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.

LM5069: LM5069 - Need to understand the dv/dt Startup circuit

Part Number: LM5069


I'm trying to design hot swap controller circuit using LM5069-2, I understand that startup circuit is required when a large amount of output capacitance is used.

I understand that the external dvdt start-up circuit at the FET gate diverts part of that charging current to slow down the rate of startup dV/dt and hence the inrush current gets reduced.

Q1: What is the minimum output capacitance needed while considering this dv/dt start up circuit.

Q2: Lets say for output capacitance around 1500uF, if I use this circuit start up circuit, I see the gate is not completely discharged, it has around 2V across capacitor during turn-off.

I would like to discharge the gate completely (Considering recommended value, Cdv/dt = 1uF).

Q3: For the above Cout, can I use RC network & a high value R parallel to C so that I can discharge the cap quickly?



  • Hi Kumar,

    Thanks for reaching out 

    Need of external dvdt soft circuit depends on various factors such as Cout, Vin, FET SOA etc. So, we recommend to use design calculator to verify when dvdt soft start circuit is must needed. Saying so, there is no harm in using dvdt startup even at lower Cout.

    To completely discharge the cdv/dt capacitor, we suggest pnp based circuit defined in app note

    Don't use just RC in large cdv/dt cases

    Best regards