lm2596 input voltage range and using method

 

The LM2596 module adopts the switching regulator chip LM2596, and the solid capacitor is integrated with 220uf/35V.

Input: DC 4-35V (The input voltage must be 1.5V higher than the output voltage.)

Output: DC 2-30V voltage is continuously adjustable, and the maximum output current of high efficiency is 3A.

The LM2596 series is a 3A current output step-down switch integrated voltage regulator chip produced by Texas Instruments (TI). It contains a fixed frequency oscillator (150KHZ) and a reference regulator (1.23v), and has a complete protection circuit and current Limits, thermal shutdown circuits, etc. The use of this device can form a high-efficiency voltage stabilizing circuit with very few peripheral devices. Provided are: 3.3V, 5V, 12V and adjustable (-ADJ) and other voltage grade products. In addition, the chip also provides external control pins for working status.

 

The main features of LM2596 series switching regulator integrated circuits are as follows:

1. Output voltage: 3.3V, 5V, 12V and (ADJ), etc., the maximum output voltage is 37V

2. Working mode: low power consumption/normal two modes. Can be controlled externally

3. Working mode control: TTL level compatible

4. Required external components: only four (not adjustable); six (adjustable)

5. Device protection: thermal shutdown and current limit

6. Package form: 5-pin (TO-220 (T); TO-263 (S))

 

Internal block diagram of LM2596 chip

Note: This picture is the internal block diagram of TO-220 package. 

LM2596 includes 150KHZ oscillator, 1.23v reference voltage regulator circuit, thermal shutdown circuit, current limit circuit, amplifier, comparator and internal voltage regulator circuit.

In order to generate different output voltages, the negative terminal of the comparator is usually connected to the reference voltage (1.23V), and the positive terminal is connected to a voltage divider resistor network. Among them, R1=1KΩ and R2 are 1.7KΩ (3.3v), 3.1KΩ (5V), 8.8KΩ (12V), 0 (-ADJ), respectively. Compare the output of the output voltage divider resistor network with the internal reference voltage of 1.23V. If the voltage is deviated, the amplifier can be used to control the output duty cycle of the internal oscillator, so that the output voltage remains stable.

 

Application:

Specific applications can be selected according to needs: LM2596 adjustable voltage type, LM2596-5V, LM2596-3.3 and other constant voltage output types. If you want to obtain +1.8V output voltage, please use Figure A, and if you want to obtain +3.3V, +5V output voltage, please use Figure B. 



LM2596 supports adjustable output. When the input is 40V, the output can be continuously adjusted to 0～37V. The typical application circuit is as follows: 



Use of LM2596

LM2596 is a common switching regulator, simple to use, cheap, and easy to buy. However, the problem often encountered is that the load capacity cannot reach 3A, in fact, sometimes even 2A is not reached. (I debugged the domestic TO-263 chip that I got from the agent, TI (NaTIonal) original may perform better.) The main factors that affect the load capacity of LM2596 are the following three factors:

First, the selection of Schottky diodes.

For the catch diode used here, the sustainable working current needs to be 1.3 times or more of the maximum load current, so when a 3A load is used, a 5A Schottky diode is used.

Second, the selection of inductors.

The rated operating current of the inductor needs to be above 3A.

Third, heat dissipation design.

TI’s technical manual "Thermal Design By Insight, Not Hindsight" mentions the heat dissipation design of TO-263: Since the bottom layer of TO-263 is thermal pad and the surface layer is plastic, the heat sink is installed on the bottom layer (directly in contact with the thermal pad or Through thermal via), the effect is better than installing on the surface.

When testing, use +24V input, +8V output, the load is provided by the electronic load, and the current is 3A.

The bottom and surface heat sinks were used at the same time without a fan. It was found that the heat sink was very hot within a few seconds. At the same time, the output voltage continued to drop slowly until it was close to 4V, and it jumped between on and off. Add a small fan to blow in the direction of the upper and lower heat sinks. Fingers can always be placed on the heat sink, the output is normal, and it has passed the 12-hour test.

When the surface heat sink is not used, even with a fan, the surface temperature of the LM2596 still feels quite scary, and it is more reliable to add up and down.

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