Question: Can VTT be tied to VREF while designing with QDR™-DDR II/II+/Xtreme devices?
VTT is the voltage to which the various signals (data, address, control) are externally terminated using a resistor. Typically, VTT is VDDQ/2. VREF, which is the reference input voltage, is also VDDQ/2. However, we do not recommend connecting VTT to VREF.
Instead, we provide a separate power for VTT. If we directly connect VREF to VTT, then any switching noise on the I/Os can be coupled to VREF. As a result, VREF could become unclean, which would create instability.
In addition, VTT has higher current requirements. To calculate the current required for VTT, note that the VTT power is 5/(16R) x VDDQ^2 x (number of inputs terminated to VTT), where R is the external termination resistor. Divide this power by VDDQ to determine the current required for VTT.
The application note cited below shows how to derive the above equation. The derivation is done using an on-die termination (ODT) part, and it shows the ODT power. You would get the same equation by inserting termination outside.