel ATA6870 and Atmel ATmega32HVB Evaluation Kit Hardware DK11 ATA6870-DK11 Features ● Evaluation of the Atmel® ATA6870 ● Monitoring 12 battery cells ● Monitoring: ● Overvoltage (every cell) ● Undervoltage (every cell) ● Overheating ● Overcurrent ● Open clamp detection ● 12-bit battery cell measurement ● 12-bit temperature measurement ● Controlling FET charge/discharge ● Status LEDs for easy evaluation ● Charge active balancing Figure 1. Atmel ATA6870-DK11 9302C-AUTO-04/15 1. Introduction The Atmel® ATA6870-DK11 is a demonstration board for the Atmel ATA6870 and offers an easy way to start evaluation of battery applications in combination with the Atmel ATmega32HVB. The included software demonstrates implementation of a 12-cell battery management system, including the function of active balancing through inductors. The supplied code serves as an example of how to use the Atmel ATMega32HVB and Atmel ATA6870 together. 2. Safety Precautions When Using Li-Ion Batteries Please observe the safety guidelines supplied with the batteries. If improperly used or defective, Li-ion and polymer batteries and packs may explode and cause a fire. 3. Demonstration Board The Atmel ATA6870-DK11 is designed to enable easy evaluation of the control software for an MCU controlling multiple Atmel ATA6870 devices. The supplied sample code demonstrates a simple permanent running measurement of voltages and temperatures. Figure 3-1. Board Concept Cell 12 Cell 11 ATA6870 ATA6870 Cell 02 Cell 01 Charge/ Discharge Control Unit Monitoring (V,T) Coulomb counting ATmega32HVB A very important characteristic of the ATA6870-DK11 is the ability to realize active balancing. Compared with the passive method, the active method is significantly more efficient with virtually lossless energy transfer between battery cells. There are usually two types of active balancing—with capacitors and with inductors. The capacitors are used for balancing currents lower than 50mA, while the inductors are for currents up to 1A or even more. On the ATA6870-DK11 the inductors with the value of 470uH are applied. Table 3-1 describes the achievable peak and average balancing currents with different inductors. 2 ATA6870-DK11 [APPLICATION NOTE] 9302C–AUTO–04/15 Table 3-1. Balancing Currents Possible Using Different Inductors f (kHz) Inductor Type Inductance (µH) Resistor of Inductor (m) Peak Balancing Current (mA) Average Balancing Current (mA) 3 Toroidal 300 130 1240 470 3 Toroidal 470 135 790 280 3 SMD 220 380 2280 890 3 SMD 330 430 1620 610 3 SMD 470 560 1500 430 The structure is shown in Figure 3-2. It is easy to see that on the demo board the active balancing can only happen between the two neighboring cells and the balancing current flows from the higher cell to the lower cell, with the corresponding Mosfet switched on. However, if it is the lowest cell of a battery stack that needs to be discharged, a transformer concept applies and the energy is transferred from the lowest cell to the top cell in the battery stack. For more information about the active balancing, please see application note “Active Cell Balancing Methods for Li-Ion Battery Management ICs using the ATA6870.” Figure 3-2. Inductive Charge Balancing between Two Stacked ATA6870s M12 Cell 12 PMOS L12 D11 M11 Cell 11 PMOS L11 D10 Cell 10 L8 D7 M7 Cell 7 PMOS L7 Balance Circuit of upper ATA6870 Balance Circuit of lower ATA6870 M6 D6 Cell 6 PMOS L6 D5 M5 Cell 5 PMOS L5 Cell 4 D4 D12 L1 D1 T1 Cell 1 M1 GND ATA6870-DK11 [APPLICATION NOTE] 9302C–AUTO–04/15 3 3.1 System Start Follow these steps to launch the system. 3.1.1 Installing the Hardware ● Connect the load/charger to J1. ● ● Connect the 12-cell stack to the J2 and J3 screw connectors. ● ● 3.1.2 For demonstration purposes it is possible to use a resistor to simulate a load. LED 1 indicates the enabled status of ATA6870-DK11 (controlled by the MCU software, see Table 4-1) In case of emulating cells such as a voltage divider, please apply sufficient voltage. Number of Cells It is possible to run the board with a reduced number of cells. The minimum voltage for each IC is 6.9V. Cell 1 and cell 6 have to be connected. The missing cells should be short-circuited to the upper cell potential of the module. For further information, see Section 7.3 “Reduced Number of Battery Cells Configuration” of the Atmel ATA6870 datasheet. For the voltage range, see Section 3.3 “Powering the Board” on page 6. If fewer than 6 cells are used per IC, the config.h file should be adjusted (CELLSIC# under General Setting). See Section 4.1 “Supplied Code” on page 7 for further information on how to configure the supplied software correctly. 