KT1025A Dual-Mode Bluetooth Audio Chip Layout Description and Design Precautions

 

1.Brief Overview

1. First, please refer to the provided BT201 test DEMO module as the standard.

2. If using the chip independently without testing the BT201 module and directly proceeding to layout, background noise is highly likely if experience is limited. Therefore, always compare with the manufacturer’s test board first.

3. The BT201 solution does not include FM functionality. Please note this. The FM circuit should be left floating without hesitation

2.Precautions

1. Background noise or interference is common in Bluetooth audio products. Do not be careless during layout.

2. If you have not worked on audio products before, study online resources thoroughly. Assuming things casually will naturally lead to noise issues.

3. The core principle is to separate analog ground and digital ground. If unclear, consult an experienced engineer.

4. Ensure a clean power supply. Use an LDO whenever possible instead of a DC-DC converter.

5. Bluetooth operates at high-frequency RF and radiates energy, so some background noise is inevitable but can be minimized. With good design, noise is imperceptible to the human ear, though it may be measurable with instruments.

 

3.Antenna Description

1. For antenna and component packaging, directly refer to the PCB files of the DEMO module, available in the document library.

2. The Bluetooth antenna has no special requirements. Follow the provided package and reference instructions.

3. The Bluetooth antenna does not require impedance matching, and standard copper thickness is sufficient. Don't over complicate it—Bluetooth has a low entry barrier.

 

4.Power Supply Description  

（1）The BT201 test board has minimal background noise, barely noticeable to the human ear.

（2）You can try powering it with a phone charger, which should not produce significant noise.

（3）Battery power is ideal as it provides pure DC and is very clean. However, consider the battery's load capacity.

（4）USB output from a desktop computer may produce significant ripple, leading to background noise. Avoid using it.

（5）If the board includes a DC-DC converter, it may cause noise. The optimal power supply is an LDO, such as a 7805.

（6）The BT201 module can drive a maximum speaker of 4 ohms, 3W with a 5V supply.

（7）When driving speakers, ensure sufficient current with a stable power supply. Insufficient current can cause speaker distortion, buzzing, etc.

（8）If possible, purchase various amplifier modules online for comparison testing. We sell Bluetooth chips, but amplifier-related noise, distortion, or other issues are extended topics that are complex and cannot be explained briefly.

 

5.How to check if the board has background noise?

（1）Use a clean power supply, ideally a battery, and disconnect all front-end power circuits.

（2）Connect the chip's headphone output and listen for noise. If none, check the rear-stage amplifier circuit.

（3）If USB playback has no noise but Bluetooth does, this does not necessarily indicate a problem.

（4）Use headphones to check for noise. If none, the issue lies in the amplifier section, so inspect it.

（5）If noise is present with headphones, disconnect the rear-stage amplifier and inspect the Bluetooth chip's peripheral circuits. Determine whether noise occurs during Bluetooth or MP3 playback. For Bluetooth noise, check if capacitors around the main controller are properly soldered and if they are too close to the Bluetooth module. Refer to other instructions for troubleshooting.

（6）For noise testing, avoid using a computer's USB or a charger's USB. Use a clean power supply with strong load capacity, ideally tested at 5V, not 3.7V.

 

6.Methods to Reduce Bluetooth Background Noise  

（1）Keep the Bluetooth antenna and Bluetooth module as far as possible from analog circuits.

（2）The chip's analog ground must be connected to the power ground at the input.

（3）Check capacitors around the chip for issues such as short circuits or poor soldering.

（4）Add multiple vias to the GND of the Bluetooth section.

 

7.Selection and index requirements of crystal oscillators  

（1）Bluetooth has high frequency offset requirements, so the crystal oscillator's quality is critical for performance. During selection, ensure consistency and stability, with a frequency deviation of ≤±10ppm and a recommended load capacitance (CL) of 12pF.

Note: Crystal-to-ground capacitors C102 = C103 = 2*CL – (4pF~6pF), where CL is the crystal's load capacitance.

（2）For designs with no size constraints, use the crystal specified in our DEMO, which is cost-effective and high-performing.

（3）For size-constrained designs, use a 24M-3225 crystal, which is slightly more expensive but performs well.

Recommendation: Use our provided crystal, which is more cost-effective and quality-assured compared to random purchases.

 

8.Simple PCB Layout Precautions  

 
（1）The input voltage to LDO_IN (Pin 18 of the chip) must not exceed +5.5V.

（2）The chip must strictly separate digital GND and analog AGND (refer to the provided BT201 PCB).

（3）During PCB routing, digital GND and analog AGND must be routed separately and connected only at the battery input. If the design includes a pre-amplifier ground, connect AGND to it.

（4）All decoupling capacitors on the main controller must be placed as close as possible to the chip pins, with short ground return paths.

（5）Prioritize the placement of the Bluetooth antenna. The RF antenna must be near the board edge (or in an open area if structural constraints apply). The antenna matching circuit must be close to the RF pin, with short routing. The antenna's ground plane should follow the yellow outline in the antenna package, and the space on both sides should be as wide as possible, as shown in the diagram avove.

（6）The 24M crystal must be placed close to the chip's clock pins (BTOSCO and BTOSCI). Its routing must be shielded with a ground plane, kept away from interference sources, and not routed parallel to other data lines.

（7）Audio signal lines (DACL, DACR, AUXL, AUXR, MIC, etc.) must be routed away from digital signals (LCD/LED signals, USB, SD, etc.).