Design of a new bipolar LDO linear regulator

　　With the continuous development of electronic products, power management solutions continue to pursue high efficiency, small footprint, and low cost, which makes low-dropout (LDO, LowDropout) linear voltage regulators increasingly popular. Applied to battery-powered products, the low leakage voltage feature ensures high battery usage efficiency, and the efficiency will increase as the battery voltage decreases; the low quiescent current feature ensures long battery usage time. The LDO linear regulator designed in this article has a typical leakage voltage of 180 mV and a quiescent current of 800 μA at a load of 100 mA. The leakage voltage at no load is only 5 mV and the quiescent current is 35 μA; and the enable switch pin is set. pin, when the enable switch pin is pulled low, the voltage regulator is in sleep mode, and the quiescent current is <1μA; in addition, the voltage regulator also has over-temperature and over-current protection functions.

　　1 Circuit topology

　　The voltage regulator includes a startup circuit, a constant current source bias unit, an enable circuit, an adjustment component, a reference source, an error amplifier, a feedback resistor network, a protection circuit, etc. In order to realize the LDO characteristics, the regulator adjustment element uses a free-collector vertical PNP tube, and the over-temperature and over-current protection circuit also adopts a special structure. The circuit topology is shown in Figure 1.

　　The basic working principle is as follows: when the system is powered on, if the enable pin is at a high level, the circuit starts to start, the constant current source circuit provides bias to the entire circuit, the reference source voltage is quickly established, and the output continues to rise with the input. When the output When it is about to reach the specified value, the output feedback voltage obtained by the feedback network is also close to the reference voltage value. At this time, the error amplifier amplifies the small error signal between the output feedback voltage and the reference voltage, and then amplifies it to the output through the adjustment tube, thus Negative feedback is formed to ensure that the output voltage is stable at the specified value; similarly, if the input voltage changes or the output current changes, this closed loop will keep the output voltage unchanged, that is:

　　If the enable pin is low, the circuit will be in sleep state.

　　2 Start-up circuit, constant current source bias circuit and enable switch circuit

　　The purpose of starting the circuit is to make the constant current source bias circuit start working, thereby establishing a normal operating point for the entire circuit. The enable high potential critical value is set to 1.4 V, and the enable low potential critical value is set to 0.5 V. When the enable voltage VEN＞1.4 V, the current source provides bias for the reference source and error amplifier, and the circuit is in a stable voltage working state; when the enable voltage VEN＜0.5 V, the startup circuit will turn off the current source. , the bias current of the reference source and error amplifier is zero, causing the entire circuit to be in a cut-off state. At this time, the quiescent current of the circuit will be very small (<1μA). This state is called the regulator sleep mode. Based on the above principles, the circuit design of this part is shown in Figure 2.

　　The microcurrent source composed of Q14, Q15, R9, and R8 serves as the starting circuit. The large resistor R9 ensures a very small pin current (usually a few μA). Q14 starts the current source, and Q5 starts the error amplifier, adjustment tube and other circuits. The current mirror composed of lateral PNP transistors Q7 and Q8 provides bias for the reference source, while Q2, the same type as the adjustment tube, provides current bias for the error amplifier. The enable switch circuit is composed of lateral PNP transistors Q9A and Q9B. Q6 makes the emitter potential of Q9B VREF + 0.7 V = 1.93 V; while the emitter potential of Q9A is VFB + 0.7 V = 1.93V. In this way, when VEN＞1.4 V, Q9A and Q9B are completely cut off, the enable circuit loses its function, and the circuit works normally to stabilize the voltage. For the same reason, Q15 can determine the regulator sleep mode when VEN＜0.5 V.

　　3 adjustment tube

　　As shown in Figure 3, the simplified structural diagram of the LDO linear regulator, the quiescent operating current (Iq) of the regulator is mainly determined by the base drive current of the adjustment tube. The smaller the value, the more current consumed by the regulator itself. The smaller the value, the higher the power supply current conversion efficiency; the leakage voltage (VDROP) refers to the minimum input-output voltage difference at which the output voltage is within the tolerance range. The smaller the value, the higher the power supply voltage conversion efficiency. For LDO linear regulators using PNP regulator tubes, Iq≈IDRV=IO/β (β is the current amplification factor of PNP), VDROP=VSAT (VSAT is the saturation voltage drop of PNP) [1]. Because under the existing technology, the general current amplification factor of lateral PNP is about 50, the parameter drift is large; the performance of VSAT is also not good. This design uses a vertical PNP with a free collector under a certain process. When the collector current is -100μA and the voltage difference between the collector and the emitter is -5 V, the standard single tube has a β of 160 and an up and down drift of 50; When the electrode current is -100μA and the collector-to-base current ratio is 10, VSAT is about 30 mV. Therefore, this free-collector longitudinal PNP is very suitable for use as a regulating tube. The layout and cross-sectional structure of a single tube is shown in Figure 4. The leakage voltage and quiescent current characteristics are shown in Figure 5 and Figure 6.

