The Eachine ET25 25mW transmitter has by default 4V camera output instead of the stated 5V. This is not enough to power my HS1177 camera so I had to adjust it to make it work on my setup.
Camera output voltage is supplied by the on-board switch mode power supply, with IC RT8259 (or equivalent) which is the step-down converter chip. Without going into details, the datasheet gives me the formula for setting the output voltage with two external resistors, Vout = Vfb(1+(R1/R2)), with Vfb (feedback voltage)=0.8V. The the resistors are denoted in the image below.
Measuring the two resistors I get R1=47K and R2=12K. By putting those values into the formula I get 0.8V*(1+(47K/12K))=3.933V which as as expected. We don’t have to calculate a new value for either of the resistors since it’s already given us in the datasheet by table 1 on page 2. It’s suggested that R1 is replaced by a 62K resistor and R2 is kept as 12K, which is convenient. All though, according to the formula this should give is 4.93V and that should be more than fine for most 5V compatible devices, you could replace R1 with a 68K resistor and get 5.33V but this is pushing it in regards to the STM8S mcu that is also supplied by the regulator circuit which has a Vmax of 5.5V.
In lack of a resistor in neither the right size or value (the resistors are 0402 size), two 0603 resistors (68K and 680K) was sandwiched to make one 61.8K resistor which set the output at 4.92V. It was easier to solder the new resistor down when the two caps on the sides was moved a bit away from the land I was aiming at.
Ideally we should replace the inductor also for one with slightly higher inductance. The datasheet suggests a 4.7uH inductor for 3.3V output and 6.8uH for 5V output. The one mounted already is 3.3uH, which seems a bit on the low side, and why this is chosen I’m not sure.
Note: The designer of this vtx did for some reason decide on 4V output voltage, a reason that I am not aware of what is. AFAIK there is no components on the vtx that cannot handle 5V, but it will run hotter than usual.