Hi,
I need to use this IC with current limit ca 30-40 mA. I have Rsens 1.3Ω and Rpwr 100kΩ.
When voltage is around 18-35V everything is ok but when voltage gets around 36,4V and current 22,7mA hot-swap disconnect power after 0,5 ms but when it's first run after long time it needs something around 1min. I've tried to add from 10nF to 20nF as Cgs but it doesn't help. V(peak-peak) on gate is around 1V when working.
What should I try to do?
Hi Jan,
Welcome to E2E! We do NOT recommend to go above a Rsense value of 100mOhms (Top of page 12 of datasheet):
The main reason for this is because of loop stability. If you use a larger Rsense, the control loop could become unstable and you would see clear signs of oscillations on the GATE. You could try to keep adding additional Cgs cap until the oscillations disappear, then double for design margin. But this will greatly reduce the response time for turn on and turn off times.
Also, I'm not sure if you're aware but our team has created these easy to use design calculator tool with this device. You can find it by going to www.ti.com/hotswap ==> 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
Hi Jan,
I'm sorry to hear that the current limit at Rsense =100mOhms is too high :-(
In terms of the max value for Cgs, we usually don't recommend anything above 100nF. This is because adding additional cap creates more energy that has to be dissipated through the 2mA gate pull-down FET (internal to the IC). Adding additional Cgs cap creates more risk for the pull-down current source to break.
Also, the FET's response time to turn off due to a current/power limit or short circuit event will be delayed dramatically. Because of the slow turn off time, the FET's SOA curve might be violated and might break.
To calculate the response time for the gate to shut off, we use I = C*dV/dt. For a current/power limit event, I = 2mA (look below for EC table in datasheet), dV = (12V + Vin), and C = (Cgs cap + FET). Usually, a FET might have around 10nF of Cgs cap. If we didn't add any Cgs cap, then the turn-off time from a current limit/power limit would be:
dt = C*dV/I = (10nF)*(12+36)/2mA = 0.24ms. This is additional time on top of the fault timer setting
If we add an additional 100nF of cap, the turn-off time would be 2.64ms, an order of magnitude more. If we were to use a 1uF Cgs cap (not recommended) then the turn off time would be 26.4ms (YIKES!!!!).
Just to reinstate, our team does not recommend a Rsense value of over 100mOhms. But, if your application needs a higher Rsense value, my colleague tested the current limit down to 143mA (Rsense = 385mOhms without adding any additional Cgs cap. Test results are shown in this E2E thread: https://e2e.ti.com/support/power_management/power_interface/f/204/p/414861/1474412#1474412
I hope this helps,
Aramis P. Alvarez
