Whether the portable electronic equipment is powered by the AC mains via rectification (or AC adapter) or by the battery pack, the power supply voltage will vary within a wide range during operation. For example, when the single-cell lithium-ion battery is fully charged, the voltage is 4.2V, and the voltage after discharging is 2.3V, which varies greatly. The output voltage of various rectifiers is not only affected by changes in the mains voltage, but also by load changes. In order to ensure that the supply voltage is stable, almost all electronic devices are powered by a voltage regulator. Small precision electronic equipment also requires a very clean power supply (no ripple, no noise), so as not to affect the normal operation of electronic equipment. In order to meet the requirements of precision electronic equipment, a linear regulator should be added to the input of the power supply to ensure constant supply voltage and active noise filtering [1].
The basic principle of LDO
The basic circuit of the low dropout linear regulator (LDO) is shown in Figure 1-1. The circuit consists of a series regulator VT, sampling resistors R1 and R2, and a comparison amplifier A.
The sampling voltage is applied to the non-inverting input terminal of the comparator A, and compared with the reference voltage Uref applied to the inverting input terminal, the difference between the two is amplified by the amplifier A, and the voltage drop of the series regulating tube is controlled to stabilize the output voltage. When the output voltage Uout decreases, the difference between the reference voltage and the sampling voltage increases, the drive current of the comparison amplifier output increases, and the series regulator tube voltage drop decreases, thereby increasing the output voltage. Conversely, if the output voltage Uout exceeds the desired set value, the pre-drive current of the comparator output is reduced, thereby reducing the output voltage. During the power supply process, the output voltage correction is continuously performed, and the adjustment time is limited only by the reaction speed of the comparison amplifier and the output transistor loop.
Figure 1-1 Basic circuit of low dropout linear regulator
It should be noted that the actual linear regulator should also have many other functions, such as load short circuit protection, over voltage shutdown, thermal shutdown, reverse connection protection, etc., and the series regulator can also use MOSFET.
2. Main parameters of low dropout linear regulator
1. Output Voltage
The output voltage is the most important parameter of the low-dropout linear regulator, and is the first parameter that electronics designers should consider when choosing a regulator. Low dropout linear regulators are available in fixed output voltages and adjustable output voltages. The fixed output voltage regulator is convenient to use, and since the output voltage is precisely adjusted by the manufacturer, the regulator has high precision. However, the set output voltage values ​​are common voltage values, and it is impossible to meet all application requirements, but the change of external component values ​​will affect the stability accuracy.
2. Maximum output current (Maximum Output Current)
The power of the powered device is different, and the maximum current required by the regulator is also different. In general, the higher the output current, the higher the cost of the regulator. In order to reduce the cost, in a power supply system composed of a plurality of voltage regulators, an appropriate voltage regulator should be selected according to the current value required for each part.
3. Input and output voltage difference (Dropout Voltage)
The input-to-output voltage difference is the most important parameter for low dropout linear regulators. Under the condition that the output voltage is stable, the lower the voltage difference, the better the performance of the linear regulator. For example, a 5.0V low-dropout linear regulator can stabilize the output voltage at 5.0V by inputting 5.5V.
4. Ground current (Ground PIN Current)
The grounding circuit IGND refers to the operating current of the regulator provided by the input power supply when the output current of the series regulating tube is zero. This current is sometimes referred to as quiescent current, but this convention is incorrect when using PNP transistors as series regulator tube components. Generally, the ideal low-dropout regulator has a small ground current.
5. Load Regulation
The load regulation rate can be defined by Figure 2-1 and Equation 2-1. The smaller the load regulation of the LDO, the stronger the LDO's ability to suppress load interference.
Figure 2-1 Output voltage and output current
In the middle
â–³Vload - load adjustment rate
Imax - LDO maximum output current
Vt——The output voltage of the LDO when the output current is Imax
Vo——Output voltage of LDO when the output current is 0.1mA
ΔV——the difference between the output voltages when the load current is 0.1mA and Imax respectively
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