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LT3471 softstart isn't very soft

Hi,

I'm doing 5V to +-16.8V with this part.  Less than 100mA output current.  I'm noticing very high in rush current, part of the problem is the amount of output capacitance I have on the board.  But I just can't seem to get the soft start to significantly reduce in rush.  I've tested 4.7k / 10uF RC time constant at the input but still in-rush is between 2.5A and 3A.  I'd like to get in-rush down to 0.5A.  I don't care how long the supply takes to come up, 1 sec is fine.  In simulation I've tried up to 100k / 10uF and I can't seem to reduce in-rush either.  The enable pin starts near 1.8V always, it doesn't have a nice RC curve to it.  I can't understand why this is, it should be changing at the start of simulation from near 0V to 1.8V.

I'm attaching my circuit simulation, which matches my actual board design fairly well.

Thanks for any suggestions!

Erik

LT3471_Dual_16V8.zip

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  • Hi Erik,

    The problem is the current sources. If the load is not an ideal current source then you can get a softstart using the RC soft-start feature. For example, if the loads are OLEDs then you can get a soft-start using the RC soft-start feature of the LT3471 because OLEDs have a resistive element and cannot be represented with current sources.

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  • Hi Erik,

    The problem is the current sources. If the load is not an ideal current source then you can get a softstart using the RC soft-start feature. For example, if the loads are OLEDs then you can get a soft-start using the RC soft-start feature of the LT3471 because OLEDs have a resistive element and cannot be represented with current sources.

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  • Thanks for the reply, I will remove the current sources.  To give you a bit more background on this... my input voltage is 5V.  My enable voltage is 3.3V.  It's hard to get a good soft start with 3.3V because the part stops the soft start at 1.8V (<1 tau).  I was thinking to use a resistive divider (with cap in parallel with lower resistor) that gives me something like 2V at the enable pin so that the slow part of the RC curve lines up better with the soft start of the part.  Does this make sense?

    At any rate, I have something like 170uF output capacitance in this design and its hard to keep in-rush down.  Ideally I get it down less than 1A, closer to 0.5A because my isolated 5V power supply cannot handle more than 3A and there are many other things in the system that have inrush.

    Would adjusting the inductor value also change the in-rush current?  I do not care about how long it takes to get to +-16.5V, even 1 sec would be okay.  Any other ideas?

    Thanks!

    Erik

  • Hi Erik,

    Having a divider feeding the capacitor for the soft start is fine. Be sure the final voltage value at the SS pin is guaranteed to be above 1.8V.

    Increasing the inductor value will help with the in-rush however this is a current mode controller and needs some inductor ripple. We don’t recommend going above 15uH.

    The only other option is to change the SS input RC to an OP-Amp integrator. This will allow a more linear and slower ramp up in voltage. Example:  https://www.electronics-tutorial.net/analog-integrated-circuits/op-amp-integrator/non-inverting-integrator/

  • Thanks, a resistive divider feeding the cap is working well for now (steady state = 2V).  The integrator makes sense- I will keep that option in mind for the future.