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Original question:

LM5008: Disableing the chip using a shotky diode.

LM5008: Output ripple/switching mode

Ahmad Farjami
Prodigy 20 points
Part Number: LM5008

Dear support team,

I am using LM5008 to regulate 18V to 95V input into 15V as depicted in the schematic below.

The problem is at some input voltages and specific loads I see the following switching and ripple behavior which is dependent on the input voltage and output load. By changing the inductor to 470uH this also happens for example at 51V on 60mA loading on the output. 

What is the reason for seeing such switching behavior? It looks that regulator switches between continuous and discontinuous mode in such cases and by increasing the input voltage that will happen on lighter loads.

Is it a normal behavior? Is there any way to prevent it?

Thanks in advance for your help and support.

Ahmad

Channel one is the ripple signal of the output and channel 2 is the switching node signal.

 For 49V and 42mA:

For 60V, 47mA:

  • 0
    Prodigy 20 points

    Please consider that I am measuring V_out on C308.

  • 0 in reply to Ahmad Farjami
    TI__Mastermind 21031 points

    Hi Ahmad,

    Pls cut the connection like below shown. I suspect the two output ripple can be overlapped and this can cause ripple out of phase ,so SW will be unstable.

    Thanks

    Colin

  • 0 in reply to Colin Chen
    Prodigy 20 points

    Hi Colin,

    Thanks for the reply.

    Clearly, those FBs are not placed, and they are used to testing them separately in our circuit. For the tests that I am now doing, those FBs are not placed, and an electronic load is connected to Vout net (across C308). 

    The point is in the datasheet of the chip we do not need to have C308, but without it, we will have high ripple on the output as the inductor is relatively small.

     Then I would be back to my first question, I do not know what is happening in such high ripple case, is it an instability state, or repeatedly switch between continues and discontinues mode, or something else?

    Thanks again for your support and help.

    Best regards

    Ahmad

  • 0 in reply to Ahmad Farjami
    TI__Mastermind 21031 points

    Hi Ahmad,

    Remove C308 can cause output cap too small, this can bring instable issue. From SW we can see the converter are working unstable condition. You can increase output cap and test again.

    B R

    Colin

  • 0 in reply to Colin Chen
    Prodigy 20 points

    Hello Colin,

    Thanks for the reply.

    I have added another 10uF capacitor to the output, but still on some loads and input voltages the same thing happens.

    The problem vanishes as I increase/decrease the load or change the input voltage. So, that is not easily traceable. I also changed the inductor to 470uH and these phenomena still happen, but at higher output current (loads) for example instead of 30mA it happens at 60mA for 51V input voltage.

    As far as I know, this pulse skipping generally happens when the minimum on time of the switch is violated, but my calculations show this will not be violated by around 260Khz switching frequency of this design as Fsw must be less than 625Khz for this regulator (acc. datasheet: minimum on-time (at maximum VIN) greater than 400 ns).

    What is happening is two consecutive pulses with the minimum 300ns off-time of the PWM signal that causes the overshoot here.

    Do you have any suggestion or idea why this happens?

    Regards,

    Ahmad

  • 0 in reply to Ahmad Farjami
    TI__Mastermind 21031 points

    Hi Ahmad,

    I think you still met unstable issue. The R552 =2ohm is too high and this may cause output ripple too big. Some  noise may become bigger as well and it may fed into FB pin. So you can reduce R552 from 2ohm to 0.2ohm and test again.

    Thanks

    Colin

  • 0 in reply to Colin Chen
    Prodigy 20 points

    Hi Colin,

    I have already added another 10uF capacitor on the output, and it reduces the ripple a lot. But, the same effect appearing on the output in very less magnitude. I will check the evaluation board of LM5008 and come back to you soon.

    Regards, 

    Ahmad

  • 0 in reply to Ahmad Farjami
    TI__Mastermind 21031 points

    HI Ahmad,

    No problem, I close this ticket temporally and you can open again, thanks.

    Colin