Practical application of integrated amplifier power IC to reduce EMI interference technology

In a flip phone, if the audio amplifier and RF receiver are both on the body, the RF receiver will affect the EMI of the audio amplifier. If the audio amplifier is moved to the flip cover, EMI can be reduced and there is no need to use a low-frequency filter. Since the speaker is also on the flip cover, the length of the connection between the amplifier and the speaker can be shortened , which can also greatly reduce EMI interference.

Moving the audio amplifier to the flip cover has another benefit, which is that it can shorten the length of the wire between the amplifier and the speaker, reduce the impedance between the two, and thus increase the output power. For example, if it is an 8Ω speaker, the output power can be increased by 3dB, and if it is a 4Ω speaker, the output power can be increased by 6dB.

Maxim's MAX8678 integrates a white LED charge pump and an audio amplifier to reduce EMI noise from the RF receiver and increase effective output power in flip-top mobile phone applications. The high-efficiency, adaptive charge pump can drive up to 4 LEDs at a constant current to obtain uniform brightness with an accuracy of 1% and a quiescent current of only 140mA. The LED current can be adjusted in a pseudo-logarithmic form in 31 steps from 0.1mA/LED to 2.4mA/LED through a single-wire serial pulse interface. High accuracy and current matching between LEDs can be maintained throughout the entire adjustment range. Each LED has independent adaptive mode switching to ensure high efficiency over a wide LED forward voltage range.

The mono Class AB audio amplifier can directly drive an 8Ω speaker with 1.1W of continuous equivalent power at 5V supply , with THD+N less than 1%. The gain can be adjusted in 10 3dB steps from -9dB to +18dB through a single-wire serial pulse interface. THD+N is 0.004% at 1kHz, PSRR is 90dB at 1kHz, and the shutdown current is 0.1mA. Clicks and pops can be suppressed during turn-on/off and all gain adjustments. Differential inputs can improve common-mode noise rejection.

The MAX8678 includes soft-start to limit inrush current, thermal shutdown, and open- and short-circuit protection, and is available in a 16-pin, 3mm-by-3mm thin QFN package with a maximum height of 0.8mm.