An article to bring you the principle and application of LDO

[[404648]]

01LDO Definition

LDO, or low drop out regulator, is a low voltage difference linear regulator. This is relative to traditional linear regulators. Traditional linear regulators, such as the 78XX series chips, require the input voltage to be at least 2V~3V higher than the output voltage, otherwise they will not work properly. However, in some cases, such conditions are obviously too harsh. For example, when converting 5V to 3.3V, the voltage difference between input and output is only 1.7V, which obviously does not meet the working conditions of traditional linear regulators. In response to this situation, chip manufacturers have developed LDO-type voltage conversion chips.

02 Characteristics of LDO

Low-dropout (LDO) linear regulators have low cost, low noise, and low quiescent current, which are its outstanding advantages. It also requires very few external components, usually only one or two bypass capacitors. New LDO linear regulators can achieve the following indicators: output noise 30μV, PSRR is 60dB, quiescent current 6μA, and voltage drop is only 100mV.

After reading the above definition, without understanding the LDO structure, you can use analog electronics knowledge to associate it with the low-dropout regulator in the figure below.

The figure above is the most basic voltage stabilization circuit. The core component is the voltage regulator tube. Its voltage stabilization working range determines the range of output voltage stabilization. Through this simple circuit, voltage stabilization within a small dynamic range and small current (hundreds of mA level) can be achieved.

Upgrade the above circuit as shown below:

The above circuit only has one more 2N3055 transistor, the purpose is to improve the output load capacity, and the transistor also introduces voltage negative feedback to stabilize the output voltage. When the input voltage Vin increases or the output load resistance increases, the output voltage Vout will increase instantly, and the emitter voltage Ve of the transistor will increase accordingly. If the base voltage Vb remains unchanged, Vb-Ve will decrease, and then the output current will decrease, and Vout will decrease.

The above picture is just a simple basic low dropout regulator. Note that it is different from the LDO we mentioned, the word "linear". It can be seen here that the output voltage Vout of the above circuit will be affected by the fluctuation of Vbe voltage, and the stability is poor. And the output voltage cannot be adjusted.

03LDO Circuit

Adding a "linear" factor to the above circuit, that is, introducing an operational amplifier, deepens the negative feedback and improves the output voltage stability. This constitutes what we call a low voltage drop linear regulator. The circuit diagram is as follows

On the basis of the basic voltage regulator adjustment circuit, an operational amplifier A and a voltage divider resistor sampling network R1 and R2 are added. When the input voltage Vin increases or the output load resistance increases, the output voltage Vout will increase instantly, and the voltage obtained by the voltage divider sampling of R1 and R2 will also increase. Since it is the reverse input, the output of the operational amplifier A will decrease accordingly, and Vb-Ve will decrease, and then the output current will decrease, and Vout will decrease. From the circuit in the figure above, we can know that LDO is divided by resistors, that is, LDO can only reduce voltage, not increase voltage. And the current cannot be too large.

04LDO Application

There are many kinds of LDO chips, I have used AMS1117, spx3819, TLV702x, etc. Let's take spx3819 as an example to briefly talk about the application of LDO.

The features of spx3819 are as follows

1. Low noise: possible up to 40uV
2. High accuracy: 1%
3. Reverse battery protection
4. Low voltage drop: 340mV at full load
5. Low quiescent current: 90uA
6. Zero cut-off current
7. Fixed output: 1.2V, 1.5V, 1.8V, 2.5V, 3.0V, 3.3V, 5.0V

Different LDOs have different performances, so you can choose according to the needs of your actual project.

The circuit diagram of spx3819 is as follows, and you can see that the LDO peripheral circuit is very simple. This is also the advantage of LDO.

spx3819 provides 3 different packages, suitable for many scenarios.

It has output rated voltage versions, which are the fixed outputs mentioned above: 1.2V, 1.5V, 1.8V, 2.5V, 3.0V, 3.3V, 5.0V, distinguished by different model suffixes.

Of course, a voltage adjustment version is also provided. The circuit and output voltage formula are as follows

05Power consumption issue

As we explained above, the principle of LDO is to divide the voltage through resistors, which makes LDO unsuitable for large current scenarios, generally not more than 1A. This also leads to the topic that LDO cannot avoid: power consumption and heat dissipation.

The first thing to consider when choosing an LDO is the maximum input voltage range of the LDO and the current output capability of the LDO.

Then the larger current or larger LDO voltage drop will lead to higher component power loss

The following figure shows the relationship between LDO voltage drop and LDO current at a specific power.

When the power consumption of LDO increases, the heat dissipation function of LDO package must also be able to cooperate. Common packages are as follows

The power consumption of an LDO is determined by the voltage drop across the LDO multiplied by the current flowing through the LDO. The power is mainly lost in the LDO's conduction components, which is the cause of the silicon crystal heating.

The power loss depends on the IC package, PCB layout, and ambient temperature.

For example: SOT23 package, the perspective view is as follows

The center pin is connected to the lead frame attached to the die. Heat dissipation is mainly to air and to the PCB through the pins.

Because the bonding wire between the outer pins and the silicon die is very thin, that is, the 4 pins in the figure below, the heat is mainly dissipated by the middle pins. As mentioned above, the middle pins are connected to the lead frame. Therefore, the PCB of the middle pins can be designed to be wider.

There is also a SOP-08 package, and the copper wire on the bottom layer can also be used for heat dissipation.

If the design is based on the following figure, the heat dissipation effect will be better

06 Conclusion

The advantages of LDOs include:

(1) Simple structure, few peripheral components, and easy to use.

(2) No switching noise, can be used in analog circuits requiring high precision and low noise.

But the disadvantages are equally obvious:

(1) It can only lower the blood pressure, not raise it.

(2) Low efficiency, especially when the input voltage is high. It is generally used when the load current is less than 1A.

This article is reprinted from the WeChat public account "Zhixiao Programming", which can be followed through the following QR code. To reprint this article, please contact Zhixiao Programming public account.