3.2 The Demonstration Board Figure 3-3. Evaluation Board with Two Stacked Atmel ATA6870 and Atmel ATMega32HVB 4 ATA6870-DK11 [APPLICATION NOTE] 9302C–AUTO–04/15 3.2.1 On-Board Features The demonstration board includes the following items: ● TwoAtmel® ATA6870 QFN 7mm  7mm ● ● ● Atmel ATMega32HVB 12 external P-channel MOSFETs, 11 inductors, 13 diodes and one transformer for active balancing Connectors ● ISP connector for programming/debugging the Atmel ATMega32HVB ● Screw connectors for connecting up to 12 battery cells Table 3-2. Table 3-3. Connector Overview Connector Function J1 Connector for charger/device to be powered J2 Bottom battery stack (cells 1-6) J3 Upper battery stack (cells 7-12) J10 ISP connector Jumper Overview Jumper Function J9 Jumper to enable/disable MISO line of Atmel ATA6870 (see details below) J13, J14, J15, J17 Current measurement for active balancing (should be closed for charge transfer) J9 should never be set while the Atmel ATmega32HVB is being programmed or while it is entering debug mode. It can be mounted as soon as AVR Studio® prompts for additional SPI lines to be connected in debug mode or after the device has been correctly programmed. Table 3-4. Test Point Overview Test Point Function J19 IC2 work status J21 MCU power and SPI status ATA6870-DK11 [APPLICATION NOTE] 9302C–AUTO–04/15 5 Figure 3-4. Connectors, Jumpers, and Test Points 3.3 Powering the Board 3.3.1 Power Supply The board supports supply voltages ranging from 13.8V (minimum 6.9V per Atmel ATA6870) to 60V. However, to run the board on voltages below 24V the ZDiode D14 needs to be replaced with a jumper to supply the Atmel® ATmega32HVB with sufficient voltage. If the jumper is mounted, be sure that stack voltage does not exceed 48V. The Atmel ATmega32HVB supports operating voltage from4V to 24V. 3.3.2 Emulating Cells Battery cells can be emulated by connecting a voltage divider to the specified clamps. Section 3.1.1 “Installing the Hardware” on page 4 describes how to connect cells. The voltage limits for this setup are the same as for the real batteries. Section 3.3.1 “Power Supply” on page 6 specifies these limits. 6 ATA6870-DK11 [APPLICATION NOTE] 9302C–AUTO–04/15 4. Software Description: Monitoring Up to 12 Battery Cells The supplied code is documented and easy to adjust for verifying the basic functions of the Atmel® ATA6870 and start BMS application development. After the board is ready as described above, the microcontroller automatically starts a cyclic measurement of voltages, temperature and current. The status is indicated by the three LEDs on the board and the functions of the LEDs are programmable according to different requirements. LED1 is switched on at the beginning of the main routine and toggles with the active balancing. LED2 indicates that for some reason the charger/load MOSFETS have been disabled. The default software disables the FETs in case of these events: ● Overvoltage (at least one cell exceeds the upper default threshold of 4.2V) ● ● ● ● Undervoltage (at least one cell exceeds the lower default threshold of 2.5V) Overcurrent (the current through the shunt exceeds the default threshold of 80mA) Overheating (the temperature exceeds the upper threshold, default value is 80°C) Low temperature threshold (the default threshold is –20°C) LED3 indicates whether the Atmel ATA6870 devices are turned on or not. An active LED indicates that the Atmel ATA6870 devices are enabled. Table 4-1. LED LED1 LED Functions Function ON indicates access in the main routine TOGGLE indicates that the active balancing is operating LED2 ON indicates disabled MOSFETs for one of the reasons listed above LED3 ON indicates active Atmel ATA6870 The ATA6870 system clock should be provided externally. The Atmel ATmega32HVB has no clock divider to provide an external clock slower than 1/2 CPU clock. The requirement for the Atmel ATA6870 is fCLK > 2 × fSPI. As a result, the clock frequency of 1MHz is mandatory to provide a 500kHz clock for the ADCs of the Atmel ATA6870 and 250kHz for SPI. 4.1 Supplied Code 4.1.1 Config.h This section refers to the config.h file provided in the demonstration source code. Only values in the user setting paragraph should be changed and other values should not be changed in the default hardware setup. For example, the variable CELLSIC should be set to match the status of the batteries actually connected. ------------- GENERAL SETTING-------------------------------CELLSIC# Selecting which cells are used Bits 0-5 -> Cells 1-6 ------------- TEMPERATURE SETTING---------------------------RES_REF# Value of the mounted reference resistor (default: 3300) T_TLS