LDO is a very simple device, but in the process of communicating with customers, I found that the technical level of customer engineers is uneven. Some engineers just follow other people’s previous designs and don’t understand any principles and design methods. I hope everyone reads this. Any article can become an LDO expert.
The first question: What is an LDO?
LDO stands for low dropout regulator, which is a low dropout linear regulator. The commonly used 7805, etc. are often called LDOs. In fact, the 7805 chip has a large dropout voltage. Take LM78L05 as an example. The minimum voltage drop of large current must be above 2V. For example, an application needs 6v to 5v, LM78L05 It would be inappropriate.
The second question: Why can LDO be stable?
The principle block diagram of the LDO is as follows. The output voltage is divided by the feedback resistor to the FB pin. When the output voltage is higher than the set value, the internal loop will change the driving voltage, which will increase the conduction voltage drop of the tube, thereby reducing the output voltage. When the output voltage is lower than the set value, the internal circuit will change the driving voltage, so that the tube conduction voltage drop is reduced, thereby increasing the output voltage. Better closed-loop negative feedback loop
The third question: What are the important parameters of LDO?
1. PSRR (Power Supply Voltage Rejection Ratio) refers to the suppression effect of LDO output on input ripple noise. This is also the reason for adding another LDO to the DC/DC rear stage in many occasions (especially when an analog sensor or ADC/DAC is connected later). The suppression effect of the high PSRR LDO on the ripple is still very obvious. For example, TPS71701 is a high PSRR LDO.
So how to judge whether the PSRR parameter of the LDO is sufficient? For a simple example, suppose the switching frequency of the DC/DC in front of the LDO is 100khz, the PSRR at 100khz is 50dB, and the front-end DC/DC ripple size is 100mv. Ripple=100mv/10(50/20)=0.3mv.
2. Noise (noise performance), different from PSRR, noise refers to the noise signal generated by the LDO itself, low-noise LDOs such as LP5907, etc. can reduce the extra noise generated by the LDO. Noise is generally calculated as the effective value (rms), but it can also be analyzed by peak to peak, and then it needs to be multiplied by a factor (for example, multiplied by 6).
3. Low Dropout Voltage (low pressure drop). This parameter was mentioned earlier. For example, when designing a circuit that requires 6v to 5v, the Low Dropout Voltage parameter should be kept <1 v. However, current LDOs generally have a low voltage drop. Like the TPS7A71 series of LDOs, the Vdropout is only 200mV.
4. Transient response (dynamic performance). In some applications, where the load changes drastically, it is necessary to increase the output capacitance and at the same time try to choose an LDO chip with good dynamic performance. For example, LP5907 is an LDO with good dynamic performance.
5. Thermal (temperature performance). Everyone knows that LDO efficiency is very low, so how to verify whether an LDO is suitable? First calculate the power consumption Pd=(Vin-Vout)Iout, and then calculate the temperature rise, here you can use the thermal resistance RθJA to calculate, the temperature rise ΔT= Pd*RθJA, and then calculate the chip junction temperature Tj= Tambient+ΔT<Tjmax(datasheet) The figure below is a basic outline of the tolerable power consumption of each package.
6. IQ (ie quiescent current) generally has higher requirements for quiescent current in battery-powered occasions. Generally, the quiescent current of the LDO chip has an inverse relationship with the other performance of the chip, such as low noise, high power supply voltage rejection ratio, The quiescent current of LDOs with good dynamic performance is a bit larger. There are also many low IQ LDOs. Like the TPS780 series of LDOs, IQ Shutdown is only 18nA.
to sum up
Regarding LDO, you only need to master the above technical points. In fact, any knowledge point is not as simple as we imagined. I hope that everyone can deepen the technology in the future, not only knowing what is happening, but also knowing why, so that in the future devices It is easy to do when choosing the model. If you think this article is helpful to you, remember to share it with your friends.