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Altera_Forum
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10 years ago

DDR2 VREF problem

Hi, My FPGA design uses 4 ddr2 devices. And I use LP2996 to output VTT and VREF for FPGA and DDR2. However, VREF measures 1.43V other than 0.9V. After I cut the VREF output trace, I found LP299...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
10 years ago

However, LP2996 datasheet says that "VREF provides the buffered output of the internal reference voltage VDDQ / 2. This output should be used to provide the reference voltage for the Northbridge chipset and memory. Since these inputs are typically an extremely high impedance, there should be little current drawn from VREF. For improved performance, an output bypass capacitor can be used, located close to the pin, to help with noise. A ceramic capacitor in the range of 0.1 μF to 0.01 μF is recommended."

Actually, both VTT and VREF pins on LP2996 ouput VDDQ/2, but they serves different functions. I'm not sure whether my problem is the same with yours.