Raspberry Pi Forums

Hi,
The 5V on BCM seems to be marked as Bat 5V, this lets think that 5V may be not 'regulated'.
Then my question is on 'A' model (without 5V USB/Ethernet chip) what is the 5V voltage tolerance for the BCM ?

Thanks

I thought that 5v didn't get on the chips, only 1.8v or 3.3v after the regulator.

Gordon77

The 5V on BCM seems to be marked as Bat 5V, this lets think that 5V may be not 'regulated'.

The pins on the schematic symbol are just labeled VDD_BAT.

I don't think the foundation have released an official postion on voltages acceptable on that pin of the chip. My gut feeling is that the chip was probablly designed to run off a single cell lithium ion battery and so 5V is likely near the upper end of the acceptable range but lower voltages are likely to be fine.

Then my question is on 'A' model (without 5V USB/Ethernet chip) what is the 5V voltage tolerance for the BCM ?

The USB hub with ethernet chip runs off the 3.3V rail as do various IO voltages. I suspect that due to the lack of that chip and the consequent lower load on the 3.3V regulator that the A will be more tolerant of undervoltage than the B but how much more tolerant I don't know (and it will probablly vary between individual devices).

I thought that 5v didn't get on the chips, only 1.8v or 3.3v after the regulator.

You thought wrong. See page 1 of the schematic.

D'oh, l missed that

It's really not the processor you need to worry about. That's powered by a 3,3 V regulator. The USB ports need 5V. Devices connected to it function best between 4,85 and 5,15 V.

The HDMI port has a 5V pass through as well.

cyrano wrote:It's really not the processor you need to worry about. That's powered by a 3,3 V regulator. The USB ports need 5V. Devices connected to it function best between 4,85 and 5,15 V.

The CPU also need 5V. I think VDD_BAT is a internal voltage regulator.
Look at Page 1 in RPi schematic

I missed the 5v as well

You're right the label is VDD_BATx.
I think also that BCM is build to work with a single cell lithium ion battery.
With a 'A' model on an embedded system that don't need HDMI nor USB it would be interesting to know the power source limit on '5V' input.
There is two points that are concerned on '5V' input limits :

• VDD_BATx inputs: We know there are used to generate core and RAM voltage (an probably other internal voltage). But did it used to other things ?
• 3.3V regulator. The NCP1117 have a dropout of 0.9V to 1.2V depending of load. It would definitively a limitation on low '5V' voltage but a external 3V3 switching supply on GPIO port can be a workaround.

Then it would be great to have BCM2835 Min/Typ/Max (Not the Absolute Maximum) rating of this input to not damage circuit and to know we are in the normal usage of this supply.