InductorsMobile Phones -Speaker/Microphone-

The market size of mobile phones continues to expand every year. Particularly, the number of multifunctional mobile phones called smart phones equipped with high performance processors has been growing remarkably more than conventional mobile phones. Generally, smart phones have built-in wireless communication functions, such as 2G/3G, wireless LAN and Bluetooth®. We believe that these smart phones will successively respond to the next generation high-speed communications standards including LTE (Long Term Evolution) in the future. Smart phones will become even more user friendly mobile phones along with the high functionalization. On the other hand, there are problems currently occurring, such as noise coupled from the baseband circuit, due to the built-in multiple wireless communication functions, and the design changes to solve the reception sensitivity deterioration caused by this noise coupling, takes a lot of time and manpower for the mobile phone manufacturers to solve these problems.

As a detailed example, there are many cases where the noise from the baseband portion of the mobile phone radiates from the FPC (flexible printed circuit board) connected to the main board, and the audio signal line can be mentioned as a typical usage location of an FPC. In the case of smart phones, they are generally connected to a speaker via an FPC or etc. from the main board. There are many cases where this FPC becomes the antenna which emits noise radio waves and causes problems.

As one solution for such a problem, it is becoming clear that the use of chip inductors for the chokes is effective. This paper introduces the inductors (LQW15Cseries/LQW18C series) which can also be used for the audio signal line, particularly the speaker line.

In order to prevent noise radiation from the audio line, the technical requirements for the components can largely be summarized in following three items.

1. Must have a large impedance value wireless frequency band
2. Must be compatible to current levels required for driving speakers
3. Must be small sized

In order to effectively reduce the spurious radiation noise of the frequency band compatible to the main band and wireless LAN of mobile phones, high impedance characteristics are demanded in each frequency band. To realize this, it is preferable to have an extensive selection of inductance ranges which can cover the wireless frequency bands. In order to achieve high impedance characteristics sufficient for these wireless frequency bands, it is necessary to control the parasitic capacitance component of the component itself to a level below the constant, in addition to an extensive lineup of inductance ranges.

Generally, a current of about 500 mA maximum, flows through the speaker line of mobile phones. In order to handle to this current level, it is necessary to reduce the DC resistance of the components as much as possible.
In addition to satisfactory reliability, it is also necessary to maintain the generation of heat, i.e., increase of temperature to below a constant level when current is applied, and also maintain the electrical performance while the current is flowing.

Although the impedance value is generally proportional to the inductance value from the physical formula, such parameters and DC resistance have a trade-off relationship. For this reason, reducing the DC resistance level of the components while considering the relationship of the trade-off will become the technical issue.

The need for small-sized components naturally becomes stronger due to the smart phone applications.
Generally, there are many users of smart phones who have requested improvements, or are dissatisfied with the duration of the battery. One of the measures to respond to the opinions of these users is to install a larger sized battery in the smart phone. However, since the size of the case is limited, it will eventually greatly limit the board size.
As a fate of the application where multiple new functions are to be added, not only numerous wireless functions, the board space assigned to each circuit block tends to increase in severity.
Therefore, it is necessary to realize miniaturization of the overall circuit by modularizing portions of the circuit block to promote the reduction of space, or adopt electronic components which are one size smaller than conventional components.

Therefore, it is necessary to realize miniaturization of the overall circuit by modularizing portions of the circuit block to promote the reduction of space, or adopt electronic components which are one size smaller than conventional components.

An example of a speaker line circuit of a mobile phone is shown in the figure.

The LQW15C series/LQW18C series offers an abundant lineup of inductance ranges. If you are considering the adoption of inductors for this application, by selecting an inductance value with a higher impedance in the applicable frequency band from the impedance vs. frequency graph (Figure 1, Figure 2) after specifying the wireless frequency which will become a problem in the band and reception sensitivity of the applicable mobile phone, a greater effectiveness can be further demonstrated.

Since the current level required for each circuit differs, it is necessary to confirm the allowable current level of the inductor (Figure 1, Figure 2) , at the same time selecting the inductance value.

As well as advancing the high definition of cameras and LCD panels installed from the beginning, there is the NFC short distance wireless communication including LTE, as functions to be installed standardly in the future. We believe that there will be no change in the trend of high performance and high functionalization of smart phones into the future.
At the same time, as a result of adding these new functions, the difficulty of designing smart phones will steadily increase in the future, such as the necessity of measures for electromagnetic interference problems between mutual wireless circuits and noise coupling from the baseband circuit.

Murata has commercialized products to improve the reception sensitivity including the LQW15C series/LQW18C series. We will continue to contribute to the high performance and high functionalization of mobile phones further, by providing design support of smart phones and solutions to solve issues.