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LM25088 - behavior at no load/light load

Per Magnusson
Intellectual 500 points
Other Parts Discussed in Thread: LM25088, LM5088

I am considering LM25088 for a design where an 18-36 V input voltage will be converted to an adjustable voltage between 2 V and 15 V (using current injection into the FB node to set the desired output voltage). The load will be between 0 mA and 2 A.

After reading the datasheet and doing some simulations in Tina TI, I am not at all sure that LM25088 will handle no load/light load well. I cannot find anything in the datasheet about pulse skipping or other means of maintaining regulation when there is little or no load and the simulation (a modified version of http://www.ti.com/lit/tsc/snvm567 with 10 kohm as the load and 500 ohms as R7) indicates that the output and feedback nodes rise far above the desired regulation point.

Can LM25088 be used when there might not be any load current?

Does it have pulse skipping built in?

Is there any other part that would be more suitable for this application?

  • 0
    TI__Guru 61461 points

    Hi 

    Looks like the issue which you are seeing(FB rising) in TINA is a modeling issue, not the issue of LM5088. 

    LM5088 has a pulse skipping. If the required pulse width is less than the minimum on-time of LM5088, LM5088 skips switching to maintain output regulation. 

    Since LM5088 has an ability to skip pulse at no load, a dummy load might be required to replenish BOOT capacitor. 

    Regards,

  • 0 in reply to EricLee
    Intellectual 500 points

    Thank you Eric.

    The voltage divider to the feedback pin will of course be a tiny load, but maybe the current through typically valued feedback resistors will not be enough to help charge the boot capacitor? Is there a way to determine what the minimum load is? I do not need a particularly clean voltage if the load is very light.

    Best regards

    Per

  • 0 in reply to Per Magnusson
    TI__Guru 61461 points

    Hi 

    Most of time,  the leak current through the feedback resistor is enough with a help from an internal pre-charge switch, but some times, feedback resistor is not enough to help charging the boot capacitor especially when input voltage is high and output voltage is high. 

    Increase the dummy load to guarantee V(BOOT-SW) voltage ripple is less than about 1.0~1.5V always at the maximum Vin, maximum Vout and no load.

    Regards,

  • 0 in reply to EricLee
    Intellectual 500 points

    Thanks.

    I will put in a dummy load in the design and see if I need to populate it.

    Per

  • 0 in reply to EricLee
    Intellectual 500 points

    By the way, I see now that you are referring to LM5088, whereas I intend to use LM25088. The parts look very similar and a quick look suggests that only the input voltage ranges are different.

    So, can I conclude that your reply about pulse skipping applies to LM25088 as well?

    Per

  • 0 in reply to Per Magnusson
    TI__Guru 61461 points

    Yes, you can. Functionally, LM5088 is the same as LM25088. Difference is absolute maximum rating. 

    Regards,