Function Description Line Driver Operation Figure 1. Cap-free Operation The APA2171’s line drivers use a charge pump to invert the positive power supply (VDD ) to negative power supply (VSS ), see figure1. The headphone drivers operate at this bipolar power supply (VDD and VSS ) and the outputs reference refers to the ground. This feature eliminates the output capacitor that is using in conventional single-ended headphone drive amplifier. Compare with the single power supply amplifier, the power supply range has almost doubled. Thermal Protection The thermal protection circuit limits the junction temperature of the APA2171. When the junction temperature exceeds TJ =+150 OC, a thermal sensor turns off the driver, allowing the devices to cool. The thermal sensor allows the driver to start-up after the junction temperature down about 125 OC. The thermal protection is designed with a 25 OC hysteresis to lower the average TJ during continuous thermal overload conditions, increasing lifetime of the ICs. Shutdown Function In order to reduce power consumption while not in use, the APA2171 contains shutdown controllers to externally turn off the amplifier bias circuitry. This shutdown feature turns the amplifier off when logic low is placed on the SDN pins for the APA2171. The trigger point between a logic high is 1.0V and logic low level is 0.35V. It is recommended to switch between ground and the supply voltage VDD to provide maximum device performance. By switching the SDN pins to a low level, the amplifier enters a low-consumption current circumstance, charge pump is disabled, and I DD for the APA2171 is in shutdown mode. In normal operating, the APA2171’s SDN pins should be pulled to a high level to keep the IC out of the shutdown mode. The SDN pins should be tied to a definite voltage to avoid unwanted circumstance changes. Under-Voltage Protection External under voltage detection can be used to shutdown the APA2171 before an input device can generate a pop. The shutdown threshold at the UVP pin is 1.25V. The user selects a resistor divider to obtain the shutdown threshold and hysteresis for the specific application. The thresholds can be determined as below: VUVP = (1.25-6mAxR3) x (R1+R2)/R2 Hysteresis = 5mA x R3 x (R1+R2)/R2 With the condition: R3>>R1//R2 For example, to obtain VUVP=3.8V and 1V hysteresis, R1= 3kW, R2=1kW and R3=50kW. Figure 2. Under-Voltage Protection
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