A temperature independent trimmable current source

A non-volatile floating-gate MOSFET based trimmable current cell suitable for use in a current steering DAC and implemented in 0.35mum, 3.3V CMOS technology is presented. The current source can be programmed to an accuracy corresponding to 12-bit resolution using either hot electron injection to add charge to the floating-gate or hot hole injection to remove charge. The output current from the circuit can be adjusted by 40% to overcome the relatively large variations in output current which arise when a MOSFET is biased with a low gate overdrive voltage (Vgs-Vt). This allows the circuit to be designed around the temperature invariant region to give an output which is temperature independent over a large trimming range. This circuit could therefore be ideally suited to operate in the uncertain thermal environment created when a DAC is incorporated within a system on chip.