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TPA3255-Q1: CLIP_OTW output is always low even no input signal, no load

Part Number: TPA3255-Q1
Other Parts Discussed in Thread: TPA3255

Hi Team,

My customer is using TPA3255-Q1, attached is the schematic design. Now the issue is the CLIP_OTW output is always issuing low constantly even no any input signal is applied and no any load is connected (pls note, though there is a transformer load on oiutput, actually during customer's test the transformer had been removed). They also measured the temperature of device on power pad, it is only 49 degree C (in 25C of room temperature). Seems there should be no reason that result CLIP_OTW warning. What is wrong?

I also noted there are several old posts in this E2E discussing similar issue. However i cannot find solution in those posts.

TPA3255design.pdf

Thank you!

  • Hi John,

    A few check points on the schematic, 

    1. GVDD/VDD power supply should be 12V. According to datasheet, the recommended range is 10.8~13.2V.

    2. Although OUT_C and OUT_D are not used, the bootstrap caps on BST_C and BST_D must be connected.

    3. It's recommended to connect the INPUT_C and INPUT_D to GND via 

    4. Could you double check the input AC coupling capacitance. Is it 33nF? That would form to a high pass filter with high cut-off frequency. Usually this cap is set to be a few uF. 

    5. I would recommend to configure the device into PBTL mode to have better thermal performance. Please refer to figure 31 on d/s.

    6. I suppose the C11 is an electrolytic capacitor, right? I think this cap should be removed cause it can't withstand reverse polarity voltage. 

    Thanks!

    Regards,

    Sam

  • Hi Sam,

    Thank you for quickly reply. Here is answer for your comments:

    1. The device customer testing is TPA3255-Q1, for which the GVDD/VDD range is 9.8V to 11.4V.

    2. However customer did not design sockets for bootstrap caps on PCB. How about setting the device in 1xBTL mode (M2=1, M1=0) and disable CD channel by connectiong INPUT_C to DVDD, connecting INPUT_D to GND. If this configuration can close the issue?

    3. Connect INPUT_C and INPUT_D to GND via capacitor? In point view of DC, connecting INPUT_C/D to GND via caps is in fact equivalent to floating i think, Why it can result any problem?

    4.Yes, input cap is 33nF. Since the application is not for audio, in stead it is for 56kHz FSK signal.

    5. They will consider PBTL mode in next version of design. They need find out the root cause of current issue and close the issue before next version of design.

    6. Is it ok to remove the C11 and connect the output to transformer directly? Is there any potential problem such as low output offset resulting high DC output current?

    Best Regards,

    John

  • Hi John,

    2. Yes, setting it into 1xBTL could be a solution.

    3. By connecting these unused input pins to GND via caps, they will be less sensitive to noise.

    4. TPA3255 is a audio amplifier, its loop is designed to take care of signals below 20kHz. It has limited bandwidth to cover higher frequency signal. Gain/noise performance may have degradation. 

    6. It's OK to remove the C11. May I ask what's the resistance of the primary transformer?

    Thanks!

    Regards,

    Sam

  • Hi Sam,

    This device has 100kHz of bandwidth based on datasheet. So there is no problem with the frequency.

    The problem is how to explain the CLIP_OTW warning signal constantly low? This warning signal issued only in two condition: over-temperature and signal clipping. However the two condition definitely did not occurred in customer circuit (no input signal, no load, temperature of device is low). Why the device issuing CLIP_OTW warning in this condition?

    Regards,

    John

  • Hi John,

    I'm running internal checking on the CLIP_OTW trigger events, will update once I get the information.

    Thanks!

    Regards,

    Sam

  • Hello John,

    Here in your schematic you don't have bootstrap capacitors between BST_C and OUT_C and BST_D and OUT_D. They are required for device operation even if those channels are not used, which is why you are seeing the clip_otw asserted. Please add the Bootstrap capacitors and the device should operate normally.

    Best regards,

    Luis