• State TI Thinks Resolved
  • Locked Locked
  • Replies 12 replies
  • Answers 2 answers
  • Subscribers 65 subscribers
  • Views 501 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.

LM2936: power designer inquiry

Gerald
Genius 9570 points
Part Number: LM2936


Hi Experts,

Our customer is using the "power designer" for LM2936.
It recommends the KRM55WR71J226MH01K as the output capacitor and states an ESR of 1.5 Ohms. When the customer looks in the datasheet of KRM55WR71J226MH01K, an ESR of 1.5 Ohms is only reached at a frequency of 1.6GHz. He wanted to know why it uses this ESR-Value?  Can you tell us, why it uses this ESR-Value?

The LM2936 recommends not using ceramic capacitors as they don't meet the ESR requirements.
He also has a question about GND-connection and GND plane concerning this IC. He likes to know if all GND-Pins shall be on the same GND-Plane with or without thermals or if there are in- and output specific gnd-pins and gnd shall be decoupled over the in-/output capacitors like the Vin and Vout voltage
I have referred him to Layout Guidelines in the datasheet but still wanted clarification on below:
Is it really not recommended to use thermals?
And is it really not recommended to connect the GND pins via the in- and output capacitors to the rest of the GND plane?
Please advise. Thank you.

Best regards,
Gerald

  • 0
    TI__Guru 50025 points

    Hi Gerald,

    Please keep in mind that we don't support Webench anymore. Also note that this is an old device and the output capacitor recommendation is from that time as well.

    I can't say definitively why that particular capacitor was selected as the recommended capacitor, but my guess at it is that the ESR (i.e. the minimum impedance of the capacitor) has a large enough value, although the frequencies of interest were not seemingly considered. In general the more correct way to look at the capacitor/ESR requirement is that the capacitor should satisfy the impedance requirements over the bandwidth of the device, which is normally in the 100kHz ~ 300kHz range. Below is an example that I made for this device, where I used 100uF as an upper limit on the output capacitor since there is no max listed in the datasheet, just for illustration purposes. This is a first-order approximated capacitor, which is what TI uses for characterizing the output capacitor requirement in simulation. A real capacitor should have its impedance within these limits across frequency within the bandwidth of the device. 

    All GND connections should be the same GND plane. I don't see in the recommended layouts that it is suggested that the input and output bypass capacitors are connected to a different ground plane. The layouts show the same ground plane connection. 

    It is not recommended to not use thermal vias. It was probably not included in these recommended layouts because the D package does not have a thermal pad, but thermal vias will nonetheless improve thermal performance so it is still recommended to use thermal vias. 

    Regards,

    Nick

  • 0
    Intellectual 453 points

    Thanks for your answer!

    1. concerning capacitor choice

    You write that it's an old device. Do you recommend to use another device? Why? And which device?

    I am not able to find any >=22uF ceramic capacitor that has an ESR between 0,3 and 8 Ohms in the frequency range of 100kHz to 300kHz.

    I guess this is only possible with tantal capacitors, am I right?

    How do I know in which frequency range the device (e.g. LM2936) works?

    2. concerning GND Layout:

    In my support Case

    CS1563360 "power designer" for LM2936 Inquiry

    https://ticsc.service-now.com/csm?id=ticket&table=sn_customerservice_case&sys_id=4452572847cd2990d4f52e61e36d432d&view=sp

    you see a screenshot (Screenshot 2023-02-13 110247.jpg) of my GND layout.

    I'm afraid you missunderstood the "thermals" term. I was not asking about thermal Vias but about the thermal connections from SMD pads to the GND-plane.

    Shall the GND pins be connected with or without thermals to the GND plane (example layout with thermals marked with yellow circle)?

    Thanks a lot in advance!

