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5V on Panda II


#1

I’ve been having some problems with the 5V line on the Panda II.

In the past I’ve been using Domino’s boards. The 5V line differs not much from that value.
On the Panda II the voltage differs some tenths of a volt. I’ve tested two Panda II boards now, and the 5V line reads 4.64 and 4.62V. For the USBizi chip it’s not a problem, cause it will run at 3.3V.

I’ve been used to have a 5V display connected to the Domino. The minimum voltage needed for the display is 4.7, and now the display sometimes works and does not work.

I’ve also been reading about using the USB port on the Panda II as a USB Host on
http://ghielectronics.blogspot.com/2011/03/usb-host-support-is-added-on-fez-panda.html
There is noted to short diode D4 that normally protects the board to get the 5V at the connected USB device. The voltage on both Panda II boards at that point is about 0.4V lower than the 5V, that will cause some USB devices that were connected to mailfunction (e.g. USB to Serial converter).

To get the USB Host working, the 5V of IC3 was directly connected to the USB port pin. Now all USB devices connected did work.

Comparing the 5V circuit from the Domino and the Panda II, they both use a LM1117MP-5. On the Panda II there is an extra diode (D2) in series, that will cause the output of IC3 to drop about 0.4V.
I’ve looked the diode STPS140 up, it’s a power schottky rectifier, but that’s the point my analogue knowledge ends.

What is the purpose of the schottky rectifier D2, and why it’s not on the Domino board?
Can it be shortened to get a real 5V on the 5V line of the board, and what problems can shortening it bring?


#2

The diode is for protection. You can short it out if you know what you are doing.


#3

Typically if I need a dependable 5V source, I use a separate supply. This particular unit has worked really well:

http://www.mouser.com/ProductDetail/Murata-Power-Solutions/OKI-78SR-5-15-W36H-C/?qs=2QmwSASSNhZqjGIOUYWBAg%3D%3D

For less than $5, you get a switching power supply that inputs anywhere from 7 thru 36 volts, and outputs a stable 5.0V at a maximum of 1.5A. Best of all, it’s tiny and completely self-contained, so you can you use it by itself without having to add additional resistors/capacitors. Since it’s a switching power supply and not merely a linear regulator, it’s also quite efficient, so you don’t have to worry too much about lots of heat heat being generated if your input voltage is higher than nominal.

With this unit, I power the Panda by wiring the 5V output into the Panda’s 5V pin, as well as any peripherals that need 5V.


#4

Adrian,

The diode is there to protect the LM1117MP-5 regulator if you enter your own 5V on the 5V signal.

So if you enter 5V directly on the board, you will have that perfect 5.0V but leave the diode.

If you will only ever power the board from the power jack, you can shorten the diode. But don’t forget to unshorten it if you ever enter again 5V directly on 5V.

I have also found that while the LM1117MP-5 is named a LDO Low Drop Out regulator, it indeed have a dropout up to 1.2V at 800mA. And even at lower current, the dropout is 1V minimum.
So as there is also a D3 diode in serie with power jack and LM input, the minimum voltage to power the Panda II on the power jack should not be listed at 6V but at least 7V.

In conclusion to get 5.0V from the on-board regulator through the power jack, you need both to

  • Use at least 7V external PSU
  • Shorten the diode

#5

Sounds like this has been pretty well covered, but if cost is an issue you can put together your own three component regulator for only a couple of bucks if you have room on whatever you are building. I’ve solved a similar problem with a 7805 regulator in either a TO=220 or TO-92 case depending on your current requirements.