title_body: How to prevent a 5V supply from sagging to 4.6V when the FPV camera is on? I have issues with my FPV camera. My camera is a RunCam split 2S (concept is similar to Caddx turtle).
  
  This camera turns off almost instantly after I have plugged in my battery. When I probe the voltage between 5V and ground, I can see the voltage drop from 4.9V to 4.6V as soon as the camera starts recording (which it does automatically when powering it). Then, when the camera shuts completely off, the voltage goes up again. Also, I can press a button and the camera will turn on but also back off again
  
  Hooking up a phone charger to the camera gets the voltage up again (and makes the camera run stably), but I obviously don't want to fly around with a power bank attached to my drone.
  
  Unfortunately, the VTX is also wired to the flight controller's 5V converter instead of its own.
  
  TL;DR: The issue is not that the voltage has too much ripple. The issue is that the voltage converter on my FC is not strong enough.
  
  Would a capacitor at the input (XT60 connector) make any sense? If yes, which size should it be? I have 220µF and 1000µF electrolytic caps.
  
  
  Links to the mentioned parts: Flight controller, Camera, VTX
  
  
  
  EDIT
  
  short answer for future people: additional, external voltage converter/BEC; use all available voltage converters (VTX, FC, (PDB)); do not make them parallel!
  
  long answer: see accepted answer
upvoted_answer: I've pulled a few specs together from the parts you linked to:
  
  
  Flight controller:
  
  
  Input: 2S to 6S LiPo (7V to 20V)
  Output: 5V 3A
  
  VTX:
  
  
  Input: 7V to 24V
  Output: 5V 0.3A
  
  Camera:
  
  
  Input: 5V 0.65A  --OR-- 12V 0.27A
  
  
  
  You don't mention what size battery you are using, but I will assume it is in the FCs range of 2S to 6S.
  
  
  The camera needs too much current (0.65A) to get it's 5V from the VTX (max 0.3A)
  The VTX should be connected to the battery
  The FC should be connected to the battery
  The camera should be connected to the FC 5V supply
  
  
  This wiring should work, if you have no other large 5V loads connected to the FC.
  
      ╔══════════════════╗  ╭───────╮
      ║ LiPo Battery     ╟──┤ 7-20V ├────┬──────────┐
      ╚══════════════════╝  ╰───────╯╔═══╧═══╗  ╔═══╧═══╗  ╭────╮
                                     ║  VTX  ║  ║  FC   ╟──┤ 5V ├────┐
                                     ╚═══════╝  ╚═══════╝  ╰────╯╔═══╧═══╗
                                                                 ║  Cam  ║
                                                                 ╚═══════╝
  
  
  Alternatively you will need a separate DC-DC converter or regulator to supply the 5V. A capacitor is great for smoothing out surges in demand, but won't hold up the 5V rail if the load is constantly too large.