Low cost DC-LED driver circuit design

DC- LED drive technology route analysis:

The LED drive circuit controls when the LED (Light Emitting Diode) illuminates the power supply circuit of the required power. The forward voltage drop (VF) of the LED changes with manufacturing error and ambient temperature, and constant voltage drive cannot be used. Therefore, it is generally necessary to use a constant current drive to keep the current supplied to the LED constant. The circuit structure of the LED driver is basically the same as that of a conventional switching regulator (DC-DC converter), and there is not much difference. But LED drivers have special requirements, and there are roughly two differences.

The first difference is that the feedback voltage is low (Figure). The feedback voltage is the voltage that is detected by the output voltage and fed back to the switching regulator IC. The LED driver IC uses constant current control to convert the output current into a voltage and then use it as a feedback voltage.

Figure 1 Schematic diagram of the technical route

Typically, the switching regulator IC sets a feedback voltage of approximately 1.2V. But for LED driver ICs, 1.2V is too large. Because a resistor of several hundred mΩ (current detecting 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 and the like, large currents such as 1.5A and 2A flow through. If a current of 1.5A is passed through the 500mΩ current-sense resistor, this alone will generate 0.75W of power loss. Therefore, the LED driver IC sets the feedback voltage to a very low 0.2V. In this way, not only a small current detecting resistor but also a large current can be used to control the power loss to a low level.

The second difference is the improvement to achieve more accurate constant current control. Because it relies solely on the detection of the output current for feedback, it is difficult to compensate for changes in the input voltage, changes in the forward voltage drop of the LED, and changes in the characteristics of the inductor and capacitor.

The representative improvement of LED driver IC manufacturers in order to achieve more accurate constant current control is "constant pulsation mode". When the above change occurs, the amplitude of the pulsating component that overlaps with the current supplied to the LED also changes, resulting in a change in the brightness of the LED. In general, the amplitude of the pulsating component is determined by the input voltage, the output voltage, the inductance, and the on-time controlled by the switch. To this end, the constant pulsation mode detects the input voltage and the output voltage, and adjusts the on-time according to the voltage. That is, by precisely controlling the frequency of the switch, more precise constant current control is achieved.

Changyuntong photoelectric DC-LED design route:

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

Figure 2 Step-Down LED Driver Reference Design

CYT34065 is an LED driver derived from the improved design of CYT34063. CYT34065 continues to retain the outstanding features of CYT34063 and also drives LEDs with high efficiency. CYT34065 is a DC driver specially designed for high power LED. It can be used for boost or buck applications. The maximum output is 1A and the efficiency is over 85%. CYT34065 has a high cost performance, high reliability, high stability and excellent constant current effect in battery DC application design.

CYT34065 is especially suitable for designing 3-12V boost design with output of 12-24V or more. It can also design a 36V-24V or even lower 12V step-down drive line, CV or CC mode, and step down to an output voltage as low as 1.25V or higher. The CYT34065 still meets the output DC constant voltage mode design and is fully compatible with the CYT34063 application. The output sampling reference needs to be recalculated, and the original design PCB line does not need to be modified. It is also possible to design voltage ranges below 1.25V or lower.

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

Figure 3 step-up LED driver reference circuit

The CYT34065 design opens up a new way for CV and CC mode compatible applications, finding a new way to design compatible CV and CC DC output applications. Changyuntong Optoelectronic Technology Co., Ltd. has been committed to LED driver IC design research. Based on the company's multi-year DC power IC design, it has found a new design direction. Next, Changyun Optoelectronics Technology Co., Ltd. will launch a full range of DC application circuits and CVs. Compatible with CC mode.