H-Bridge Adjustable Slew Rate: Why is a decoupling capacitor needed?

Question

I am designing a simple 3-phase inverter for BLDC Motor control. For that purpose, i plan to use a IFX007T from Infineon. The slew rate is controlled via a resistance connected in series to SR pin.

I do not understand how this resistance is used to adjust the slew rate but i suppose the analog magic behind it far too complex and beyond the scope of this question. What i would like to know is: What is the purpose of the capacitor connected parallel to this resistance in the reference design? As far as I know, its a decoupling capacitor that basically sends any high frequency components to GND.

The reason I am asking is; in older versions of this H-Bridge such as BTN7930 there is only the resistance connected to the SR pin and no caps what so ever. Since I am a little tight on space, I wonder if it would be alright to omit it with IFX007T.

Answer 1

You need to read the application notes.

Starting with page 39, it explains how different values of R11 influence the slew rate.

C2 is mentioned a few times, and seems to be non-critical - but it seems you really do need it.

The most important comment about C2 is that it and R11 need to be as physically close to the chip as possible and that the ground connection needs to be very short as well.

I would assume that C2 is there to keep the voltage on R11 steady. That voltage sets your slew rate, so variations would cause the slew rate to vary - maybe pushing you into territory where you cause too much interference.