At present, mobile phones generally use white light LEDs as the backlight components of the display screen, and the corresponding white light LED driver has become an indispensable IC in the design of mobile phones. The white light LED driver uses a switching power supply topology, such as an inductive boost converter. While the converter is switching at high speed, the EMI interference generated by the use of inductance will bring difficulties to the design of other functional modules of the mobile phone. With the increase of LCD screens, the output capacity required by the driver increases accordingly, and EMI interference will also become serious. Therefore, EMI considerations must be taken seriously when designing white LED drivers.
In addition to the drive capability of 10 LEDs, the TPS61161 boost converter introduced by Texas Instruments also has corresponding design considerations on EMI issues. Its typical application is shown in Figure 1. In the TPS61161 switch design, two switching processes are adopted to effectively reduce the EMI radiation intensity, thereby avoiding the impact of the driver on other modules of the mobile phone. As shown by the black curve in Figure 2, when the TPS61161 turns on the internal MOSFET switch, the drain-source voltage Vds of the MOSFET changes from a high voltage to a low voltage close to zero in a short time, that is, a large voltage change rate dv/dt; And in the initial stage of switching on, due to the characteristics of the MOSFET, the rate of change of the current flowing through the MOSFET switch tube is also very large, that is, di/dt. Taking into account the effects of dv/dt and di/dt on EMI, in the initial stage of MOSFET turn-on, slow down the switching voltage change rate dv/dt to reduce EMI intensity, as shown in the red curve in Figure 2.
The actual EMI test results of traditional switching technology and secondary switching technology prove that the secondary switching technology of TPS61161 reduces EMI radiation energy. In the EMI test experiment, TPS61161 drives 10 LEDs in series with a battery voltage of 3.7V. Figure 3a shows the white noise in the EMI test environment, Figure 3b shows the EMI test results of the TPS61161 using traditional switches, and Figure 3c shows the EMI test results of the TPS61161 using the existing secondary switching 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 on-current of the LED to change the luminous intensity of the LED. This dimming method effectively avoids EMI interference caused by LED dimming. Such interference often occurs in PWM dimming mode.
Of course, good PCB layout and routing in the specific application of TPS61161 design also helps to better reduce the interference of EMI on the mobile phone system. In addition to TPS61161, Texas Instruments' TPS61160 white LED driver and TPS61165 high-brightness LED driver all use secondary switching technology. For details, please refer to the product catalog of Texas Instruments.
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