• State TI Thinks Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 2 answers
  • Subscribers 62 subscribers
  • Views 1024 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

LMZ23605: Extending minimum output voltage SMPS

Marc Tierney
Prodigy 50 points
Part Number: LMZ23605

I'm trying to generate less than .8 volts with the LMZ23605 Simple Switcher. I have a negative voltage rail. My questions are:

1. Can this be done?

2. If not, Why?

3. If this was attempted, what would be the behavior of the device? In other words, what would the output regulate to?

From the block diagram in the data sheet, it seems this should be possible considering the FB node is closing the internal erroramp FB loop.

Surly the device will be in dcm mode at this extemely low voltage, but an average value should be attainable with excessive filtering (bigs-ass Vo cap).

  • 0
    TI__Mastermind 47060 points

    Hi Marc,

    I might not be understanding your initial condition. Are you saying you want to generate a positive voltage less than 0.8V from a negative input voltage rail or are you saying you want to generate a negative voltage rail? Can you tell me what you expect of Vin,Vout and Iout? 

    For generating a negative voltage rail, most converters should be able to handle it. The LMZ23605 can accurately regulate a voltage as low as 0.8V so I expect it to also be the case for negative voltage rail. An example of converting a buck power module into an inverting "negative" rail can be seen in the following application note(SNVA807). When configured as a negative voltage rail, it's expected that the converter will not be able to load the same amount of current as an unmodified buck converter.

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 50 points
    Let me clarify. I would like to extend the range of the LMZ23605 to lower, but still positive, than the 0.8V limit. I've only mentioned that I have a negative rail to work with because I am suspecting I will need a negative voltage to make this work. In other words, I would like an adjustable output range of 0V to 6V @ 4A.
  • 0 in reply to Marc Tierney
    TI__Mastermind 47060 points

    Hi Marc,

    Can you tell me what application you plan on using this in? 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 50 points

    Thermoelectric cooler driver for a laser application.

  • 0 in reply to Marc Tierney
    TI__Mastermind 47060 points

    Hi Marc,

    From my understanding, I don't know of any external circuits that could effectively reduce the voltage reference of the power module. The lowest the LMZ23605 can go is 0.8Vref. Anything lower than that might cause instability in the control loop since the part is internally compared to a fixed Vref as shown in the functional block diagram. 

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 50 points
    Suppose we put a pull-up resistor to Vcc selected such that the FB node is 0.786V when Vo equals 0V? The control loop can be compensated with appropriate external passives. Note: there is no requirement for fast transient response for my application; very very slow loop would suffice.
  • 0 in reply to Marc Tierney
    TI__Mastermind 47060 points
    Hi Marc,

    I see where you are getting at. What I can say is that it could be done since as long as FB sees roughly 0.8V the regulator's control loop will be happy, but it would not be recommended since we have not tested for such an application nor spec it in the datasheet.

    In your case that you mentioned, it would only apply for Vo equals 0V and would not be dynamic enough to range between 0V to 6V that you desire, unless you control the resistor divider to have different voltages where the external voltage and Vout voltage add up to roughly 0.8V at the feedback for adequate Vout regulation.

    Regards,
    Jimmy
  • 0 in reply to Jimmy Hua
    Prodigy 50 points

    I understand why the SIMPLE SWITCHER would be limited in the way it is due to the internal reference. What I'm proposing makes the SIMPLE Switcher less "Simple". Anyway I've attached a simplified schematic of what I had in mind. Vin would be driven from DAC. The other resistors would be tailored for appropriate scaling needed. In principle, this should work. However, I'm concerned about weird behavior of the device operating under this mode (lock-out, failure of the device, overheating, instability, ect.). I just don't have an evaluation kit to try this. For my design, I can use a device from Linear Tech which I have experience with, but the SIMPLE Switcher would be a much simpler solution. I would really appreciate it if I could get some feedback on this un-specified operating regime. Thank you.

  • 0 in reply to Marc Tierney
    TI__Mastermind 47060 points
    Hi Marc,

    I can provide some evaluation kit for you to try this out if you'd like. Please can contact me directly at j-hua@ti.com.

    Regards,
    Jimmy