Single TL431 constant current/current limiting techniques and improvements in LED

Due to its characteristics in impedance, TL431 is often used to replace diodes . As a precision voltage regulator, it is widely used in various circuit designs. In LED driving, TL431 plays a role in constant current and current limiting of the circuit. There are many reference materials on the Internet about the principle. This article will not go into details, but focus on the constant current/current limiting techniques of a single TL431.

First, let's get familiar with the circuit diagram. The flyback drive method is used here.

Circuit Description

TL431 itself is an excellent voltage regulator, so we will continue to use it as a reference voltage here to achieve constant current. Figure 1 is the simplest TL431 constant current solution. Because it does not require debugging, this solution is still used by a large number of small factories. And as long as the quality of TL431 is reliable, the product quality depends on the selected resistor .

From the perspective of constant current principle, the current flow in Figure 1 is determined by the sampling resistor and the reference voltage. The current through the LED circuit is determined by the sampling resistor and the reference voltage of TL431. The current passing through is as shown in the ICC formula in Figure 1 above. The current flow is divided into two parts. One is directly consumed by the sampling resistor, and the other provides uA-level current for the input op amp bias of TL431. This part of the current can basically be ignored. So as long as the reference is fine and the change is small, the current is constant.

The reference voltage value of TL431 is 2.495V. A certain deviation needs to be considered, and the constant current of some circuits cannot be too large, otherwise the inherent loss of the sampling resistor will be amazing.

Usage tips

RCS uses high-precision, low-temperature drift resistors. If the volume is acceptable, plug-in resistors are preferred. After testing the actual 300mA circuit, the heat generated by two 1206 SMD resistors is still relatively large. More 1206 SMDs can be used to evenly distribute the loss.

If the output voltage is within 30V or not very high, it can be used directly. Optocoupler power supply does not need to be provided separately. Due to inherent losses, the lower the power, the less advantage. After actual measurement, the efficiency gap between 3W and 5W is quite large, so a higher power can "make up" for this inherent loss.

The number of LED loads can be changed at will without affecting the current accuracy. A smaller current is advantageous. In addition, the power should not be too large, with a current of 400mA (less than 250mA in practice).

ways to improve

If TL432 is used as a benchmark, the loss can be reduced by half, but the cost will increase accordingly due to the amount of material used.

By dividing the 2.5V reference voltage again, some losses can be reduced. The schematic diagram is as follows:

After such improvement, the pressure on Rcs can be relieved. You can set the reference voltage above by yourself, and then carefully adjust R3 and R4 to ensure that the output voltage meets the standard while the current remains constant. The disadvantage is that there are certain requirements for the design, and the influence of the TL431 reference on the output voltage must be fully considered. The resistance matching of R3 and R is more troublesome, and it is not suitable for occasions where the number of LED lamp loads changes.

This article introduces the current limiting/constant current techniques of a single TL431 in an LED driver circuit, and proposes a method to improve the loss problem in the circuit. It can be said to be an entry-level article that is very suitable for novices to read. I hope that after reading this article, you can master more knowledge about TL431.