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LM2596 Circuit: Adjustable Regulated DC Power Supply

  • Contents

I Description

This blog has designed a step-down DC switching power supply (its voltage can be adjusted from 0V) for neutron tube storage. The power supply adopts the BUCK topology, and the rail-to-rail LMV358 operational amplifier voltage feedback circuit is designed to cooperate with the LM2596 internal error amplifier to overcome the shortcomings that the minimum output voltage of the LM2596 step-down chip cannot be lower than 1.2V, thereby the output voltage can be adjusted from 0V.

This video reviews an LM2596 DC to DC, adjustable, step down regualtor

Catalog

I Description

II Introduction

III LM2596 Basic Circuit

IV Design of Zero-starting Peripheral Circuit

4.1 Voltage Reference Circuit

4.2 Voltage Feedback Circuit

V Power Inductor Parameter

VI Analysis of Experimental Results

VII Conclusion

FAQ

Ordering & Quantity

II Introduction

Before we start reading this blog, please take a few seconds to look at the following two questions:

① What is the neutron tube storage?

② What is the relationship between the neutron tube storage and the DC power supply that requires the voltage can be adjusted from 0V?

lm2596

Figure 1. LM2596

  • What is the neutron tube storage?

Neutron tube storage is generally composed of powder with strong hydrogen absorption capacity and heating wire, used to store deuterium and tritium gas for the nuclear fusion reaction. By controlling the current of the heating wire, the outgassing volume of the reservoir and the air pressure in the tube are indirectly controlled, thereby controlling the neutron yield. The cold resistance of the reservoir is about 3Ω. When the passing current reaches 0.35A, the reservoir starts to release deuterium and tritium gas. The addition of the ion source voltage will ionize the gas in the ion source, and the acceleration high voltage will cause the ionized deuterium-tritium ions in the ion source to be extracted and accelerated, and a deuterium-tritium fusion reaction occurs on the target to produce neutrons.

  • What is the relationship between the neutron tube storage and the DC power supply that requires the voltage can be adjusted from 0V?

In order to accurately control the heating current of the storage, the power supply of the neutron tube storage requires that the voltage can be adjusted from 0V.

After we took a quick look at one of the circuit protagonists, neutron tube storage, discussed in this blog, let's take a look at another equally important role: the LM2596 chip. The voltage reference of the LM2596 integrated voltage error amplifier is 1.2V so that its lowest output voltage is not less than 1.2V.

Does it seem that the LM2596 chip itself cannot be adjusted from 0V? How can this defect be resolved?

We can use the rail-to-rail op amp LMV358  powered by a single power supply to design the voltage feedback circuit. By isolating the sampling terminal and the feedback terminal, the linear adjustment interval of the op amp can be used to compensate for the variation of the feedback voltage, which can solve the problem of the lowest output of LM2596 that cannot be lower than the problem caused by 1.2V.

After experimental tests, the design of the power supply feedback circuit is simple and efficient. Not only the power output voltage can be adjusted from zero, but also the output voltage adjustment accuracy is high, so the actual application needs can also be solved.

III LM2596 Basic Circuit

LM2596 is available in fixed and adjustable versions. Among them, the output voltage of the adjustable version ranges from 1.2v to 37V, with the maximum input voltage does not exceed 45V. Under load conditions, the circuit shows the following characteristics:

  • Voltage regulation ±4%.
  • Do not need too many components for peripheral circuits.
  • Low power bypass mode.
  • The typical static current is 80 μ A.

LM2596 minimum output voltage 1.2V application circuit is shown in Figure 2. As can be seen from Figure 1, the feedback resistors R1 and R2 determine the voltage divider ratio of the power supply output voltage, which can be adjusted by adjusting R1. When R1 is adjusted to 0 Ω, the output voltage is the minimum, which is the internal voltage reference of the LM2596 chip.

lm2596 circuit

Figure 2. LM2596 Basic Circuit

The internal voltage reference at the feedback end of the LM2596 chip is 1.2V, which limits the output voltage of the power supply to be adjusted from zero.

IV Design of Zero-starting Peripheral Circuit

First, try to connect the FB terminal and the Vs terminal in Figure 1, and adjust the R1 resistance to 0Ω. At this time, the Vout is at least 1.2V. What should we do to achieve the purpose of adjusting from zero? We can disconnect Vs, design an independent voltage feedback circuit to adjust the FB terminal voltage, and change the error variation of the LM2596 internal voltage error amplifier through the external input voltage to increase or decrease the  PWM  output duty cycle, which in turn enables the output voltage to be adjusted from zero.

