View Full Version : Vdd current maximum
02-18-2005, 12:43 AM
I have·a BS2 which is·controlling a mostly TTL circuit.· The circuit, so far, consists of a 555, a 74191 (binary counter),·an AND gate (7408), an LM335 temperature sensor,·an ADC0831 (analog to digital converter), and a 2x8 parallel LCD display.· Each of these peripheral devices is being supplied by the Vdd pin.· So far, I am drawing·around 80mA.· I·still have a few more devices to include.· I expect to at least double my current draw from Vdd, let's just assume 250mA to be safe.· Does anyone know·the recommended current limit for·Vdd?· Eventually, I will be using two supplies, a 5V supply for the peripherals and a 9V battery for the stamp.· But until then, I want to make sure I don't overload the chip.·
02-18-2005, 01:08 AM
You are at the limit of the current specification on Vdd.· 80mA is max for Vdd.· You'll need to add a separate power supply with a common ground.· You could damage the stamp if you go over the 80mA.
02-18-2005, 01:17 AM
· Thank you very much.
02-18-2005, 01:46 AM
·· I just wanted to confirm that you are not using a carrier board with it's own regulator, correct?· You are running the stamp on a breadboard or something?
02-18-2005, 01:47 AM
Do you mean 80mA out of Vdd, or sourced through the BS2's pins? Vdd is provided by the regulator, not the PIC, correct? The spec sheet for the LM2940 regulator states that it can provide up to 1 Amp given an ample power supply:
Not only that, but I have a project that drains close to 200mA off Vdd when it's going "full bore". What gives?
02-18-2005, 01:50 AM
·· That's why I am trying to confirm if the OP is using a carrier board.· Those ratings are for the regulator on a carrier board, such as the BOE.· If the OP is running circuitry right off the BS2's Vdd pin, then he'll be limited.
02-18-2005, 02:03 AM
That is correct for the through hole version of the regulator, but the surface mount version on the BS2 is limited to about 100mA and we recommend 80mA for a safety net.· I can't tell you how many people have damaged their stamps due to over voltage on the Vdd pin.
02-18-2005, 02:17 AM
1. The "normal" use of Vdd on the BS2 is as a place to connect +5 volts, supplied from an external regulator.
2. The "other" use of Vdd on the BS2 is when you don't HAVE a regulator, but just hook 5.5 to 16 volts directly to Vin (NOT to Vdd!). The Vdd pin then CAN be used to supply current to your circuit, using the on-BS2 regulator. HOWEVER, the on-BS2 regulator is very small, and very current limited. 100 mA is the current limit for this regulator, and that includes the BS2 as well as any external circuity. Parallax (from above) recommends 80 mA MAX.
3. IF you put more than 5.5 volts on Vdd (as I just told you NOT to do) THEN you will destroy the BS2. That is the 'over-voltage' comment from above. I don't know what happens if you try to pull more than 100 mA out of your regulator on the BS2 -- possibly it goes into thermal shutdown, possibly it burns up.
02-18-2005, 02:18 AM
Now, to Agentile: You might try to use "HCT" circuits, instead of plain TTL. HCT draws MUCH less current for the same function.
Having said that :· You have a 555, AND an LCD display, and you expect that poor little BS2 regulator to drive all that?· Man, spend the $1.00 and get a 7805 with a couple of capacitors.· Save the BS2 for things worth $50.00.
02-18-2005, 04:25 AM
Thank you all for your help.· I will be using a secondary power supply to drive the peripheral circuitry, so the Vdd pin will have the day off.
02-19-2005, 02:01 AM