LED series-parallel drive design

　　Please note that VF values ​​need to be divided into different grades. LEDs with the same VF value should be used on the same product as much as possible. The product can ensure that the error current is within 1mA and the LED works in a relative constant current state.

　　The integrated transistors in the figure below can keep the current of each LED channel consistent. These transistors have the same β value produced under the same temperature environment and the same process conditions, which can ensure that the current of each channel is basically the same. The constant current part can be designed like this under conditions where the requirements are not very high. A stable voltage or a stable PWM volt value drives the bias of the transistor after voltage stabilization to achieve basic constant current.

　　The figure below uses a high-precision IC as a constant current reference source. R can set the IC output current. Once the R resistance value is determined, it can be replaced by a fixed resistor. The use of multi-transistor integrated devices can reduce the number of ICs used, thereby reducing the cost of design products.

　　Linear high-power LED constant current output can be used in parallel. In product design, we often cannot find driver ICs with larger currents. Generally, more than 2A is rare, and ICs with a nominal current of 2A may not be used to the limit. The reason for the process cost of ICs greater than 1A is that MOS tubes are external, and the circuits of external MOS tubes are complex and the reliability is reduced. Parallel use is an effective design method.

　　The figure below uses the DD312 parallel reference design to directly drive three 6W LEDs. The PWM control signal needs to be properly isolated to avoid mutual interference and drive capability issues. The EN enable voltage must meet the requirements of the specification, and the voltage should not be too high to damage the EN pin. Generally, the IC withstand voltage refers to the load and power supply. If the excitation voltage is not specified, please do not design it to be greater than 5V.

　　Like this, the LED constant current driver IC can also be designed in parallel for detection at one end of the LED. In fact, the IC works alone and finally flows in parallel. The DC-DC method works at a higher frequency. It should be noted that when laying out the PCB, cross design should be avoided. The respective filter and bypass capacitors should be close to the IC, and the load current will finally be summed.

　　Of course, you can design 2 in parallel, 3 in parallel, or more in parallel, but you should try it more often!