Internal Structure of Lithium Battery Protection Circuit:

In this circuit, P4, P5, P6, P7, N3, N4, and N6 form a second-order operational amplifier (op-amp) as a reference voltage source. The bias voltage for the op-amp is supplied by the reference source, simplifying the circuit and reducing extra power consumption. By adjusting the MOSFET dimensions, the op-amp achieves higher gain and lower offset voltage. The reference source employs a cascade of diodes, with Q1 and Q2 having equal emitter areas, as do Q3 and Q4. To minimize power consumption, the area of Q3 is doubled that of Q2. The cascade diode configuration effectively reduces the impact of op-amp offset on the output reference voltage accuracy.

Detection voltages used in protecting circuits are generally low, such as the overcurrent detection voltage of about 0.15V, and the secondary overcurrent detection voltage of about 0.6V. However, a typical bandgap reference circuit can only output about 1.2V. The introduction of resistor R5 addresses this issue by stopping the voltage division of the output reference voltage.