If I use the ADV8005 just as a HDMI phy chip to change video format from TTL to HDMI format ，is there a DDR2 needed to use？
Is there a DDR2 needed to use？
No, Its not required.
External DDR2 memory is required only for motion adaptive de-interlacing, Scaling, Frame Rate Conversion (FRC), and OSD bitmap overlay. If you are not using these feature then you can drop off DDR.
Depending on the feature required, the appropriate memory device can be used.
DDR2 configuration is required only when we use the DDR2 memory.
For your requirement(TTL to HDMI) these writes are not required.So you can skip these DDR writes.
Please note that configuring this manually allows the user to have very flexible control over the external DDR memory.