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LM25011: Some questions for LM25011

Tomoaki Yoshida91
Genius 5255 points
Part Number: LM25011

Hi team,

Our customers will begin to design from using the LM25011.

There is a question regarding the current limit especially when looking at the data sheet.

#1

Is the function of current limit same as setting On time to 40%?

#2

Typical ON time of the LM25011 at the current limit is 100ns in the data sheet, but there are not written minimum and maximum on time.

How much will the on time 40% at the current limit fluctuate?

Because I think that the peak current during current limit is determined by the on time, I would like to know.

#3

What does it  mean minimum current limit threshold written in datasheet page 18 Rs section.

Does the threshold of the current limit change?

#4

It is written to use low ESR ceramic capacitors, please tell me specific value.

#5

Although it is written to use a Schottky diode in the section of DI, I do not know how influence will be caused by ultra fast recovery diode.
Is there any document that I can understand?

Best regards,

Tomoaki Yoshida

  • 0
    TI__Prodigy 480 points

    1. As mentioned in the datasheet, the current limit reduces the on-time by 40%. Therefore, whatever on time you have, it will be reduced by 40%.

    2. The current limit affects the on time, but the on time does not necessarily affect the current limit.

    3. The current limit can change depending on your selection of Rs.

    4. There are some recommended low ESR capacitor values on page 19 of the datasheet. Additionally, the Webench tool can also be used to find specific values based on your inputs.

    5. I found a TI document, which pretty much restates the same information as the datasheet. I also included two links about reverse recovery in case you were confused about that part. Overall, the reverse recovery is worse for the ultrafast recovery diode compared to that of the Schottky.

    slva477b.pdf

  • 0 in reply to Eric Sato31
    Genius 5255 points

    Hi Eric-san,

    Thank you for your support.

    I would like to know more about No. 1 in a bit more detail.

    The minimum on-time during normal operation is 100ns and it is described in the datasheet.

    It is described in datasheet as follows, "The on-time must be chosen greater than 90 ns for proper operation."

    Does this mean that the on-time during current limit will be less than 90 ns and becomes 40% of the normal on-time?

    You said that the on time does not necessarily affect the current limit with previous answer.

    I understand that on-time does not affect the current limit threshold.

    However, I believe that the peak current will be determined by the on-time.

    It is necessary to estimate how far the peak current will reach in the worst condition.

    I would like to know maximum on-time during current limit.

    Best regards,

    Tomoaki Yoshida

  • 0 in reply to Tomoaki Yoshida91
    Genius 5255 points

    Hi Eric-san,

    Any update on this issue?

    I want to estimate the Ipk in the figure below.

    Best regards,

    Tomoaki Yoshida

  • 0 in reply to Tomoaki Yoshida91
    TI__Prodigy 480 points
    Hi Tomoaki,

    Currently, a product engineer is looking into the maximum on-time during current limit. I will let you know when he gets back to me.

    Also, yes, you are correct that the on-time will be less than 90ns and becomes 40% of the normal on-time. It will no longer be in normal operation as in the figure you sent, but it will be in current limited operation.

    Thank you.
  • 0 in reply to Eric Sato31
    TI__Prodigy 480 points

    Hi Tomoaki,

    Thank you for your patience. The product engineer came back with the following:

    Maximum Ton during I lim occurs at cold. Below is the data at cold based on 240 units sample size.

    Max= 138.174ns
    Avg= 131.079ns
    Min= 124.804ns
    Sigma= 2.22542ns

    Please note that this data is only typical. It’s not guaranteed.

     

  • 0 in reply to Eric Sato31
    Genius 5255 points

    Hi Eric-san,

    Thank you for your support and I'm sorry for my late reply.

    I think that the variation is smaller than our assumption.

    I would like to know the design value of on-time is such a value.

    Could you tell me the design value such as Vin, temperature condition, on-time, etc. at the above measurement?

    Best regards,

    Tomoaki Yoshida

  • 0 in reply to Tomoaki Yoshida91
    TI__Prodigy 480 points

    Hi Tomoaki,

    The maximum on-time above was calculated under the following conditions:
    1. VIN = 12V
    2. RT = 50KΩ
    3. Ta = -40C

    Hopefully, this was what you were looking. Thank you.
     

  • 0 in reply to Eric Sato31
    Genius 5255 points

    Hi Eric-san,

    Thank you for your reply.

    I calculate ON time from the condition taught is 187.5 ns.

    Ton = (4.1 * 10^(-11) * (RT + 500 Ω)) / VIN +15 ns = 187.5416 ns

    In other words, during the current limit at low temperature, the ON time may reach 70% over 40%.

    This is important information to consider how much margin should be taken.

    Best regards,

    Tomoaki Yoshida

  • 0 in reply to Tomoaki Yoshida91
    TI__Prodigy 480 points

    Hi Tomoaki,

    The Ton equation that you mentioned is based on typical conditions (25degC). Therefore, it might not be accurate for low temperature at -40degC.

    Can you probably elaborate on what you mean by 70% over 40%? I am not sure where this 70% is coming from.

    Thank you.

  • 0 in reply to Eric Sato31
    Genius 5255 points

    Hi Eric-san,

    Thank you for your support.

    I calculated Ton as 187.5ns in typical conditon as I wrote in the previous post.

    I know it is the design value at typical conditoin and the Ton during normal operation will change under worst conditoin. 

    Because the measured maximum Ton at the wotst condition during current limit, I calculated as follows : 138 ns / 187.5 ns = 0.74.

    In order to estimate the maximum peak current roughly, we only want to know the maximum on-time during the current limit and how much to consider margin.

    Since not only the  Ton of LM25011 but also the external components such as the  inductor affects the peak current, we will design with sufficient margin.

    Best regards,

    Tomoaki Yoshida

  • 0 in reply to Tomoaki Yoshida91
    TI__Prodigy 480 points
    Hi Tomoaki,

    Glad to be of service. Having a sufficient margin sounds like a good idea. If you need anymore support, you can always contact us again. Thank you.