    Matthias

  • 0 in reply to Matze
    TI__Guru 50025 points

    Hi Matthias,

    1. What specs are important for your application? I can look for another device for you if you provide more information. Newer devices come on newer technology nodes with usually better Iq, often in smaller packages, and newer devices are almost always designed for use with a ceramic output capacitor, which are generally much cheaper and smaller. 
      Regarding the question about the output capacitor ESR: It is not necessary that the resonant frequency is within the bandwidth of the LDO. What is important is that the capacitor impedance meets the requirements that are bound by the min/max output capacitor value and the ESR range. Below is another similar example, where I have used the minimum capacitor along with max ESR and the maximum capacitor (I used 200uF because the datasheet doesn't specify an upper limit) along with the minimum ESR to bound the impedance requirement. The capacitor that is chosen should have its impedance characteristics fall between these two curves at frequencies less than ~300kHz. 
    2. I am not able to view the support case inquiry and I also cannot view the .jpg for some reason. Can you embed the image in a comment? You can simply copy-paste the image into the text box. 

      I'm not sure what is meant by the "thermal" connections. Are you asking if the GND pins (pins 2, 3, 6, 7 for the D package) should be connected to the same GND plane? Perhaps the image will help to explain what you mean.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 453 points

    Hi Nick,

    1.

    I need output of 50 - 100 mA, input of round about 9-30V (12V and 24V vehicles) and would be glad if there is any newer device that is pin compatible to LM2936M-3.0, LM2936MM-3.0 or LM2936MP-3.0 so I don't need to change my layout. And of course it is important that the new device is available in a sufficient quantity (as the LM2936 is).

    Concerning the capacitors with correct ESR (300 mOhms to 8 Ohms): Can you name (some) example(s) (partnumbers) for capacitors 22 uF (or bigger)?

    2.

    I embedded the image in the comment above. Didn't know that this simple copy paste option works. Hope it gets clearer what I want to ask.

    Thanks!

    Matze

  • 0 in reply to Matze
    TI__Genius 12511 points

    Hi Matze,

    Thanks for the update and the photo, unfortunately Nick is OoO, he will get back to you when he returns after the holiday on the 21st.

  • 0 in reply to John Schneider
    Intellectual 453 points

    Hi ,

    have you returned from holiday?

  • 0 in reply to Matze
    TI__Guru 50025 points

    Hi Matze,

    If you are looking for a P2P part, there aren't any devices that are much newer than LM2936. I don't know of any specific part numbers, but is it possible in your application (i.e. do you have the budget and board space) to use a series resistor with a ceramic capacitor? That is often the easiest solution.

    I think I understand your question regarding "thermal connections" now. I believe you are asking whether the bypass caps need to be connected directly across the input or output to specific GND pins on the device. Is that correct? If that is correct, in general for an LDO it is better to connect the bypass capacitors as close to the input and output pins as possible to the GND pins of the device to minimize the loop path. For this device, all GND pins should be connected to the same GND, and the bypass capacitors for input and output should be connected to the closest GND pin. All GND pins for this device are true GND; they are all part of the lead frame and the IC GND is down-bonded to it. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 453 points

    Hi Nick,

    1.

    yes, I have put a series resistor + ceramic capacitor as an option into my layout.

    2.

    Yes, this was one part of my question. the other part was about the thermal pads that you can see (marked with yellow pencil) in my screenshot (from February 15) .

    As an explanation I add the following screenshot:

    Is this "thermal pad layout" recommended for the GND-Pins of the LM2936 as you can see in my screenshot (from February 15) (marked with yellow pencil) on the GND pins (pin 6 & 7) of the LM2936M?

  • 0 in reply to Matze
    TI__Guru 50025 points

    Hi Matze,

    1. In that case, I think this is the easiest and most straightforward approach.
    2. I see. We call those "thermal relief connections" to distinguish them. Transient performance is most-optimized when there are no thermal relief connections because each one contributes some ESR and ESL to the control loop. So in regards to performance, using full connections is better than thermal relief connections, but if the device will need to be reworked the thermal relief connections are preferred for easier soldering. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 453 points

    Thanks a lot for your answers!

    Finally we came to a point without missunderstandings Slight smile

  • 0 in reply to Matze
    TI__Guru 50025 points

    Hi Matze,

    Great! I'll close this post.

    Regards,

    Nick