By Luis Cordova, Texas Instruments
When you test the video feed from your newly designed camera module, have you ever noticed slow-moving bars, discoloration or flickering in your video, or no video at all?
There can be many different reasons for video issues like these: switching noise from a switcher, voltage ripple during frames or rows, elevated system temperature, or even damage to an imager. In this blog post, I will address three design techniques that reduce voltage ripple caused by load steps from an imager and image signal processor (ISP) using the Automotive 1MP Camera Module Reference Design with YUV422 Output, FPD-Link III and Power Over Coax and Automotive 1MP Camera Module Reference Design with YUV422, FPD-Link III and 4V-36V Power Over Coax. Below is the block diagram for the 4-36V Power Over Coax camera module reference design. Previous design approaches for camera modules only included considerations for just the imager. However, introducing an ISP created a new set of challenges.
Because of frame and row transitions, the dynamic loading of both the imager and ISP can create considerable voltage ripple on the analog rails. For example, during the transition to a new frame or row, the current draw resembles a load step, requiring 100-200mA during a row or frame and 10-20mA during row or frame dead time. Figure 2 shows the actual 2.8V rail load current waveform in yellow, and the 2.8V rail voltage ripple in pink. Figure 2 was taken on the first revision. I had to address this unexpected ripple.
Figures 3 and 4 are oscilloscope screenshots that show the fast Fourier transform (FFT) of the load-current waveforms for 2.8V and 1.8V rails, respectively. I measured the load current waveforms by replacing ferrite beads between the supply and rail decoupling capacitors with a loop of wire, and then placing a grounded…