Constant Current Load

The constant current is implemented with a 14-bit DAC, differential amplifiers and two MOSFET transistors operated in their linear mode. The reference is 5V, step size is approximately 305µV. Shunt resistor is two of 0.5Ω in parallel. Resolution is 1.22mA and max current is 20A.

The load can draw high currents for a short time and is mainly limited by thermal overstress. The current drawn by the load is controlled from the PC software. If the current is kept below 2A for single cell and 1A dual cells applications the load should not overheat (the higher the voltage of the battery stack the lower should the current be).

The USB1287 monitors the temperature of the load, and will shut it down if it reaches 125°C to avoid damage to the MOSFET's.

The load has fairly good accuracy at higher currents, while it may be somewhat inaccurate at lower currents. The constant current stability is very good and it can therefore be used to test and characterize battery cells and application. The load can be calibrated to get better accuracy and linearity by rewriting the firmware for SB200.