LM5069: how to set inrush current lower than load current

Hi Akshat,

Great question and you're in luck! The LM5069 has a method to control the slew rate (dv/dt control) of the gate, which in turn sets a constant load current during startup (the circuit diagram of dv/dt circuit is below).

The dv/dt circuit limits the slew rate of the gate and the output voltage (Vgs is constant during startup), which in turn limits the inrush current. Meaning that if we can control dv/dt by adding a cap on the gate, we can control a constant current flowing through the FET to any value we want. By doing so, the FET never reaches the set power limit or current limit. Make sure that the Plimit divided by Vinmax is around 2x greater than the constant inrush current of 2A so that the device does not go into power limit mode (which starts the fault timer). 

Page 6 of the “Robust Hot Swap Design” app note explains this concept very well. You can find this at www.ti.com/hotswap ==> then click on “Technical documents”. 

One last thing, our team has created these nifty and fantastic calculator tools to help with finding an appropriate value for Cgate (as well as others like power limit, fault timer, etc.), which will help determine the SOA margin of the FET chosen. You can find it by going to www.ti.com/hotwap ==> click on “Tools & software” ==> then click on “Hot Swap design calculator tools”

For help on filling out the cells on the excel spreadsheet, our team created video tutorials on how to fill out the hotswap design calculator spreadsheet. They explain cell by cell what each one means and how to enter the correct value. The videos are at www.ti.com/hotswap ==> Click on “Support & Training” tab.

I hope this helps :-)

Best Regards,

Aramis P. Alvarez