When the dimmer EMI radiation and response
Generated when the dimmer of EMI radiation and deal with the
SW pin output signal of the EMI radiation is mobile phone design personnel to address the concerns of more, but they often found that even already takes a lot of energy, reduce the output signal of the SW pin EMI radiation, but EMI problems still exist. Inductance type booster backlight driver chip in the PWM dimming output voltage VOUT may produce a large output ripple. It is also an EMI radiation source, but easily ignored by mobile phone design personnel.
figure 3 is a way of using common PWM dimming type inductor step-up backlight driving in PWM dimming can make pin ( EN) And the output waveform of VOUT. Can see from figure 3, with 10 KHZ 50% duty ratio of PWM dimming signal, the output voltage ripple of up to 4 v VOUT. And we found that move light frequency is lower, the output voltage ripple is larger. In PCB design, output VOUT need backlight driver module receives the screen backlight LED anode, go line will be long, so go VOUT line of the output ripple is also a serious EMI source!
figure 3: a model of inductance booster type backlight drive PWM dimming output VOUT ripple ( 10 KHZ, 50% duty cycle) 。
on the output voltage amplitude will spend big on the output capacitance whistlers cacophony. This is because the output caused by the vibration produced by the piezoelectric effect of the MLCC capacitor. Normally ripple amplitude exceeds 0. 5 v can hear squeaking noise. Improve the light frequency is a kind of solution, but improve the dimming frequency affects the GuangXianXing degrees of PWM dimming, even disable the dimming function, but also did not fundamentally solve the problem.
Shanghai ai is the inductance of booster type backlight driving AW9920STR/DNR turn an innovative way of using PWM constant current dimmer. PWM switch constant-current dimmer receiving ordinary PWM dimming signal, after the internal circuit transformation, the final output of a constant output current. The size of the output current is proportional to the duty ratio of PWM dimming. Output current is constant, the output voltage ripple is very small. Figure 4 is AW9920 under the same conditions of test results, and the output of the AW9920 voltage ripple does not exceed 100 mv. Output VOUT EMI radiation and capacitance on the noise problem got the perfect solution! And AW9920 support a higher light frequency. Dimmer, the higher the frequency, EMI performance and capacitance the noise problem to improve the better, at the same time also does not affect GuangXianXing degrees.
figure 4: AW9920 PWM dimming output VOUT ripple ( 10 KHZ, 50% duty cycle) 。
the slope variable drive technology and PWM constant current dimming technology, making Shanghai ai as the second generation of the series backlight driving EMI performance improved significantly. The PCB design requirements are greatly reduced, but also eliminate the output capacitance of whistlers. But EMI radiation is a very complicated and difficult to perceive problems, when in the PCB design mobile phone designers pay special attention to: connect to SW pin connection short, small area as far as possible, as far as possible to reduce the SW go online EMI radiation; Use shielding inductive inductance as far as possible; Input output VIN and VOUT bypass capacitors as close to the corresponding pin chip; Power supply after chips to go line according to the current wiring, minimize parasitic resistance and parasitic inductance; Backlight driver module power supply and other modules power line as far as possible, use the stars connection; Ground take paved the way as much as possible, and module and other vulnerable to interference to separate; Backlight driver module proposed use shield block, to reduce EMI radiation as much as you can.
edit: Steve