(1) Start the circuit

In the integrated voltage regulator, many constant current sources are often used. When the input voltage V1 is turned on, these constant current sources are difficult to conduct by themselves, so that the output voltage is difficult to establish.Therefore, a start-up circuit must be used to provide base current to the BJT T4 and T5 of the constant current source.The start-up circuit consists of T1, T2, and DZ1.When the input voltage V1 is higher than the stable voltage of the voltage regulator tube DZ1, a current flows through T1 and T2, so that the base potential of T3 rises and turns on, and the constant current sources T4 and T5 also work.The collector current of T4 passes through DZ2 to establish the normal working voltage. When DZ2 reaches the voltage regulation value equal to that of DZ1, the whole circuit enters the normal working state, and the circuit starts up.At the same time, T2 is turned off because the voltage of the emitter junction is zero, which cuts off the connection between the start-up circuit and the amplifier circuit, so as to ensure that the ripple and noise on the left side of T2 will not affect the reference voltage source.

(2) Reference voltage circuit

A reference voltage circuit T4, DZ2, T3, R1, R3and D1, D2 composition, is from the reference voltage circuit
wherein the stabilization voltage VZ2 of DZ2, the VBE emitter of T3, D1, D2 emitter (D1, D2 by The forward voltage value of the diode).In circuit design and process, R1, R2, DZ2 with positive temperature coefficients and T3, D1, D2 emitter junctions with negative temperature coefficients are mutually compensated, so that the reference voltage VREF basically does not change with temperature.At the same time, the constant current source is used to supply power to the voltage regulator tube DZ2, so as to ensure that the reference voltage is not affected by the fluctuation of the input voltage.

(3) Sampling comparison amplifier circuit and adjustment circuit

     This part of the circuit is composed of T4~T11, in which T10, T11 form a composite adjustment tube; R12, R13 form a sampling circuit; T7, T8 and T6 form a differential amplifier circuit with a constant current source; T4, T5 form a current source as its active load.
The functions of T9 and R9 are explained as follows: if there is no T9 and R9, the current IC5=IC8+IB10 of the constant current source tube T5, when the regulating tube is fully loaded, IB10 is the largest, while IC8 is the smallest; and when the load is open, IO=0, IB10 also tends to zero, at this time almost all IC5 flows into T8, which makes the variation range of IC8 large, which is not allowed for the comparison amplifier circuit. For this reason, the buffer circuit composed of T9 and R9 is connected.When IO decreases, IB10 decreases, and IC8 increases. When IC8 increases to >0.6V, T9 turns on and plays the role of shunting.In this way, the excessive burden of T8 is relieved, and the variation range of IC8 is narrowed.

(4) Protection circuit

Current reduction protection circuit

The current reducing protection circuit is composed of T12, R11, R15, R14 and DZ3, DZ4, and R11 is a current-sensing resistor.The main purpose of protection is to make the adjustment tube (mainly T11) work within the safe area, and special attention should be paid to make its power consumption not exceed the rated value PCM.Consider first a simple case.Assuming that DZ3, DZ4, and R14 in Figure 1 do not exist, both ends of R15 are short-circuited.At this time, if the voltage regulator circuit works normally, that is, the PCAt 0.6V, the T12 tube is turned on.Due to its shunt action, the base current of T10 is reduced, thereby limiting the output current.The disadvantage of this simple current-limiting protection circuit is that it can only limit the output current within the rated value.Since the dissipated power PCM=ICVCE of the regulating tube, only considering the current passing through it and its tube voltage drop VCE value, and making the PC(VZ3+ VZ4), then DZ3 and DZ4 break down, causing the T12 tube emitter junction to be turned on under the forward voltage.The value of VBE12 is clearly sorted out. The larger (VI – VO) is, that is, the larger the VCE value of the pass tube, the smaller the IO, so that the power consumption of the pass tube is limited within the allowable range.Due to the reduction of IO, the above protection is called flow reduction protection.

Overheat protection circuit

The circuit consists of DZ2, T3, T14 and T13.At normal temperature, the voltage drop on R3 is only about 0.4V, and T14 and T13 are cut off, which has no effect on the circuit operation.When the chip temperature rises to a certain limit for some reason (overload or ambient temperature rise), the voltage drop on R3 increases with the increase of the working voltage of DZ2, while the emitter junction voltage VBE14 of T14 decreases, resulting in the conduction of T14. On, T13 is also turned on.The base current IB10 of the adjustment tube T10 is shunted by T13, and the output current IO decreases, so as to achieve the purpose of overheating protection.The function of R10 in the circuit is to give a shunt path to ICEO10 of T10 tube and ICBO11 of T11 tube to improve temperature stability.