Prototyping a Texas Instruments BQ25504
The TIBQ25504 is a high efficiency battery management system that requires a certain amount of configuration to achieve desired values at the VBAT and VSTOR pins. In addition, the boost converter has a built in status signal that can be connected to a micro-controller and utilized as desired.
Configuring the over-voltage and under-voltage thresholds for the attached battery simply reduces or expands the range of voltages that the device can produce under varying degrees of load and levels of battery charge. The device also allows the programmer to set a MPP to maximize the power produced by the connected sustainable source.
Due to limitations of the Evaluation Module for the battery management device, a slightly more complicated procedure was used to test the hardware. This process is outlined as follows:
- Download TI's spreadsheet tool and enter the desired parameters depending on the battery being used. A good MPPT configuration to start off with is 80% of maximum voltage. This can be tweaked later on in the prototyping process.
- Download the SPICE model for the device and run a transient analysis after replacing the included resistors with the values calculated in the spreadsheet tool. This can be done using OrCad Cadence or other SPICE software.
- Verify that VBAT and VSTOR values are as expected, and make any necessary modifications, adjusting the spreadsheet and simulating with the new values.
- Obtain a QFN to DIP converter from a reliable source. The example pictured was purchased from dEcolectrix, a company in Edderitz, Germany that specializes in custom PCB design. Ensure that the model chosen has a 3x3mm body and 0.5mm pitch between pins. Purchase 16x breakaway headers, the bq25504 and the required resistors, capacitors, and inductors as specified in the device manual and the spreadsheet.
- Use any re-flow soldering technique necessary to adhere the QFN chip to the PCB. Since the leads are on the bottom of the chip, traditional means of soldering are not an option. This can be done through use of professional facilities, or by creating a customized soldering oven. Be sure to align pin one with the correct pin on the board for proper pin numbering.
- Build the circuit on a breadboard and check voltage characteristics at the VBAT and VSTOR outputs without load or battery. Make any necessary adjustments to obtain values similar to those obtained in OrCad. Keep in mind that the breadboard will never perform as well as a custom PCB. Also, the over-voltage threshold will dictate the highest voltage produced by the boost converter, and this value should not exceed the maximum voltage allowed by the storage element. For example, a Li-Ion battery should not be charged at higher than 4.2V to prevent electroplating.
- Connect the battery (to the VBAT pin) before and after cold-start to verify cold-start functionality.
- Perform cold-start with the battery connected, and connect the desired load to the VSTOR pin. The voltage should drop at both pins slightly as voltage is exchanged for current. Note that if the voltage drops below the under-voltage threshold, the boost converter will cut power to the load, and voltage will drop on the VSTOR pin. If this occurs, the under-voltage threshold should be reconsidered to allow for the voltage drop.
- The VBAT OK signal should be configured to hold high for the interval from VSTOR high when load is connected to VSTOR low when load is connected, and the under-voltage threshold is almost reached. Keep the battery chemistry in mind when choosing this low threshold as well. For instance Li-Ion batteries should not be discharged below a certain voltage (3.0V - 3.2V) with load attached.
- Run the prototype for extended periods. If possible, run until over-voltage and under-voltage thresholds are reached. Check battery voltage when fully depleted and fully charged to characterize the battery for MPPT. Use formulas in the data sheet to figure out the maximum power point, and enter this into the spreadsheet to tweak its corresponding resistor net.