• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 967 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

TPS61193-Q1: About Sub-harmonic instability

Tsukui Yusuke
Guru 21930 points
Part Number: TPS61193-Q1
Other Parts Discussed in Thread: TPS61193

Hi,

Customer is considering using TPS61193-Q1.
I will describe thecustomer conditions.

Fsw : 330kHz
Iout(LED current) : 30mA
Vout : 33.8V (Vf=3.7V×7pcs)
Vin : 5V to 20V

When the input voltage is set to 13.5 V, the switching waveform oscillates.
Please check the attached waveform.

TPS61193.pdf

I got two questions from customers.

・Is the attached waveform a normal waveform?

・Is the waveform of Vin=13.5V a sub harmonic state?
  Please tell me the cause and countermeasures to make such a waveform.

Could you g

Best Regards,
Yusuke / Japan Disty

  • 0
    TI__Expert 3370 points
    Tsukui-san,
    we will review your question and provide response either today or tomorrow. thanks!
  • 0
    TI__Expert 4565 points

    Hi Tsukui-san,

    Both waveforms show normal operation of boost DC/DC.

    First waveform at 10V shows CCM(continuous current mode) operation when load or conversion ratio is high, so boost needs to keep switching.

    Second waveform at 13.5V shows DCM(discontinuous current mode) operation on pulse skipping switching at lighter load or lower conversion ratio, so boost doesn't need to switch all the time.

    This pulse skipping+DCM mode SW is fundamental behavior of boost converter unless boost is specially designed to avoid this mode.

    Pulse skipping + DCM mode is not oscillation, and lowers boost efficiency by stopping SW occasionally, but often cause audible noise on output caps.

    It seems it's very difficult for you to prevent this SW mode as Vin range is very wide. In general, using higher boost SW freq and inductor value will lower this symptom which means boost will operate at CCM mode for wider load/conversion ratio conditions.

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    Guru 21930 points

    Sung-san,

    Thank you for your support.
    I understood your explanation.
    I have one more question.
    Is there a problem with the customer circuit?

    Best Regards,
    Yusuke / Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Expert 4565 points

    Tsukui-san,

    I don't see any critical issue. One thing is that apps condition may hit min value of max SW current(1.2A).

    33.8V*0.03(A)*4(string)/0.8(efficiency) = input power = 5.07W

    5.07/5V(input) = 1.014A, if ripple current on inductor is considered, Isw will be much higher than 1.2A.

    Is there any chance to increase Vin level to 6V or higher?

    FB divider resistor value also need to be changed. 680kohm will limit max Vout to 33.9V which is very close to Vout max apps condition.

    There can be variation on LED driver itself and resistors, so please use 750khom or higher value to have some extra room.

    And please make sure V, I ratings of each boost component has more than 30~40% margin on top of worst case conditions.

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    Guru 21930 points

    Sung-san,

    Thank you for your kind support.
    "Is there any chance to increase Vin level to 6V or higher?"
    →Input voltage can not be changed due to the battery voltage.

    Let me ask you one more thing.
    Customers want to know the DCM frequency to calculate the inductor current.
    Please tell me how to calculate DCM frequency.

    Best Regards,
    Yusuke / Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Expert 4565 points

    Hi Tsukui-san,

    Sorry for the late answer. I just got this thread.

    DCM freq on our LED drivers can't be calculated as it will vary with load size, SW freq, output cap, Vout, and so on.

    SW mode is automatically decided by monitoring Vout level cycle by cycle.