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Introduction

Microcontroller unit, also known as a single-chip microcomputer, is a small computer on an integrated circuit (IC) chip, which appropriately reduce the frequency and specifications of the CPU, and integrates memory, timer, USB, A/D conversion,  UART PLC DMA, and even LCD driver circuits on a single chip. The MCU  of the embedded control system usually needs a stable working voltage.

However, designers are used to using linear voltage regulators (such as 78xx series three-terminal voltage regulator) to convert high DC voltage into working voltage required by MCU. The linear adjustment mode of the linear regulated power supply will cause large "heat loss", and its working efficiency is only 30% - 50%.

The switching power supply regulator works in a completely on or off mode. Therefore, either the high current flows through the switch with low conduction voltage, or it is completely cut off without current flow. Therefore, the power consumption of the switching power supply is very low, and its average working efficiency can reach 70% - 90%. Under the same voltage drop, the switching power supply regulator has much less "heat loss" than the linear regulator. Therefore, the switching power supply can greatly reduce the size of the heat sink and the area of the PCB board. Even in most cases, there is no need to install a heat sink. It is helpful to reduce the harmful impact on the MCU  working environment.

This article introduces the design scheme of using the LM2576 series switching power supply to replace the linear power supply as MCU power supply. It has two advantages. One is the high-frequency on-off characteristics of switch tubes. Another advantage is the use of a series filter inductor. They have a strong suppression effect on the high-frequency interference from the power supply. In addition, due to the reduction of the "heat loss" of the switching regulated power supply,  it is helpful to the ability of the AC voltage to resist drop interference if increased the input voltage of the regulated power supply.

LM2576 series switching regulator integrated circuits have reliable working performance, high working efficiency, and strong output current driving ability, which provides a strong guarantee for the stable and reliable operation of MCU.

Catalog

Introduction

Catalog

I Application Design

II Working Mode

FAQ

Ordering & Quantity

I Application Design

The basic voltage stabilizing circuit composed of LM2576 only needs four peripheral devices, and its circuit is shown in Figure 1.

Figure 1. voltage stabilizing circuit

The selection of inductance L1 depends on the output voltage, maximum input voltage, and maximum load current of  LM2576.  First, the electric voltage·microsecond constant (E·T) can be calculated according to the following formula:

E·T=(Vin - Vout)×Vout/ Vin×1000/f (1)

Vin is the maximum input voltage of LM2576, Vout is the output voltage of LM2576.  and f is the operating oscillation frequency value of LM2576  (52kHz).

Generally, the input capacitance Cin in the circuit should be greater than or equal to 100 μF. when installing, it is required to be close to the input pin of lm2576  as far as possible, and its withstand voltage value should match the maximum input voltage value.

The value of the output capacitor Cout (unit μF) should be calculated according to the following formula:

C≥13300 Vin/ Vout×L (2)

Vin is the maximum input voltage of LM2576; Vout is the output voltage of LM2576; L (unit μH) is the value of inductance L1. The withstand voltage value of capacitor C should be 1.5 ~ 2 times of rated output voltage. For 5V voltage output, it is recommended to use a capacitor with a voltage withstand value of 16V.

The rated current of diode D1 should be 1.2 times the maximum load current. Considering the state of the short circuit and load, the rated current of the diode should be greater than the maximum current limit of LM2576. The reverse voltage of the diode should be greater than 1.25 times the maximum input voltage.

The selection of Vin should consider the input voltage value of LM2576 corresponding to the lowest AC voltage drop (Vac-min) and the minimum input allowable voltage value Vmin of LM2576.  Therefore, Vin can be calculated according to the following formula:

Vin≥(220Vmin/Vac-min)

If the minimum allowable AC voltage drop is 30% (Vac-min=154V) and the voltage output of the LM2576  is 5V (Vmin=8V), when Vac=220V, the input DC voltage of the LM2576  should be greater than 11.5V, usually 12V.

II Working Mode

The pin 5 input level of LM2576 can be used to control the working state of LM2576.   The pin 5 input level is compatible with the TTL level. When the input is low level,  LM2576  works normally; when the input is high level,  LM2576 stops output and enters a low power consumption state.

Figure 2. Schematic diagram of controllable circuit

In Figure 2, The pull-down resistor can ensure the normal operation of LM2576 when the control terminal of mcu-con is low. The control end signal of shutdown input comes from MCU.  When the terminal of shutdown input is at a low level, LM2576 stops output, and the system enters into a low-power consumption state. When the terminal is high level, the transistor will make LM2576 work again. When the control terminal of MCU  -CON is high level and the triode is on, the resistance R will not damage the output control terminal of MCU due to over-current.


FAQ

  • What type of voltage regulator is used to convert high DC voltage into working voltage required by MCU?

78xx series three terminal voltage regulator

  • What is the working efficiency of the linear regulated power supply?

30% - 50%

  • What is one advantage of using LM2576 series switching power supply?

High-frequency on-off characteristics of switch tubes

  • Where is LM2576 used?

LM2576 is usually used as a voltage stabilizing device when the input and output voltage difference is large and the output current is also large. Because it is a switching regulator, it has a higher conversion efficiency and low heat generation than a linear regulator.

  • Whats the difference between LM2576T-ADJ and LM2576S-ADJ?

LM2576T-ADJ is the package of TO-220, LM2576S-ADJ is the package of TO-263-5, there is no difference in their functions.

  • What is the difference between LM2940 and LM2576? Which circuit are they applicable to?

LM2940 is a low-dropout linear stabilized integrated circuit. The linear stabilized power supply is characterized by a relatively simple circuit, high precision, and small ripple coefficient. It is suitable for precision power supplies with high voltage requirements. The disadvantage is that the efficiency is very low and the output The current is relatively small (relative to the switching power supply)

LM2576 is a switching power supply integrated circuit. Switching power supply, the circuit is more complicated, but the output current is large, the efficiency is high, the disadvantage is that the accuracy is lower and the ripple coefficient is larger.

  • Why do switching power supply chips LM2576 and LM2596 have diodes, inductors and capacitors behind the output pins?

The function of the diode and the inductance is that the output current can be continuous when the LM25XX is in the off state, and the function of the capacitor is to prevent the output voltage from sudden changes when the LM25XX is turned on and off. In fact, it is filtering.

  • Why the higher the switching frequency of LM2576 and LM2596, the smaller the output inductance and capacitance value?

Quite simply, the capacitive reactance of a capacitor decreases as the frequency increases, and the inductance of an inductor increases as the frequency increases. That is to say, the effect of using an inductance of 33uH in the case of 150Khz is basically the same as the effect of using an inductance of 100uH in the case of 52khz, and the principle of capacitance is the same. LM2596 is an upgraded version of LM2576. But LM2576 also has the advantage of less switching loss and less interference.

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