This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC27524: abnormal voltage waveform in the EN pin

Part Number: UCC27524

Hi team,

It is nice to know you here. My customer is evaluating our device as below, they find that the EN pin will have Vpp=400mV voltage fluctuation when they apply the PWM signal to the INA/INB pin in below two cases. Customer suspect that our internal circuit causes this issue, could you please help share with me what may cause this abnormal waveform? Thanks. 

1.NC R186 and R187, C115=150pF

2.NC R186 and R187=47k, C115=150pF

  • Hi Charles,

    Thanks for reaching out.

    From the schematic, EN seems to connect to VDD externally through a small decoupling capacitor. To be effective, C115 must be connected close to pin 1 and 8 with a typical value of 100nF. Please confirm connect point.

    From the waveforms, VCC has quite a bit of ringing which is likely the source of disturbance of ENx signals. What are the values of decoupling caps on VDD? Can you confirm that they are directly across VDD pin?

    Regards,

    -Mamadou

  • Hi Mamadou,

    Thank you for your help here. The VCC decoupling caps is 10uF and directly across the VDD pin. The previous waveform has quite a bit of ringing because of that they did not use the Minimum loop principle. Please see the latest waveform below. They are still voltage fluctuation on the EN signal. After we increase the C115  capacitor value to 1nF, the issue is gone. But the customer needs us to explain this phenomenon, would appreciate if you can share your professional explanation here. Thanks.  

    BR,

    Charles Lin

  • Hi Charles,

    Given a specific frequency operation, the capacitance reactance of 150pF is likely inadequate to filter out noise and other parasitic (stray inductance from traces) from the PCB board. Generally speaking, assuming frequency range of 10kHz to 500kHz, our drivers work best with >=1nF to 100nF on ENx pins and 2 parallel decoupling capacitors 0.1uF AND >=1uF on VDD pins to effectively handle noise.

    Regards,

    -Mamadou 

  • Hi Mamadou,

    Thank you for your repsonse here. The customer cannot accept that the noise comes from PCB board, there are no high frequency signal around the EN PIN. Our datasheet said that we can left this PIN open. They suspect the noise comes from our internal circuit. How to verify that the noise is not from our device?

    BR,

    Charles Lin

  • Hi Charles,

    My name is Aaron Grgurich and I am also an applications engineer.  In the meantime before Mamadou gets back to you on Monday, I have a couple comments.

    The Enable pins are internally pulled up to V_DD to allow for the "floating/open" condition and they therefore can see noise that V_DD has; this will be mitigated by keeping the additional connections with the recommendations Mamadou gave before.

    The reason for the sinusoidal waveform on the Enable pins could be due to parasitic or insufficient capacitance around the circuit as those capacitances could have voltages on them charging up and relaxing as strong switching is going on nearby (along with trace inductances too).  A good starting point to see where the noise is coming from, would be to check if the Enable pin waveform is at the same frequency as the switching frequency of the circuit. If so, the problem can likely be pointed at the decoupling capacitors as Mamadou mentioned before; to properly filter the noise that typically comes with switching large currents.

    Thanks,

    Aaron Grgurich

  • Hi Aron,

    I have checked with the customer that the switching frequency is 70kHz. As you said the internal circuit may charge up and relax the capacitance, but why we cannot see the waveform from the VCC waveform? This is a concern from the customer. Thanks.

    BR,

    Charles Lin

  • Hi Charles,

    I will follow up with you offline.

    Meanwhile, marking this thread as "resolved".

    Regards,

    -Mamadou