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Question about Standby mode for LM5122

Paul Wu1
Prodigy 90 points
Other Parts Discussed in Thread: LM5122

Hi,

 I am designing a multi-phase boost controller with 4 x LM5122. All will be in Slave mode (90deg apart), with the FB disabled. I am providing the control loop feedback directly into COMP of the 4 chips. I have a external power supply constantly providing 12v to the VCC pin.

However, I will need to stop some or all of these chips from switching, but need it to go into switching mode immediately, upon request.  There are 2 ways. Either use UVLO, to get it into Standby or even into shutdown, or to use COMP, and pull it all the way below 1v. I do not need pass-thru mode at all when not switching.


Can advise the pro and con of the various ways to stop switching? 

Regards,
  Paul Wu

  • 0
    TI__Guru 58171 points

    Hi

    If you are controlling COMP externally now, I think the best way is controlling COMP to stop the switching.

    UVLO toggling is also okay, but the device will have to decide its mode (Master/ Slave) at every UVLO toggling.

    Regards,

     

  • 0 in reply to EricLee
    Prodigy 90 points

    Thanks Eric for the heads-up. So what you mean is, even when merely switching from stand-by to active mode, the device will re-decide its Master/Slave mode?

    In addition, I have further concerns about supplying low-voltage into the device. There was another post that says it is okie to apply a separate low-voltage into the Vin, as oppose to applying it into Vcc. I do have a 9v or 12v supply which is available earlier than Vin. Should be be applied to Vcc or Vin? Pro and Con?

    Another question is the deviation of voltage of COMP to CSP/CSN. I measured 2mV delta between the master and slave on my eval board. Am concerned with current-sharing on this delta. What is the maximum expect deviation between devices, with regards to COMP-voltage to CSP/CSN voltage.

  • 0 in reply to Paul Wu1
    TI__Guru 58171 points

    Hi

    Mode selection (Master/slave) occurs when UVLO pin voltage crosses over 1.2V from low to high. This means if you toggle UVLO, the device shoud decide its mode at every toggle.

    External supply helps you to save some power loss and decrease IC temperature. If any chance your main power supply turns off when the external VCC supply turns on, you need a block diode at VIN ( please refer Fig 19 in the datasheet.)

    Current-sharing between master and slave has not been characterized. I don't have a good answer.

    Regards,