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LM25088: Soft Start maximum programmable time? Webench limits to 10ms...

Jim Smith1
Prodigy 160 points
Part Number: LM25088
Other Parts Discussed in Thread: LM5088

Hello,

Is it possible to utilize the SS pin of LM25088/LM5088 for longer soft-start times than what Webench allows?

Webench has a field for entering a millisecond value, but if we enter anything higher than 10ms it says "Please enter a valid Soft Start Time."

For 1ms SS, a 10nF value is implemented for capacitor Css. For 10ms, a 100nF is implemented. Clearly there is a basic relationship. Is it possible to simply use a higher value cap to continue increasing the SS duration, or does the IC simply have a limit on SS duration?

I'm interested in a longer pause, such as 500ms. 

Thanks!

  • 0
    TI__Mastermind 22299 points

    Hi Jim,

    You should be able to use any soft start capacitor you want, I think WEBENCH wont allow it due to limits in their simulation time.

    The soft-start equation (19) can be found in section 9.4.9 of the datasheet. 

    I think you will need 4.5uF for 0.5s soft start. 

    It will not be a pause it will be a linear ramp up.

    Hope this helps

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 160 points

    Thank you! And just so I understand all of the impacts that a larger SS cap value will have on the total circuit (LM5088-2), a value such as 4.5 (or 4.7) uF will also impose an approx 500 msec linear ramp up whenever the LM25088-2's overload feature is triggered to, correct? So as opposed to the system quickly attempting to start back up and provide power, it will take a half second for each successive attempt. Correct?

    Lastly, does the SS cap only ever encounter 1.2V? Or does it relate to higher in/out voltages? Just wondering for when choosing the cap's voltage rating. 

    Thanks again!

  • 0 in reply to Jim Smith1
    TI__Mastermind 22299 points

    Jim, 

    Yeah if you have LM25088-2 with hiccup restart, the soft-start is reset during hiccup current limit. It will do a 0.5s restart. See datasheet section 8.3.9.

    The section 8.2 shows the functional block diagram, looks like the soft start 11uA current source is charged from 5V, expect the soft start cap to reach 5V.

    Note that the output voltage will be climbing linearly during the 0.5s softstart.  This voltage can still provide power (to a resistive load for example)

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 160 points

    Ok thanks! 

    About capacitor choice and placement... Is this SS lead a particularly sensitive trace, and do i need to locate it immediately next to the IC? Or, would it be ok to locate the capacitor a couple inches away, with a two inch trace from the SS pin and then a local connection to ground on the other side of cap. 

    I had read somewhere in these forums that it's important to use an SS cap with low leakage specs, so am wondering if this sub-circuit is very sensitive and if any small fluctuations of mV are going to have impact on the IC's total output voltage stability/ripple, since the ramp to 5V is a guide for the IC's output of say 0 to 15V. 

  • +1 in reply to Jim Smith1
    TI__Mastermind 22299 points

    Jim,

    I read section 8.3.10 of the datasheet, heres what I found:

    SS voltage is clamped to FB + 120mV.  The VREF and nominal FB voltage is 1.205V, so SS voltage should be clamped to ~1.33V and not 5V as I said before.

    The SS cap should have low leakage otherwise the 11uA will leak to GND and not charge up the voltage.

    I dont think the long traces will affect it, I ran some sims adding ESR and ESL and it seems to charge up ok. I wouldnt route it near any noisy nodes though.

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 160 points

    Terrific! Thank you for all of your help!!