High-reliability MCU power supply design based on LM2576
The MCU of an embedded control system generally requires a stable operating voltage to work reliably. Designers are accustomed to using linear voltage regulators (such as the 78xx series three-terminal voltage regulators) as voltage regulators and voltage regulators to convert higher DC voltages into the operating voltage required by the MCU. This linear adjustment working mode of the linear voltage regulator will cause large "heat loss" (the value is V voltage drop × I load) during operation, and its working efficiency is only 30% to 50% [1]. In addition, the embedded industrial control system is often placed in a closed container when working in harsh environments such as high dust, which also exacerbates the harsh working conditions of the MCU, making the stability performance of the embedded control system even worse.
The switching power supply regulator works in a fully on or off mode. Therefore, when working, either a large current flows through the low conduction voltage switch tube, or it is completely cut off and no current flows. Therefore, the power consumption of the switching power supply is extremely low, and its average working efficiency can reach 70% to 90% [1]. Under the condition of the same voltage drop, the switching power supply regulator has much less "heat loss" than the linear voltage regulator. Therefore, the switching power supply can greatly reduce the volume of the heat sink and the area of the PCB board. In most cases, there is no need to install a heat sink, thereby reducing the harmful effects on the MCU working environment.
Another advantage of using a switching power supply to replace a linear power supply as the MCU power supply is that the high-frequency on-off characteristics of the switch tube and the use of a series filter inductor have a strong inhibitory effect on high-frequency interference from the power supply. In addition, due to the reduction of the "heat loss" of the switching power supply, the input voltage of the power supply can be increased during design, which helps to improve the ability of the AC voltage to resist drop interference.
The LM2576 series switching voltage regulator integrated circuit is a substitute for linear three-terminal voltage regulator devices (such as the 78xx series three-terminal voltage regulator integrated circuit). It has reliable working performance, high working efficiency and strong output current driving capability, thus providing a strong guarantee for the stable and reliable operation of the MCU.
1 Introduction to LM2576
The LM2576 series is a 3A current output step-down switch integrated voltage regulator circuit produced by National Semiconductor Corporation. It contains a fixed frequency oscillator (52kHz) and a reference voltage regulator (1.23V), and has a complete protection circuit, including current limiting and thermal shutdown circuits. This device only requires very few peripheral devices to form an efficient voltage regulator circuit. The LM2576 series includes two series: LM2576 (maximum input voltage 40V) and LM2576HV (maximum input voltage 60V). Each series of products provides multiple voltage grades such as 3.3V (-3.3), 5V (-5.0), 12V (-12), 15V (-15) and adjustable (-ADJ). In addition, the chip also provides an external control pin for the working status.
The main features of the LM2576 series switching regulator integrated circuits are as follows [2]:
●Maximum output current: 3A;
●Maximum input voltage: 40V for LM2576, 60V for LM2576HV;
●Output voltage: 3.3V, 5V, 12V, 15V and ADJ (adjustable) are available;
●Vibration frequency: 52kHz;
●Conversion efficiency: 75% to 88% (efficiency varies at different voltage outputs);
●Control mode: PWM;
●Operating temperature range: -40℃ ～ +125℃
●Working mode: low power consumption/normal mode can be externally controlled;
●Working mode control: TTL level compatible;
External components required: Only four (non-adjustable) or six (adjustable)
●Device protection: thermal shutdown and current limiting;
●Package type: TO-220 or TO-263.
The internal block diagram of LM2576 is shown in Figure 1. The pin definition of this block diagram corresponds to the five-pin TO-220 package.
The LM2576 contains a 52kHz oscillator, a 1.23V reference voltage regulator circuit, a thermal shutdown circuit, a current limiting circuit, an amplifier, a comparator, and an internal voltage regulator circuit. In order to generate different output voltages, the negative end of the comparator is usually connected to the reference voltage (1.23V), and the positive end is connected to the voltage divider resistor network. In this way, different resistance values can be selected according to different output voltages, where R1=1kΩ (open circuit when adjustable -ADJ), R2 is 1.7 kΩ (3.3V), 3.1 kΩ (5V), 8.84 kΩ (12V), 11.3 kΩ (15V) and 0 (-ADJ). The above resistors have been precisely adjusted inside the chip according to different models, so there is no need for users to consider them. Compare the output of the output voltage divider resistor network with the internal reference voltage regulator value of 1.23V. If the voltage is biased, the amplifier can be used to control the output duty cycle of the internal oscillator to keep the output voltage stable.
From Figure 1 and the characteristics of the LM2576 series switching voltage regulator integrated circuit, it can be seen that the switching voltage regulator with LM2576 as the core can completely replace the MCU voltage regulator composed of three-terminal voltage regulator devices.
