Power modules are used in many scenarios in life, such as communications, industrial automation, power control, railways, mining, military industry and other fields. DC power conversion modules are divided into two types: low voltage dropout linear power supply (LDO) and switching power supply (DC-DC).

1.LDO

Traditional linear voltage regulators, such as the 78xx series of chips, require the input voltage to be more than 2V~3V higher than the output voltage, otherwise they will not work properly. But in some cases, such conditions are obviously too harsh. For example, when converting 5V to 3.3V, the voltage difference between the input and output is only 1.7v, which obviously does not meet the conditions. In response to this situation, LDO power conversion chips are available.

Linear power supplies mainly use LDO (low dropout regulator), which is a low-voltage linear DC regulated power supply module.

LDO maintains a stable output voltage by adjusting the impedance change of the circuit tube. When the output voltage increases from the input voltage or the load current decreases, and there is a tendency for the output voltage to deviate from the set value and increase, the impedance of the adjustment tube is adjusted through negative feedback. Increase, reduce the output voltage to restore the set value and maintain stability. When the input voltage decreases or the load current increases, and the output voltage tends to deviate from the set value and decrease, the impedance of the adjustment tube is reduced through negative feedback, and the output voltage is increased to restore the set value and remain stable.

Today's LDOs are highly integrated and generally only require 2 capacitors (usually 2 1uF) and an LDO chip. The circuit is simple, as shown below:

2. DC-DC

The switching power supply DC-DC (Directcurrent-Directcurrent) converter is composed of a regulating chip, a pole electromagnetic coil, a diode, a transistor and a capacitor. The switching power supply DC-DC is a voltage that can effectively output a stable working voltage after being converted into an input working voltage. converter. A switching regulated power supply (DC-DC) is a pulse generator that uses a switching power supply circuit to output an adjustable duty cycle or operating frequency. It uses high-frequency voltage regulator tubes, inductors, and capacitors to form a DC output voltage. By changing the duty cycle or The output voltage is adjusted according to the operating frequency.

Switching power supply DC-DC includes three types: BUCK (step-down), BOOST (step-up), and BUCK/BOOST (step-up and step-down). Three types of adjustment can be selected according to market demand. The PWM adjustment type has high working efficiency and good output voltage ripple and noise. The PFM adjustment type has the characteristics of low power consumption even when used for a long time, especially at small loads. PWM/PFM conversion implements PFM regulation when the load is small, and the system automatically switches to PWM regulation when the load is heavy.

A typical DC-DC BUCK circuit includes input and output capacitors, FREQ frequency setting, EN enable pin, FB feedback resistor, freewheeling diode and inductor on SW, BST capacitor, COMP frequency compensation, etc., as shown in the figure below:

3. Selection criteria

The main difference in performance indicators between LDO and DC-DC is that the output voltage ripple of a linear regulated power supply (LDO) is small. However, when the input operating voltage and output voltage are relatively large, the conversion efficiency is low, and only the voltage reduction is insufficient. Can boost. The ripple of the switching regulated power supply (DC-DC) is large. However, when the distance between the input operating voltage and the output voltage is relatively large, the conversion efficiency is higher and the voltage can be increased or decreased. The specific details of the difference are as follows:

LDO has few peripheral components, simple circuit, and low cost; DC-DC has many peripheral components, complex circuit, and high cost;
LDO load response is fast and the output ripple is small; DC-DC load response is slower than LDO and the output ripple is large;
LDO efficiency is low, and the input-output voltage difference cannot be too large; DC-DC efficiency is high, and the input voltage range is wide;
LDO can only step down; DC-DC supports step-down and step-up;
The quiescent current of LDO and DC-DC is both small, depending on the specific chip;
LDO output current is limited, the highest may be a few A, and reaching the highest output is related to the input and output voltage; DC-DC output current is high and the power is high;
LDO has small noise; DC-DC switching noise is large. In order to improve the accuracy of switching DC-DC, many applications will connect LDO at the back end of DC-DC;
LDO is divided into adjustable and fixed types; DC-DC is generally adjustable and adjusted through FB feedback resistor.
For boosting, you must choose DC-DC. For bucking, whether to choose DC-DC or LDO must be compared in terms of cost, efficiency, noise and performance.