• Applications Overview

·General of Audio Amplifiers
• Technologies

AKM Activity for Audio amplifier devices

Asahi Kasei Microdevices has been supplying many LSI into Mobile phone market. The lineup covers RF devices to Power-supply widely, and AKM has been given high marks specially in the digital-analogue mixed LSI field.
AKM has advantage in audio amplifier field, and has been developing not only the Dynamic speaker amplifier, but also the Piezo speaker amplifier.

Dynamic speaker and Piezo speaker

In mobile terminal application, two types of speakers are mainly adopted. One is the dynamic speaker composed of coil and magnet, the other is the piezo speaker made of ferroelectric ceramic.
The electrical characteristics are different between dynamic speakers and piezo speakers. Dynamic speaker has an inductive behavior, while piezo speaker has capacitive behavior. So, the suitable driver for each speaker is different.
AKM has the both line-up, resistant and capacitive load, which feature low power consumption and saving space.

Amplifier for Dynamic speaker

Generally, mobile equipments have 4 to 32ohm dynamic speakers inside, and 16 to 32ohm speakers are for receiver, 4 to 8ohm speakers are for louder. The reason to select the impedance is the sound volume and current consumption.
Also, to keep the reliability of speaker, it is necessary not to exceed the rated condition of speaker. ALC(Auto Level Controller)function may help to protect speakers, and maximize the performance.

Amplifier for Piezo speaker

Piezo speaker, called a ceramic speaker sometimes, has a characteristic of capacitive. It is used in various applications, such as buzzers, MIDI melody, receivers. Lately, Piezo speakers have been developed and some of them have achieved super thin speakers and the audio quality speakers are there.
Piezo speaker needs to be driven at high voltage, but it doesn’t consume so much electric power. The total power consumption is influenced by efficiency of the Voltage boost circuit and the output driver circuit.
The capacitive loads consume more current with higher frequency. It is sometimes difficult to estimate the enough drivability of amplifier for the target signal source. For example, music source is sometimes lighter than single tone. Please try to evaluate the combination of speakers, drivers and target sources.
AKM has several devices placing on the market, and are developing the suitable amplifier for capacitive load with low power consumption, which are called AKM Class-P amplifier. Please find your best driver in AKM line-up.

Class-D Amplifier

Class-D is the digital method of driver amplifier. This has the feature of less power consumption and higher power efficiency than Class-AB. On the other hand, the digital method tends to cause more EMI, but this problem has been settled by advanced driver technology. So, mobile phones are employing Class-D amplifier these days.
AKM has been developing Class-D amplifier targeting for mobile phone and is supplying the high performance devices including the Filter Free technology and Low EMI technology.

Boost Circuit (DCDC converter, Charge Pump converter)

AKM has the devices employed power boost circuit.
Power boost circuit generates the voltage required for the elements want to drive. This provides these merits,
- To enable higher voltage output than supplied power
- Without external boost converter circuit
- To keep the output voltage level regardless of supply power
- To get high PSRR performance
The methods of boost are mainly a Charge Pump converter and a DCDC converter, AKM has both circuit technologies. Generally, Charge Pump method is higher efficiency and requires less external parts than DCDC converter, while fixed gain of Charge Pump method is twice or three times. DCDC converter method is used in case of higher boost requirement.

Issue of Small speaker for Mobile equipment, and BE technology

The trend of thin and high functional mobile devices limits the space of speaker. This debases the audio quality like cheap bass sound, because it is difficult to keep the area of the speaker size and cavity that is the important factor of speaker.
To resolve this problem, AKM has developed Bass Enhancement technology (BE), which educes the full performance of speaker and leverages the illusion of hearing, and achieves the rich bass and much volume sound.