Just finished soldering my first smd board and all seems to be working apart from the voltage regulator. The voltage regulator is supposed to be fixed output at 5v however it seems to be tracking input voltage minus 0.2V so if I input 5v I get 4.8V out, if I input 7V I get 6.8V out and so on, the board powers fine over usb or with a regulated 5V input. I have double checked all connections and all seems fine.
I have vin and enable hooked up to the input voltage, gnd hooked up to ground, vout is where I am measuring and byp is open as per the manufacturers notes.
Does this kind of behaviour usually point to a faulty regulator?
Not looking forward to trying to desolder and replace as the board was reflowed with a skillet.
Thanks,
Mark
Any idea's or is it simply just a faulty part.
Do you have the fixed or the variable output device. If the latter and the byp terminal is connected to a potential divider, then if the lower resistor (the one connecting to gnd) is not actually connected to groubd, the output will track the input (minus the voltage drop across the series transistor)
The part could be bad, or you might have the wrong part. It's available in various voltages, or as an adjustiable version. I wonder if you have the adjustable version. (I assume you've wired it as 5V fixed?)
Under Product Information on the page you linked,
The 2nd line says: "Output Voltage Fixed: 5V"
The last line says: "Voltage Regulator Type: Adjustable"
According to the spec sheet the 5V version should be marked "LG50" and the variable version is marked "LGAA".
Thanks for the replies... since posting I had also noticed that the description on the farnell website was misleading they list the item as fixed output but also further down the page as variable, checking the case code it notes LGAA which turns out to be adjustable. Becuase the adjust pin is not connected to a circuit the output floats around near input. Luckily when I first powered the board I only gave it just over 6V so the 1284p and ft232rl seem to have survived if I had powered it up with 12v things may have been different.
I just spoke to customer services at farnell and they agree the description is at fault so they are going to credit me for the regulators.
Thanks for the tips on the desolder, I think I will try cutting the pins as suggested and then clearing up after, before replacing with the correct part. This is my first frying pan smd board and the only issue apart from the wrong part was a couple of solder bridges that took a few minutes to sort out. The main learning point was to maybe use larger components than 0603 in the future for ease of pick and place.
Applied solder paste with a toothpic, placed components with tweezers, then in a pan over a gas hob, 3 minutes from 0c to 160c, then 2 minutes from 160c to 220c then hold at 220c until the usb and jst connectors fully flowed then heat off, cool the bottom of the pan slowly with water until it was less than 160c then lift the board off to a cooling rack. Had a couple of solder bridges to clear with wick apart from the wrong regulator part the board works without issue.
