A High Accuracy Current Sensing Circuit With Built-In Offset Self-Zeroing Operational Amplifier

To solve the problem that the load current sensing precision of transistor power switch chip was easily affected by the offset voltage of operational amplifier(op-amp), current sensing mode, process and environmental change, a highprecision current sensing circuit was designed. The sensing transistors was used to sense the load current in equal proportion, and the sensing precision was high. The design of the auto-zero amplifier could reduce the offset voltage caused by the process fabricating of the operational amplifier, and then increased the loop gain, improved the precision of the circuit current sensing. The circuit was fabricated with a 40V 0.35 µm BCD process. Simulation results show that the average offset voltage of the operational amplifier in the sensing circuit is about 1µV under the condition of 27°C temperature and 13V supply power. With load current ranging from 50mA ཞ 4.5A, the current sensing precision can achieve 99.88%. In the temperature range of −40°Cཞ150°C, the offset error is not more than 2%, and the sensing precision error is not more than 0.74%.