2 LM2576 Application Examples
2.1 Basic application design
The basic voltage regulator circuit composed of LM2576 requires only four peripheral devices, and its circuit is shown in Figure 2.
The selection of inductor L1 should be based on the output voltage, maximum input voltage, maximum load current and other parameters of LM2576. First, calculate the voltage·microsecond constant (E·T) according to the following formula:
E·T=（Vin - Vout）×Vout/ Vin×1000/f (1)
In the above formula, Vin is the maximum input voltage of LM2576, Vout is the output voltage of LM2576, and ? is the operating oscillation frequency of LM2576 (52kHz). After E·T is determined, the required inductance value can be found by referring to the corresponding voltage·microsecond constant and load current curve provided in reference [2].
The input capacitor C2 in this circuit should generally be greater than or equal to 100μF. It should be installed as close to the input pin of LM2576 as possible, and its withstand voltage should match the maximum input voltage. The value of the output capacitor C1 should be calculated according to the following formula (in μF):
C ≥ 13300 Vin/ Vout × L (2)
In the above formula, Vin is the maximum input voltage of LM2576, Vout is the output voltage of LM2576, L is the value of inductor L1 selected by calculation and table lookup, and its unit is μH. The withstand voltage of capacitor C should be greater than 1.5 to 2 times the rated output voltage. For 5V voltage output, it is recommended to use a capacitor with a withstand voltage of 16V.
The rated current value of diode D1 should be greater than 1.2 times the maximum load current. Considering the load short circuit situation, the rated current value 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. Reference [2] recommends using 1N582x series Schottky diodes.
The selection of Vin should take into account the LM2576 input voltage value corresponding to the lowest AC voltage drop value (Vac-min) and the minimum input allowable voltage value Vmin of LM2576 (taking 5V voltage output as an example, the value is 8V). Therefore, Vin can be calculated according to the following formula:
Vin≥（220Vmin/Vac-min）
If the minimum allowable drop of AC voltage is 30% (Vac-min=154V) and the voltage output of LM2576 is 5V (Vmin=8V), then when Vac=220V, the input DC voltage of LM2576 should be greater than 11.5V, and can usually be selected as 12V.
2.2 Application design with controllable working mode
The input level of LM2576's 5-pin can be used to control the working state of LM2576. The input level of 5-pin is compatible with TTL level. When the input is low level, LM2576 works normally; when the input is high level, LM2576 stops output and enters low power consumption state. Figure 3 is the schematic diagram of the controllable working mode circuit of LM2576.
In Figure 3, the pull-down resistor R2 can ensure the normal operation of LM2576 when the MCU-CON control terminal is low, and its value is 1~10kΩ. The control terminal signal of MCU-CON comes from MCU. When this terminal is high, LM2576 stops outputting and the system enters a low power consumption state. Closing switch K will make LM2576 work again. The selection of R1 is related to the resistance value of R2. When designing, it is ensured that when the MCU-CON control terminal is high and K is closed, R1 will not damage the output control terminal of MCU due to overcurrent. Similarly, when the MCU-CON control terminal is high and K is disconnected, the voltage divider on R2 should be greater than the minimum value of TTL high level (2V).
2.3 Coordination design with linear voltage regulators
Higher output voltage ripple (generally greater than 20mV) is an unavoidable problem in the design of switching regulated power supplies. In some occasions where there are special requirements for power supply ripple voltage (such as a high-precision A/D converter inside the MCU, etc.), a switching regulated power supply can be used to improve the working efficiency of the regulated power supply or a linear regulated power supply can be used to reduce the output ripple voltage of the regulated power supply. Therefore, the combination of a switching regulated power supply and a linear regulated power supply can provide a better way to power MCUs with special requirements. Figure 4 is a schematic diagram of a low ripple output voltage regulated circuit.
The first half of Figure 4 is similar to Figure 2. In order to improve the overall working efficiency of the voltage stabilizer, when IC2 uses 7805, since the minimum input voltage of 7805 is 7.5V, the switching voltage stabilizer integrated circuit in Figure 4 uses an adjustable output chip (LM2576-ADJ). In the figure, the relationship between the output voltage Vort of the switching voltage stabilizer integrated circuit and R1 and R2 is as follows:
Wart=1.23×（1+ R2/ R1）
3 Conclusion
It has been proved in actual use that using LM2576 series switching voltage regulator integrated circuit as the core device of MCU voltage regulator can not only improve the working efficiency of voltage regulator, reduce energy loss, reduce thermal damage to MCU, but also reduce the interference of large fluctuation of external AC voltage on MCU, and at the same time reduce high-frequency interference entering through power supply, which can achieve twice the result with half the effort in ensuring the safe and reliable operation of MCU.
提升卡
变色卡
千斤顶
京公网安备 11010802033920号
