Set depth buffer for z test on mobile platform

Hi, I am doing the optimization for our translucent objects. Here is what i am doing: 1. Render translucent objects into a small render texture. 2. Upsample the small translucent texture into scene color buffer with some algorithm. Then the problem is in the step 1. The z test is not working since there is no valid z buffer. And I don't want to redraw all the opaque meshes to get a new z buffer using a replace shader or whatever, due to the performance issue on the mobile platform . Then i tried to downsample the main camera's depth texture(_CameraDepthTexture) to get a small depth texture, but i don't know how to set it into the low translucent camera. I tried DEPTH semantic in the fragment shader. But it seems unity doesn't recognize it. Is it possible?