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TPS61193-Q1: Escape from DCM

hideyuki sakai
Mastermind 6250 points
Part Number: TPS61193-Q1

Dear, Sir.

My customer is evaluating TPS61193-Q1, now. They are facing a strange symptom.

fSW & Vout are set by resistors to 401kHz $ 33.7V, however 375kHz & 35.4V are observed.

<Condition>

Vin = 12V

LED current = 73.5mA *3 = 220.5MmA

LED strings = 3p9s

LED Vf Total = 32.4V (Cold), 26.5V (Normal)

L = 33uH

I checked the captured SW node waveform & E2E, then found this thread as

"About Sub-harmonic instability".

SW node behavior looks exactly same as the thread contents.

So, I would like to confirm the escape method from DCM to CCM.

fSW change to higher will be impossible considering radio noise.

Is it possible to urealize that only to change L value for smaller?

If yes, Could you please give your recommendation value.

Sorry to ask this kind of matter. I am hoping to get your help at this time.

Best Regards,

H. Sakai

  • 0
    TI__Expert 5475 points
    Hi,

    What do you mean by switching frequency is changed? If it is the DCM and pulse skip operation that the thread "About Sub-harmonic instability" mentioned, it is true that some switching cycle may be skipped. But I think the period time between two consecutive cycles is not changed. Is that what customer is seeing?

    What is customer's concern if the device works in DCM or pulse skipping mode? As the other thread said, it's difficult to prevent the device from working in DCM all the time since the input voltage range may be large and the device may need to work in light-load condition.

    Use a larger value inductor may reduce the possibility and range of DCM operation, but it's not easy to avoid DCM completely. And the recommended inductance value in the datasheet is the optimal value for the specific frequency. We do not recommend to change the value a lot.