Detailed Interpretation of Low-Cost DC-LED Drivers - CYT34065
Changdong Optoelectronics Technology Co., Ltd., an integrated circuit design company dedicated to power supply and LED driver design, recently released the best economical DC- LED driver - CYT34065.
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.
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.
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