General Technical Discussion

[zgora]

Hi,

I’m wondering is there some on-chip (inside package) bypass capacitance for FPGA VCCINT and VCCAUX supplies?

The reason I’m asking is because the inductance of power supply pins/pads seems be too high. Without some internal on-chip capacitance power supply pins/pads cannot work as an adequate power path for high frequency logic switching circuits like 1GHz SerDes or PLL. Let’s take TQFP144 package for example. According to IBIS file its pins have typical inductance of about 8nH. The impedance of 5 parallel VCCINT pins for 1GHz wave signal would be about 10 Ohm. That is too high. So I guess there should be some intentionally added internal on-chip capacitance for VCCINT and VCCAUX. Is that true? Could you clarify?

Thank you,

Sergey

[mcgett]

The answer will depend on them FPGA family and package technology. Current Xilinx families with FF packages will have internal package decoupling capacitors on some, but not all rails. The PCB design user guide for each family will document the decoupling capacitors for each die/package combination (MGT decoupling is documented in the MGT user guide)