Comparator circuit diagram suitable for headphone detection

Detect the type of headphones connected to the circuit in the picture . In the figure, the 2.2kΩ resistor RMIC-BIAS is connected to the low-noise reference voltage (VMIC-REF) provided by the audio controller. When the audio jack is plugged into the accessory, the VMIC-REF voltage acts on the equivalent resistance (not shown in the figure) between the plug and ground through RMIC-BIAS, thereby generating the voltage VDETECT at the non-inverting input of the MAX9063. For stereo headphones, this resistance is small (8Ω, 16Ω, or 32Ω); for microphones, the current source sinks a fixed current that varies from 100µA to about 800µA depending on the microphone type, so the resistance is larger. Since VDETECT changes with the type of headphones plugged into the jack, VDETECT can be monitored through a comparator to determine the type of headphones.

As shown in the figure, assuming the microcontroller's reference voltage (VMIC-REF) is 3V, a 32Ω headphone load will produce a VDETECT voltage of 43mV. A fixed current microphone load of 500µA will produce a voltage of 1.9V. Note that in most applications, direct connection to VDETECT will cause problems. Assuming that the CMOS input of a typical microcontroller port requires logic levels above 0.7 × VCC and below 0.3 × VCC, then the input logic level of a microcontroller powered by 3.3V should be above 2.3V and below 1V.

The 1.9V level produced by the 500µA microphone load is not a valid logic "1" level. A microphone bias current of 100µA to 800µA will produce a VDETECT of 2.78V to 1.24V, anything below 2.3V does not meet the controller's VIH (input high, assuming RBIAS is 2.2kΩ) requirement. To get 2.3V or higher, the microphone bias current must be 318µA or less. Otherwise, the 2.2kΩ bias resistor must be changed, thereby changing the microphone's detection threshold. Since headphones with a typical load of 32Ω can easily pull the level to near ground, generating logic low levels of 1V or even lower is easy to achieve.

In order to detect the headphone type, you need to connect VDETECT to one input of the comparator and the reference voltage to the other input. The comparator output represents the headphone type.

The comparator for this portable headphone detection application should have a small size and consume very low power. The size of the comparator shown is only 1mm × 1mm, and the maximum supply current consumption is only 1µA. It has strong anti-interference ability against mobile phone frequencies and provides extremely high reliability. The comparators also feature internal hysteresis and low input bias current. These characteristics make it an ideal choice for headphone detection circuits in battery-powered products that are extremely sensitive to space and power consumption, such as mobile phones, portable media players, and notebook computers .