High efficiency and high precision constant current solution for LED lighting

　　OZ8022 is a non-isolated, high-efficiency, precise constant-current driver chip from O2O Technology, using a buck architecture. The most attractive feature is the three-stage dimming function achieved by a wall switch. With this function, users can use lighting products that can adjust the brightness of the light source that best suits them without making any changes to their home. Below we will introduce the features of the OZ8022 chip:

　　Valley switching technology for high efficiency

　　OZ8022 uses valley detection technology inside, and the near zero current opening effectively improves the conversion efficiency (as shown in Figure 1a). Gate is the output waveform of OZ8022, Vds is the voltage across MOS DS, and IL is the current waveform of the output inductor. From the figure, we can see that when the Gate state is "on", the voltage drop across Vds is very small and is at a low level, and the output inductor current begins to increase; when the Gate state is "off", the output inductor begins to discharge, and when the inductor discharge is completed, the Vds of MOS reaches the valley bottom. At this time, the OZ8022 ZCD pin detects the Vds valley bottom through the auxiliary winding and the Gate will be turned on again. At this time, the switching loss is minimal. Through experiments, it is assumed that a 5-8W LED driver can achieve an efficiency of more than 90%.

　　High precision constant current

　　In the traditional Buck circuit, current detection is shown in Figure 1b. When the Gate is "on", current I2 is formed. I2 flows through R1 and forms a voltage across R1. Sense controls I2 to make it output constantly. However, when the Gate is "off", L1 reverses and forms a discharge loop I1 with the LED through the freewheeling diode D1. However, the IC cannot detect this discharge current, and the energy of I1 is determined by the inductance of L1. Therefore, the constant current accuracy of this traditional Buck circuit is not ideal.

　　Based on the shortcomings of the traditional Buck circuit, O2Micro has launched the OZ8022 control chip. As shown in Figure 1c, when Q1 is turned on, the current passes through R1, L1 and LED to form a current loop I1. At this time, OZ8022 detects the current I1 flowing through R1 and LED through the sense pin and controls it. When Q1 is turned off, L1 reverses, and the energy passes through the LED and the freewheeling diode D1 to form a current loop I2 again. Moreover, this current can still control the size of I2 from the voltage drop formed at both ends of R1. Therefore, in this way, whether Q1 is turned on or off, the current at the LED end can be accurately detected, thereby achieving high-precision constant current control. This control method has been patented by O2Micro.

　　Three-stage dimming technology

　　The so-called three-stage dimming technology means that when you turn on the power switch continuously, the IC will automatically switch to the next brightness. The OZ8022 has three brightness output modes, namely 100%, 60%, and 20%.

　　When the AC input is turned on, the VDD voltage starts to rise to the start-up voltage of 15V, and the IC starts normal output (Figure 2). At this time, the Sense ADJ reference voltage is 250mV (100% brightness). When the AC is disconnected, the VDD voltage starts to drop, and the IC stops working when it drops to 9V. The VDD voltage continues to drop, but as long as the AC power supply is turned on again during the period when VDD drops to 9~4V, the IC will enter the next brightness. Therefore, if the AC input is disconnected and then turned on again, the IC will switch to the second brightness, that is, the 150mV ADJ reference (60% brightness). Switching the AC on and off again, the input IC switches to the 50mV ADJ reference (20% brightness). By repeatedly switching the AC on and off, the input voltage IC will change the ADJ reference voltage in sequence, that is, 250mV, 150mV, and 50mV, so the corresponding output brightness is 100%, 60, and 20%, thereby realizing three-stage dimming.