Adding soft-start for LDO LT3080


I have a question about the below circuit of LT3080, what is the purpose of D1?

Is that for discharge C2 when the Vin disappeared? 

Because D1 will affect the accuracy of Vout, I do not want to add D1。

Best Regards,


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    •  Analog Employees 
    on Oct 28, 2020 3:59 PM

    Thanks for the post.  Yes, the diode D1 you show will affect accuracy.  I show how much a silicon diode can leak in the image below.  As far as I know, the purpose of D1 is to prevent SET from being held up if IN=VCONTROL is forced low.   Actually, if IN=VCONTROL is forced low, OUT will also be forced low because there is a parasitic diode from OUT to IN on the LT3080, similar to the parasitic diode I discuss in this post titled "Disable LT3083".

    1N4148 leakage_1.png

  • Hi,

    Very thank you for your reply.

    ABS.max of SET pin voltage is just ±0.3V relative to OUT.

    it seems that If I use a little big C2 for soft-start, that means it is a must to use D1 to protect SET pin when IN=VCONTROL is forced low. this will be a weak point.

    Best regards,


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