Simple design description of MIC circuit of ECM mobile phone

MIC： A device that converts an acoustic signal into an electrical signal .
The current market MIC It is mainly divided into ECM And MEMS Two types .
However , What we are talking about today is ECM, Electret condenser microphone . In mobile applications , It is mainly used in headphones MIC In circuit . Other main and auxiliary MIC Then for MEMS type .
ECM Type of MIC The interior can be simply understood as a diaphragm capacitor and a FET constitute .

When there is sound transmission , It will drive the membrane to vibrate , Thus, the air gap changes . Change electric capacity and electromagnetic field , Generate electrical signals ,E=Q/C.
FET High impedance . One end is in contact with one pole of the capacitor . One end of the capacitor is fixed , One end is movable . The distance between the two ends is related to the input of sound . The size of the sound 、 Frequency causes the change of amplitude and frequency of metal sheet vibration , In the view of capacitance, it is the change of the distance between the two poles of capacitance , Then it is equivalent to the change of capacitance charge accumulation .

MIC The working current is generally 500uA.

The circuit adopts differential circuit . It has a strong function of restraining interference .
AU_VIN1_N And AU_VIN1_P Access CODEC End .
C201 And C202 Main filtration
C203 It plays the role of filtering differential mode interference in the line .
C204 And C205 It plays the role of filtering common mode interference .
Three kinds of filtering are for MIC Provide a clean power supply .
ACCDET To detect the earphone type . It is connected to CODEC There are two comparators inside . Plug in headphones and three segment headphones , The voltages of the four segment headphones are different .
AU_MICMIAS1 by MIC Provide bias voltage .R204 Is the bias resistance . Mostly 1K. In order to make FET Working at saturation .R203 Yes MIC The sensitivity of plays a certain role . The resistance value should not be too large .R203 Too much leads to MIC Input current is too small . And then affect the dynamic range of the output level . for example , When R203 When the resistance value is too large , The input current is small , The dynamic range of the output level will be smaller as a whole . When there is sound input , Some will be mistakenly given up because of noise or no input , Sensitivity will decrease . When inputting a smaller sound , Will not be available . therefore , When designing hardware , The parameters of this resistor , Consider and choose based on noise and sensitivity .
And then HPH_MIC_N And HPH_MIC_P Get into MIC End .
Suppose the current flowing is I.I=I0+i.I0 It can be understood as DC bias , It is the current under quiet conditions .i It is the current when there is sound input . According to the fluctuation of sound , It is changing . ideally , It is a sinusoidal signal .
Strictly speaking , This circuit is not really differential . Because the common mode voltage of the differential signal is the same , And the above differential connection ,P and N The common mode voltage of is different . Is therefore ,Vbias The fluctuation of will make the common mode voltage change into differential mode voltage , Form noise .