TAS5806MD: Should the differential output traces of the audio amplifier be impedance controlled?

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Hi,

The core purpose of using wide traces is not impedance matching, but to reduce DC resistance (DCR), handle high current, and minimize heat generation.

• Reducing DC Resistance (DCR): The output side of a Class-D amplifier is effectively connected to a power inductor (the output filter inductor). If the output traces are too thin, their parasitic resistance forms additional series impedance with this inductor. This leads to two issues:

  1. DC Voltage Drop: When high DC bias current flows, thin traces create significant voltage drops, reducing the actual operating voltage at the speaker terminals and thus decreasing output power.

  2. Heat Generation: High current flowing through thin traces generates Joule heating (P=I2R). This not only wastes energy but can also affect the stability of the chip and inductors (especially in high-temperature environments).

• Handling Peak Current: Although Class-D amplifiers are highly efficient, they can experience instantaneous high peak currents in the output pins and traces when delivering high power. Wide copper pours provide a low-resistance path to ensure smooth current delivery.

Hi,

Here are additional design tips for your reference. When routing these pins, adhere to the following principles:

• Trace Width: To balance low resistance and interference immunity, it is recommended that the output trace width meet or exceed 20 mil (approx. 0.5 mm)​ according to standard fabrication capabilities. If your device outputs high power (e.g., >10W into a 4Ω load), it is advisable to increase the width to 40 mil (approx. 1 mm) or 60 mil (approx. 1.5 mm), depending on your expected peak current calculations.

• Strict Length Matching: This is the most critical requirement​ for differential pair routing. OUT_A+ and OUT_A- must be strictly symmetrical, with length mismatches controlled within a minimal range (typically recommended within ±5 mil). Similarly, OUT_B+ and OUT_B- should also be strictly matched. Under no circumstances should these two differential pairs cross each other, as this would disrupt the differential signal balance and increase common-mode noise.

• Keep Away from Noise Sources: The output traces of a Class-D amplifier act as high-frequency antennas. During layout, keep trace lengths as short as possible and route them away from sensitive analog signals (such as audio input interfaces, I2S/I2C lines, reference voltage pins, etc.) to prevent high-frequency noise coupling into the audio system, which causes distortion or noise.

• Reference Plane: Maintain a solid ground plane beneath the differential pairs to provide a stable return path and reduce electromagnetic radiation.

• Connection to LC Filter: Traces should terminate directly at the external LC filter (typically a ferrite bead or power inductor + ceramic capacitor). Ensure the pads connecting to the inductor are sufficiently large to further reduce connection impedance.

Please let me know if you have additional questions. Thanks.

Hi Peter,

Thank you for your insights.

Could you please confirm whether the differential traces need to be designed for a specific impedance, such as 100 Ω or 90 Ω?