Low-cost DC-LED driver circuit design

Analysis of DC-LED driving technology route:

The LED driver circuit controls the power supply circuit for lighting the LED (light emitting diode). The forward voltage drop (VF) of the LED changes with the manufacturing error and ambient temperature, and constant voltage drive cannot be used. Therefore, a constant current drive is generally required to keep the current supplied to the LED constant. The circuit structure of the LED driver is basically the same as that of an ordinary switching regulator (DC-DC converter), with no significant difference. However, LED drivers have special requirements, and there are roughly two differences.

The first difference is the low feedback voltage (Figure). Feedback voltage refers to the voltage that is fed back to the switching regulator IC after detecting the output voltage. The LED driver IC uses constant current control, which converts the output current into a voltage as the feedback voltage.

Figure 1 Technical route schematic diagram

Normally, the feedback voltage set for a switching regulator IC is about 1.2V. However, 1.2V is too high for an LED driver IC. This is because a resistor of several hundred mΩ (current detection resistor) that converts the output current into a voltage needs to be connected in series with the LED. For high-brightness LEDs used in lighting fixtures, large currents such as 1.5A and 2A flow through them. If a current of 1.5A flows through a 500mΩ current detection resistor, this alone will generate a power loss of 0.75W. Therefore, the LED driver IC sets the feedback voltage to a very low level of about 0.2V. This allows not only a smaller current detection resistor to be used, but also power loss to be kept low when a large current flows through it.

The second difference is the improvement to achieve more accurate constant current control, because it is difficult to compensate for changes in input voltage, changes in LED forward voltage drop, changes in inductor and capacitor characteristics, etc. by simply relying on output current detection for feedback.

The representative improvement made by various LED driver IC manufacturers to achieve more accurate constant current control is the "constant pulsation method". After the above changes occur, the amplitude of the pulsation component superimposed on the current supplied to the LED will also change, causing the LED brightness to change. Generally speaking, the amplitude of the pulsation component is determined by the input voltage, output voltage, inductance, and the conduction time of the switch control. For this reason, the constant pulsation method detects the input voltage and output voltage and adjusts the conduction time according to the voltage. That is, by precisely controlling the frequency of the switch, more accurate constant current control is achieved.

Changyuntong Optoelectronics DC-LED design route:

Changyuntong Optoelectronics has been operating CYT34063 for many years, and its market share ranks among the top. Based on this theory and combined with the understanding of LED driving technology, it has designed a DC driver with high constant current accuracy and officially launched the DC-LED driver-CYT34065 in batches.

Figure 2 Buck LED driver reference design

CYT34065 is an LED driver derived from CYT34063 with improved design. CYT34065 retains the excellent features of CYT34063 and can drive LEDs with high efficiency. CYT34065 is a DC driver specially designed for high-power LEDs. It can be used for boost or buck applications. The maximum output typical value is 1A, and the efficiency is as high as 85%. In the battery DC application design, CYT34065 has a high cost performance, high reliability, high stability, and excellent constant current effect.

CYT34065 is particularly suitable for designing 3-12V boost designs, with outputs of 12-24V or above. It can also design 36V-24V or even lower than 12V buck drive circuits, CV or CC mode, and step down to an output voltage of more than 1.25V. CYT34065 still meets the output DC constant voltage mode design and is fully compatible with CYT34063 applications. The output sampling reference needs to be recalculated, and there is no need to modify the original design PCB circuit. And it can also design a voltage range below 1.25V or lower.

CYT34065 is available in 28V and 40V specifications. It is designed with step-up and step-down constant current LED drive circuits, built-in 1A power Darlington tube, switch-type high-efficiency converter, lightweight SOP8 or DIP8 package, and operates at a frequency of 100KHz. The sampling efficiency can reach 85% as low as 300mV, achieving high cost-effectiveness in DC drive constant current design.

Figure 3 Boost LED driver reference circuit

The design of CYT34065 has pioneered the application of CV and CC mode compatibility, and found a new way to design CV and CC DC output applications compatible. Changyuntong Optoelectronics Technology Co., Ltd. has been committed to the design and research of LED driver ICs. Based on the company's many years of DC power IC design, it has found a new design direction. Next, Changyuntong Optoelectronics Technology Co., Ltd. will successively launch a full range of DC application circuits that are compatible with CV and CC modes.