EMI Interference Issues of White LED Drivers in Mobile Phone Design

 At present, mobile phones generally use white light LED as the backlight element of the display screen, and the corresponding white light LED driver has become an indispensable IC in the design of mobile phones. White light LED drivers use a switching power supply topology, such as an inductive boost converter. While the converter switches at high speed, it will cause EMI interference due to the use of inductors, which will bring difficulties to the design of other functional modules of the mobile phone. As the LCD screen increases, the output capacity required by the driver also increases accordingly, and EMI interference will become serious. Therefore, EMI considerations must be taken seriously when designing white light LED drivers.

　　In addition to providing the ability to drive 10 LEDs, the TPS61161 boost converter launched by Texas Instruments also has corresponding design considerations for EMI issues. Its typical application is shown in Figure 1. The TPS61161 switch design adopts two switching processes to effectively reduce the radiation intensity of EMI, thereby avoiding the impact of the driver on other modules of the mobile phone. As shown in the black curve in Figure 2, when the TPS61161 turns on the internal MOSFET switch tube, the drain-source voltage Vds of the MOSFET changes from high voltage to low voltage close to zero in a very short time, that is, a large voltage change rate dv/dt; and in the early stage of the switch, due to the characteristics of the MOSFET, the current change rate flowing through the MOSFET switch tube is also large, that is, di/dt. Considering the role of dv/dt and di/dt in the generation of EMI, in the early stage of the MOSFET opening, the EMI intensity is reduced by slowing down the switch voltage change rate dv/dt, as shown in the red curve in Figure 2.

　　The actual EMI test results of traditional switch technology and secondary switch technology prove that the secondary switch technology of TPS61161 reduces the EMI radiation energy. In the EMI test experiment, TPS61161 drives 10 series LEDs through the battery voltage of 3.7V. Figure 3a shows the white noise of the EMI test environment space, Figure 3b shows the EMI test results of TPS61161 using traditional switches, and Figure 3c shows the EMI test results of TPS61161 using existing secondary switch technology. The test results show that the secondary switch reduces the EMI radiation intensity by 10db.

　　In addition, TPS61161 supports linear dimming technology - by adjusting the LED's on-current, the LED's luminous intensity is changed. This dimming method effectively avoids EMI interference caused by LED dimming. Such interference often occurs in PWM dimming mode.

　　Of course, when designing a specific application of TPS61161, a good PCB layout can also help to better reduce the interference of EMI on the mobile phone system. In addition to TPS61161, Texas Instruments' TPS61160 white light LED driver and TPS61165 high brightness LED driver all use secondary switching technology. For details, please refer to the Texas Instruments product catalog.