Hello,

Does anybody know how much current the Vout pin (3.3Vdc regulated) can supply?

I can't find this info anywhere on the site.

Maybee i can see what power chip is used in the schematic and find out but maybee someone here knows.

/Mateo 

Hi Mateo,

The Vout pin is supplied by a separate LDO (LD1117s33) on the board which is rated at 800mA.

If you are supplying your mbed from Vin then you can, in theory, draw the full 800mA through Vout assuming your power supply can source this. In practice this is limited by the heat dissipation of the LDO, and therefore by the actual voltage you are applying. The higher the voltage on Vin, the higher the heat dissipation for a given load, which might lead to thermal shutdown.

If your mbed is being powered by USB, the maximum you can draw through Vout is around 350mA. The mbed has a current limiter on the USB power supply that cuts off at 450mA to prevent damage to the host PC. The mbed itself take around 100mA, so a load of more than 350mA will cause the current limiter to step in.

Hope this helps,

Chris

I have a related problem. If I connect an ordinary red LED in series with a 150 ohm resistor and connect it between VOUT and P5 like this :

then I can turn it in and off by configuring P5 as an output and toggling it. When P5 is low the LED lights, when high it turns on, although it does seem a little dull. I assumed that the voltage drop across the LED would be around 2V and was aiming for a current of 10 mA. If VOUT is 3.3V then there should be 1.3V across the resistor and so I should get 8.67 mA with a 150 ohm resistor.

However, when I tried it, what I got was 1.00 V across the resistor, 1.95V across the LED and a current of 5.97 mA. This doesn't make sense. If I have 1V across a 150 ohm resistor then I should have a current of 6.67 mA. More worrying is the fact that the VOUT voltage has dropped from 3.3V to 2.95V. How can this be? The regulator (IC4) should be able to cope with 10 mA.

Or can it? On further investigation I discovered that the voltage in the VU pin (USB supply, pin 39, should be 5V) is actually only 4.81V. This voltage is fed to the input of the regulator (IC4) via a BAT60A diode (D1) and I reckon if the mbed is drawing 200 mA (worst case, I am using the Ethernet hardware) then I could get a voltage drop of between 0.2V and 0.3V, which means the regulator is only being supplied with 4.51V. Since the regulator has a dropout voltage of around 1V this should still be enough, although the electrical characteristics on the regulator's datasheet all use test voltages high than that I'm not entirely sure.

Has anyone else had similar problems?

Would I be better off using an external 5V supply connected across pins 1 (GND) and 2 (VIN)?

Has anyone else noticed such a low USB voltage on their PC?

Hi Richard,

Some USB ports are "unpowered", but most PCs will have at minimum one port that is powered and can supply 500mA fine. Not sure if this has anything to do with your case, but may be worth checking.

Simon

Here's an update.

I decided to power the mbed from pins 1(GND) and 2(VIN) using 4 AA batteries in a holder. Now the 3.3V supply holds up OK, so I reckon the 3.3V regulator just wasn't getting enough headroom from my USB supply of 4.81V.

Now I measure 3.29V at VOUT, 2.30V between the resistor and the LED and 0.34V at the digital output (P5) and a current of 5.71 mA flows.

Should the voltage at P5 be so high? The micro datasheet (p45) quotes a maximum low output voltage of 0.4V when sinking 4mA, but I am sinking more than that so it may be OK.

Also, the mbed is drawing 220 mA from the batteries, with the LED turned on, but as I say I am using ethernet. So if you are using ethernet, and powering from the USB supply, then this means you're probably only going to get 230 mA from the VOUT pin before the current limiter kicks in.

According to Wikipedia, the USB supply voltage is permitted to be between 4.75V and 5.25V and USB 2 "low powered hub ports" are allowed to go as low as 4.4V so the 4.81V I am getting is actually within spec.

You might have a look at this app note from Maxtor that discusses USB supply voltage limits. http://www.maxim-ic.com/app-notes/index.mvp/id/3241

They point out that after allowing for worst-case tolerances and for real-world cable resistance, "the minimum available voltage from a USB host or powered hub at the peripheral end of the cable is 4.5V".

So your 4.81v is actually within the expected range.

For your LED, you could just take a page from the design of the mbed itself: connect the LED to ground instead of +3.3, then drive it through a 220 ohm resistor. That is what is done with LED1- LED4, anyway.