　　4 reference circuit

　　It can be seen from equation (1) that the reference voltage is crucial for the LDO linear regulator. This design uses a bandgap reference source structure with an output of 1.23 V, high precision and low temperature coefficient, which also means that the output of the voltage regulator will also have high precision and low temperature coefficient characteristics. According to the basic principles of bipolar bandgap reference circuit [2, 4], the designed bandgap reference source structure is shown in Figure 7.

　Q15, Q18, and R13 constitute a VT (VT=KT/q, called thermal voltage) generator. The current source composed of Q19, Q16 and Q17 accurately ensures that the collector currents of Q15 and Q18 are equal. The area ratio of the emitter areas of Q15 and Q18 is 1/10, so the ratio of their junction saturation currents IS is 1/10. Therefore, the BE junction voltage difference ΔVBE of Q15 and Q18 is:

　　Among them: VBE is the negative temperature coefficient, and VT is the positive temperature coefficient.

　　5 error amplifier

　　Voltage regulation rate and load regulation rate are important quality parameters of the voltage regulator. They respectively represent the ability of the voltage regulator to maintain the output at the specified value when the input voltage changes and the output load changes. According to the basic principles of LDO linear regulators [1, 3], they are inversely proportional to the DC open-loop gain of the error amplifier. Therefore, the larger the transconductance of the error amplifier, the better the voltage regulation and load regulation performance of the voltage regulator. In addition, it can be seen from Figure 1 that the output current of the error amplifier directly drives the PNP tube, so the error amplifier must be able to provide a large enough output drive current, and the output drive current must be able to follow changes in the load. The bias current source of the error amplifier must also be It can change with the load, and the error amplifier itself must still be in the amplification state when the load changes, maintaining strong negative feedback to achieve stable output.

　　Based on the above, this article gives the design circuit shown in Figure 10. The voltage drop of the error amplifier output current on the small resistor R6 controls the dynamic load of Q4. When the output load current of the voltage regulator increases, the error amplifier output current increases, the voltage drop on R18 increases, which increases the dynamic load, so that a larger drive current can be provided to the adjustment tube. The differential input pair tube and the adjustment tube of the error amplifier designed in this design are both vertical PNPs with free collectors, which enables the error amplifier to have high transmission transconductance and low input offset [5].

　　6 Over-temperature and over-current protection circuit

　　Over-temperature and over-current protection circuits are necessary for LDO linear regulators. When the operating temperature of the voltage regulator exceeds the maximum allowable junction temperature, the over-temperature protection circuit stops the voltage regulator, thereby eliminating power consumption, achieving cooling, and preventing the voltage regulator from burning out; when the voltage regulator is short-circuited or When the output current is too large due to other reasons, the overcurrent protection circuit will quickly reduce the current of the voltage regulator to prevent damage to the voltage regulator due to excessive current. The maximum operating temperature of the voltage regulator in this design is 125°C, and the output current limit is 200 mA. The circuit form is shown in Figure 11. At normal temperature, the BE junction voltage of Q12 is set lower than its conduction voltage drop. When the temperature rises, the conduction voltage drop of the NPN tube decreases by about 2 mV/℃, so the potential of point A increases with the temperature. It is high and continues to rise until the Q12 tube is turned on. At this time, the bias current of the error amplifier is completely pulled to Q12. In this way, the error amplifier will stop working and the adjustment tube will have no drive and the output will be zero.

　The overcurrent protection circuit is somewhat similar to the overtemperature protection circuit. Within the operating current range of the voltage regulator, Q11 is cut off. When the output current of the voltage regulator increases to 200 mA, the base current of the adjustment tube will reach 2 mA. , the voltage drop across the current sensing resistor R7 will turn on Q11, forming negative feedback and limiting the output current to this value.

　　7 Conclusion

　　LDO linear regulators are mainly used in portable electronic products, and are becoming increasingly widespread. CMOS-type LDO linear regulators are also under development, but they have weaknesses caused by the CMOS process itself, and because the PMOS regulator tube has a large gate parasitic capacitance, stability compensation is difficult to control. This article starts from the topology of the voltage regulator and conducts a detailed analysis and design of each module. Using a certain bipolar process, the LDO linear voltage regulator implemented has low leakage voltage and low quiescent current characteristics, and will have very good application prospects.