Fig.1: the add-on circuit uses just two transistors plus a few resistors and capacitors. Its job is to modify the sense signal to the LTC1325 IC.
During finalisation of the firmware for this project, we were
surprised to discover that under certain conditions, the 111kHz PWM section of
the circuit became unstable. In fact, with high cell counts, high charge
currents and low input voltages, such as could be the case in an automotive
environment, the switching frequency halved to about 50kHz.
When low cell counts (1-2 cells) and low charge currents are
combined, a lower operating frequency is of no consequence. However, at high
power levels, it will cause inductor L1 to overheat and damage the PC board and
surrounding components. It may also eventually cause MOSFET Q2 to fail.