4.1 Voltage Reference Circuit

So, how to ensure that the FB terminal voltage of U1 is around 1.2V? Since the sampling voltage feedback terminal needs a voltage regulator for compensation, we can use a simplified circuit design method, that is, using TL431 to achieve this goal. What is TL431 TL431 has the following characteristics:

  • TL431 is a typical three-terminal precision voltage regulator
  • The output voltage from 2.5 to 36V can be arbitrarily set with 2 resistors
  • Its dynamic impedance is 0.2Ω
  • The accuracy of the voltage reference is 0.6%

The voltage reference circuit based on TL431 is shown in Figure3.

tl431 voltage reference circuitFigure 3.  TL431 Voltage Reference Circuit

In this circuit, the voltage reference has two functions: it can both be used as the external input voltage reference and the compensation voltage of op amp in phase. The external input voltage is used to change the output voltage, while the compensation voltage is used to compensate for the change in voltage at the feedback terminal.

4.2 Voltage Feedback Circuit

The non-inverting end of U1's internal voltage error amplifier  is integrated with a 1.2V voltage reference and the output voltage feedback resistor R2 is connected to the inverting end of the error amplifier  :

  • When the voltage at the feedback terminal is greater than 1.2V, the error amplifier  generates a negative error signal, the  PWM output duty cycle decreases, and the output voltage decreases;
  • When the voltage at the feedback terminal is less than 1.2V, the error amplifier generates a positive error signal, the PWM output duty cycle increases, and the output voltage increases.

Regardless of whether the output voltage of the power supply becomes larger or smaller, the chip can keep the output voltage stable by controlling the on time of the switch tube, but the minimum output voltage is 1.2V.

The voltage feedback circuit designed by LMV358 is shown in Figure 4.

Figure 4.  LMV358 Voltage Feedback Circuit

Here is a question: What is the key to realizing the output voltage adjustment from zero? In fact, it is very simple. The key is whether the lowest voltage of FB in the operational amplifier feedback circuit can be 0V. In the case of a single power supply, the op amp has integrated transistors, and its minimum output voltage is about 0.6V (which obviously does not meet the above requirements). If we want to solve this problem, we can choose LMV358 op amp powered by the rail-to-rail single power supply, with a minimum output voltage of 65mV, so as to meet the requirement of power adjustment from zero.

make:

  • M=R14/R13
  • N=R16/(R16+R18)
  • Q=R19/R20

Then the voltage feedback circuit parameters are calculated as follows:

                                                                               (1)  Vo1=Vs    

                                                                               (2)  Vo2=(1+M)×Vref-Vad×M   

                                                                               (3)  Vo3=N×(Vo1+Vo2)×(1+Q)       

                                                                               (4)  Vo4=k×Vo3=Vfb      

In the above formula:

  • k is a variable coefficient.
  • Let FB=1.2V. When the power is initially powered on, Vad=0V, Vref=2.5V, but the output voltage of the operational amplifier does not necessarily meet FB=1.2V, causing LM2596 to malfunction. In order to avoid this situation, we can use R21 to correct the voltage value of Vfb at the initial power-on, so that FB=1.2V.
  • Increase the Vad terminal voltage, the Vo2 terminal voltage decreases and the output voltage increases, so that Vo1 increases. Conversely, Vo2 compensates the variation of Vo1 to ensure that the voltage at the feedback terminal of the chip is equal to 1.2V, so that the sampling voltage Vs follows the change of the Vad voltage, and the power supply can be adjusted from zero.

The test data of the voltage feedback circuit is shown in Table 1.

Table 1. Test Data

From the above test data, we can see that we can make use of the linear output characteristics of the operational amplifier to achieve the purpose of linear adjustment of the output voltage. The Vs changes with the Vad, the chip reference terminal voltage Vfb remains unchanged, and the output voltage range is determined by the resistance partial voltage ratio. Under the condition of full load,  when the Vad input voltage changes from 0 to 5V, the sampling resistor Vs terminal voltage output range can be 0 to 5V, and the power supply output voltage is adjustable from 0 to 35V through the resistor divider ratio.

V Power Inductor Parameter

There are 3 operating modes for the inductor current in the BUCK circuit. This blog designs the inductor according to the CCM working state, and fine-tunes the inductor parameters according to the actual test results. When the power tube is turned on, the inductor current rises linearly, and the current increment expression is:

(5)  ΔION=(VON×TON)/L      

VON, TON, and L are the inductor conduction voltage, turn-on time and inductance, respectively. The current decrement in the turn-off phase of the power tube can be expressed as:

(6)  ΔIOFF=(VOFF×TOFF)/L       

VOFF and TOFF are the inductor voltage and turn-off time when turn-off. In a switching cycle, the increment and decrement of the inductor current are equal, and the volt-second law can be used to obtain:

(7)  VON×TON=VOFF×TOFF       

In the buck topology, VON=VIN-VO, VOFF=VO, the above formula is transformed to derive the duty cycle equation of BUCK topology work:

(8)  D=VO/VIN            

How to determine the duty cycle? The power supply input voltage is a fixed value of 40V, and the output voltage range is 0-35V. From this, the duty cycle D=0-0.875 can be calculated. Here, we design the inductor parameters according to the principle of maximum duty cycle and maximum output power. So how is IL determined? The maximum load capacity of LM2596 is 3A. For the BUCK topology, the average inductor current IL is equal to the load current IO. We can set the current ripple rate r to be 0.3, and the choice of r affects device selection and circuit cost. The expression of r is defined as follows:

(9)  r=ΔI/IL  

In the above formula: ΔI is the change of the inductor current in a period, and the parameters are substituted into the above formula to obtain ΔI=0.9A.

