We have some stability problems with our DC-DC-regulator design using TPS40041.
At high current the system gets unstable and the TPS40041 turns off (Short Circuit Protection?), also the calculated max current wasn't reached.
The max current is depending very much by the switching MOSFET device IRF7910. Changing a "good" with a "bad" IRF7910 on the PCB causes a factor 2 in max current.
- Changing the COMP R-C-network does not solve the problem. (Thank you, Na, for the link to the compensation design tool).
- Adding a 2,7 Ohm bootstrap capacitor series resistor, the regulator works with all IR7910 until the calculated max current. This would solve our problem.
- We also tested the different Short Circuit Current Limit Thresholds using a resistor at the COMP pin. Here we found that the regulator switched off at lower current using a 12-kΩ resistor, and at higher current using s 2.4-kΩ resistor. This is the inverse behaviour, than we expected from the datasheet.
Can this behaviour give you a hint, what is wrong with our design? Is there a typo in the datasheet?
From your description, it's possible that the SW ringing cause the error in short-circuit trip point. Due to the small gate charge of the IRF7910, the turn-on speed of high-side FET is too fast. The fast di/dt on the parasitic inductance may introduce switch node voltage ringing. The current limit comparator has a blanking time of 100ns (not tested) after high-side is turned on. If the ringing does not die out within the blanking time, the ringing will cause error for the short-circuit trip point. One of the most effective solutions is to add boot resistor as you mentioned to slow down the turn-on speed.
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