Temperature belonging to the first value in the lookup table (index 0, default: -20) T_TLE Temperature belonging to the last value in the lookup table (default: 80) T_TLSZ Temperature step size used in the lookup table (default: 1) T_LOWERTHRESHOLD Lower temperature threshold T_UPPERTHRESHOLD Upper temperature threshold ATA6870-DK11 [APPLICATION NOTE] 9302C–AUTO–04/15 7 ------------- COULOMBCOUNTER SETTING------------------------SHUNT_RESISTANCE Value of the shunt resistor in mOhm RCC_CONVERSIONPERIOD The cycle times for the Regular Current Check 0x00 - 256ms (default) 0x01 - 512ms 0x02 - 1s 0x11 - 2s RCC_DIVIDEDSZ 0x01 to enable divided Voltage (Current) stepsize RCC_CHARGETHRESHOLD Threshold for charging current, exceeding the threshold will turn off the Mosfets RCC_DISCHARGETHRESHOLD Threshold for discharging current, exceeding the threshold will turn off the Mosfets. Balancingthreshold Threshold for the active balancing. 4.2 Voltage Measurements The standard software loop measures the voltage ADC value and the offset ADC value for every cell and checks for overvoltage and undervoltage once per cycle. Further information about acquiring voltages can be found in Section 7.5.1 of the Atmel® ATA6870 datasheet. The formula for calculating the voltage is: V acq – V offset Voltage (Cell) = 4V   --------------------------------- 3031 – V offset 4.3 Temperature Measurements The default software only measures channel 1 of chip 1. The temperature sensors are based on a resistor divider using a standard resistor and an NTC resistor. This resistor divider is connected to the reference of the ADC for temperature measuring. Because the ADC shares the same reference value, the output of temperature measurement with ADC is ratiometric. Further information can be found in Section 7.5.3 “Temperature Channel” of the Atmel ATA6870 datasheet. For this application Atmel recommends using Res_Ref1 = 3.3k and RES_NTC1 R25 = 4.7k, B = 3500. The software supplied for this board uses these values as default. The function uses a lookup table to determine the temperature. This table has to be edited if an NTC other than the recommended one is used. The values in the lookup table range from –20°C (index 0) to +80°C (index 100). These values can be edited via the config.h file in the user settings section. More Information about this file can be found in Section 4.1 “Supplied Code” on page 7. The calculation of RES_NTC is carried out based on the formula provided in Section 7.5.3 of the Atmel ATA6870 datasheet: RES_NTC(1) 8 8 adc (out) = 2048   1 + ----------------------------------------------------------------------------  ------ – ------   RES_NTC(1) + RES_REF(1)  15 10 When using another NTC, the LookupADC.txt has to be edited to match the NTC used. 4.4 Gas Gauging Gas gauging is a commonly used method for estimating how much capacity and runtime are left in the battery. By combining its very accurate VADC and CCAVC the Atmel ATmega32HVB can achieve high accuracy gas gauging. Here the voltagebased gas gauge is for the remaining capacity initialization, while the coulomb-counter-based voltage is to keep track of how much current is being used to charge and discharge the battery. Because the gas gauging calculation needs characterization data for the specific battery cell, the algorithm does not fall within the scope of this application note. See the other AVR® application notes for further details. 8 ATA6870-DK11 [APPLICATION NOTE] 9302C–AUTO–04/15 4.5 Overcurrent Protection The current through the shunt is calculated by measured voltage drop. The limit can be set via the CADRDC/CADRCC register. The step size depends on the settings of the CADCSRC register and the shunt used. For further information about the limiting current see Section 19.4 “Regular Current Detection Operation” of the Atmel ATmega32HVB datasheet. The supplied software allows the feature to be tested by adjusting the values in the config.h file. More Information about this file can be found in Section 4.1 “Supplied Code” on page 7. Values/part of the code should only be changed if you are aware of possible consequences. The default implementation continuously measures the current and generates an interrupt if the entered thresholds are exceeded. The thresholds are defined in the config.h file. The thresholds are written to the registers in the CCinit function in the Atmel ATA6870_func.c file. Refer to the features of the Atmel ATmega32HVB in the coulomb counter section to learn more about the time the controller waits for the values to be written. Table 4-2. C Code Example C Code Example CADRCC = RCC_CADRCC; while(CADCSRA & (1