Challenges in Headphone Connections
Modern devices often incorporate headphone jacks, but space constraints and low supply voltages pose challenges. Integration and miniaturization leave little room, and low battery voltage further complicates matters. Without special techniques, output power and headroom are severely limited.
Solution: Maxim’s MAX4410
Maxim’s MAX4410 offers a solution to these challenges. Its compact size and internal supply inverter (charge-pump) address space limitations. With just two external ceramic SMD capacitors (C6 and C7), it creates a symmetrical supply voltage for the output stage. This symmetrical supply eliminates DC offset, eliminating the need for large output capacitors to prevent DC voltage reaching the headphones.
Overcoming Limitations of DC-Coupled Output
While a DC-coupled output could be achieved using two bridge amplifiers, most stereo headphone plugs are asymmetric and use 3-pole connectors (common ground). These connectors cannot be connected to a bridge output. Maxim’s design circumvents this issue, providing a practical and efficient solution for headphone connections.
Individual Channel Control and Power Management
The MAX4410 offers individual channel control, allowing each channel to be turned off using jumpers JP1 and JP2. During normal operation, these inputs are connected to the positive supply. When both channels are off, the charge pump is also deactivated, reducing current consumption to approximately 6 μA. The IC incorporates thermal and short-circuit protection. It enters standby mode under low supply voltage conditions and prevents power-on and off pops at the outputs. The recommended supply voltage ranges from 1.8 V to 3.6 V, and the IC can deliver around 80 mW per channel into a 16 Ω load. Adequate power supply output, at least 200 mA, is crucial, requiring a reserve of 300 mA when powering other circuits simultaneously. The amplifiers operate in inverting mode with a gain determined by the ratio of resistors (R3/R1 or R4/R2), and the input impedance is set by R1 and R2.
Decoupling and HF Filtering for Audio Precision
Capacitors C1 and C2 decouple potential DC-offsets from the inputs. While tantalum capacitors are used in the MAX4410 evaluation kit, they are not recommended for audio applications. Plastic film capacitors are a better choice, although they occupy more space. HF decoupling is achieved with 100 pF capacitors parallel to R3 and R4, setting the amplifier bandwidth slightly above 150 kHz. Typical distortion stands at 0.003%. Detailed specifications can be found in the MAX4410 datasheet and the associated evaluation kit documentation. The selection, positioning, and layout of capacitors, along with careful attention to construction, are critical due to the 14-pin TSSOP package with a 0.65 mm pin spacing and SMDs in 0402 packages. Additionally, the IC is available in a more challenging-to-solder UCSP 16 package, utilizing ball grid array technology with dimensions of 2.02 by 2.02 mm.