According to the law of electromagnetic induction, the inductance is calculated as the following formula:

(10)  L=(VON×D)/(r×IL×f)

It is known that the switching frequency of the LM2596 chip is 150kHz, and the calculated parameters are substituted into the above formula to calculate L = 259 μ H. In the actual debugging, the theoretically calculated inductance parameters can not meet the practical application requirements. According to the actual debugging results, the high frequency power inductor with rated input current 4A and inductance value 330 μ H is selected.

VI Analysis of Experimental Results

Make a prototype according to the design parameters. Under the condition of full load, given 40V input voltage, the output voltage of the power supply under different input voltage is measured by adjusting the external Vad voltage. Table 2 shows the output voltage test data at load 10.1 Ω. In practical application, the efficiency of the power supply is the first concern. Table 3 shows the data for measuring voltage and current at the input and output of the power supply, respectively. As can be seen from tables 2 and 3, the input and output voltages are proportional to each other, and the regulation accuracy of the output voltage is about 0.05V. The output voltage of the power supply can be adjusted from zero, which solves the deficiency of 1.2V of the lowest output voltage of the LM2596 chip. Using the above calculation method, the average efficiency of the power supply is 93.44%. Using the test data under no-load and full-load conditions, it is calculated that the load adjustment rate of the power supply at 35V output is 1.3%. In addition, the power supply can not only work for a long time, the temperature rise is normal, but also the performance is stable.

lm2596 dc regulated power supply  (10.1Ω)

Table 2.  Test Data: LM2596 DC Regulated Power Supply  (10.1Ω)

 

Table 3.  Test Data: LM2596 DC Regulated Power Supply Efficiency

 

VII Conclusion

In order to solve the shortcoming that the output voltage of LM2596 cannot be adjusted from zero, this blog adopts the method of isolating the sampling voltage and the feedback end of the chip error amplifier.  That is: a voltage feedback circuit by designing a rail-to-rail op amp to make the output voltage capable of adjusting from zero. In addition, we analyzed the design of the inductance parameters of the adjustable BUCK power supply in principle, and make a prototype according to the parameters to verify the correctness and reliability of the design.

The experimental results show that the load adjustment rate of the power supply is 0.88%, the maximum working efficiency under rated load is 95.08%, and the full load power is 105W, which can meet the needs of practical applications.


FAQ

  • How does a LM2596 work?

The LM2596 operates at a switching frequency of 150 kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators. Available in a standard 5−lead TO−220 package with several different lead bend options, and D2PAK surface mount package.

  • What is the purpose of Neutron tube storage?

Deuterium and tritium gas.

  • What is lm2596?

The LM2596 series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving a 3-A load with excellent line and load regulation.

  • What is the difference between LM2596 and LM2595?

LM2596: The highest output current is 3A, 1PIN-VOUT, 2PIN-VIN;

LM2595: The highest output current is 1A, 1PIN-VIN, 2PIN-VOUT.

  • What is the voltage of each pin of LM2596-12? What is the function of each pin?

Pin 1 is the input terminal, 12V, the normal working voltage range should be 14V~37V under the output condition; pin 2 is the output terminal, here is the high-frequency oscillation output, not DC voltage, so there is no definite voltage; pin 3 is grounded, 0V ; Pin 5 is the enable control terminal, LM2596 works normally when connected to a low level, and no output is turned off when connected to a high level, usually directly grounded; Pin 4 is the feedback control signal input terminal, connected to the middle connection point of the proportional resistor, where the voltage It is proportional to the actual output.

  • Can LM2596 output negative voltage?

LM2596 can output negative voltage.

LM2596 has different application circuits, which can output positive voltage or negative voltage, but it has no boost function, and the absolute value of its output voltage must be less than the absolute value of the input voltage.

  • In the circuit with adjustable output voltage of lm2596, can the output voltage be greater than the input voltage?

The LM2596 switching voltage regulator is a step-down power management monolithic integrated circuit, which can output 3A drive current and has good linearity and load regulation characteristics. The output voltage will not be greater than the input voltage. If the output voltage is greater than the input voltage, it is best to use a step-up power chip. Such as XL6009, VT1050.

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