LTM8005 - require some user guidance

Hi ADI experts,

There are a few aspects about LTM8005 that are confusing me.

1. In the LTspice simulation circuit, if I connect SYNC to a 3.5V voltage source as required by datasheet, the part will not regulate. It works well if this pin is floated. What did I do wrong?

2. If spread spectrum switching is prefered, is it OK to drive this pin with a voltage divider formed by two resistors?

3. Datasheet mentions the part is not designed to "load share". What is load share?

4. The recommended switching frequency is 750KHz for 2.5V output. If output ripple is primary concer, can I increase switching frequency to 1.8MHz which is recommended maximum. Is this maximum value limited by switching loss? If load current is about 2A, can I increase fSW further?

Looking forward to your reply!

  • 0
    •  Analog Employees 
    on Nov 13, 2020 12:59 AM 22 days ago

    Hi,

    I would like to ask if you are really using the LTM8005 since when I checked the datasheet there were no SYNC pin and when I also checked the LTspice directory there were no SYNC pin also. I checked on a different IC named LTM8050 and it fits the description you are saying. Let me just confirm if you are actually referring to the LTM8050 instead of the LTM8005? Thank you.

  • Hi Siglo,

    There is a typo. The part number is LTM8003. Sorry.

    In addition, I am confused with EMI features such as "low EMI", "Silent switcher", "Silent switcher 2" and "EN55022 Class B Certified". What are the differences among them? I think the scope of the aforementioned features overlaps with each other. For example, parts with "EN55022 Class B Certified" feature are all "low EMI" parts.

  • 0
    •  Analog Employees 
    on Nov 18, 2020 3:19 AM 17 days ago in reply to Yubing

    Hi,

    I think this is an LTspice issue. I also tried simulating it with an external voltage supply of 3.5 and 3.3v (as recommended in the data) and I got the same output. I'll contact the LTspice team about this for further investigation.  In theory yes you can use a voltage divider circuit. The recommended switching frequencies in the data sheet are tested to work with no problem regarding noises and losses. You can try on your demo board to confirm if you can increase further the switching frequency. Yeah I think you are right. The aforementioned features just overlaps with each other.

  • Hi,

    Thanks for your reply.

    As for current design, I will leave SYNC floating and have a fixed switching frequency.

    Does "load share" stand for applications that have multiple converters work in paralell in order to have higher driving capability?

    Can you explained in more detail what is Silent Switcher (SS) 1 and 2 and what is the difference between them?

    In my opinion, SS1, in comparison with a conventional buck converter, places input capacitors on both sides of the IC, resulting forms a magnetic dipole (two current loops in opossite directions) around the IC and thus reduced EMI emission. In addition, SS1 is packaged by flip-chip technique and reduces parasitic inductance at VIN pin. As for SS2, it is a buck module that have SS1 IC, input and output capacitors, etc packaged together.

    SS1 and SS2 are all "low EMI" buck converters. Some of them are "EN55022 certified" and some of them are not.

    Am I correct?

  • 0
    •  Analog Employees 
    on Dec 1, 2020 8:37 AM 4 days ago in reply to Yubing

    Hi Yubing,

    My answers after each of your questions:

    Does "load share" stand for applications that have multiple converters work in paralell in order to have higher driving capability? Fil: Yes.

    Can you explained in more detail what is Silent Switcher (SS) 1 and 2 and what is the difference between them? Fil: The difference is the technique used to achieve low EMI performance. Silent Switcher 2 has Internal Bypass Capacitors to Reduce Radiated EMI.

    SS1 and SS2 are all "low EMI" buck converters. Some of them are "EN55022 certified" and some of them are not. Am I correct? Fil: Yes.