separating a 3.3V chip from USB's 5V

[galed]

would this be a safe way to do it?

the leads that run off to the left go straight to I/Os on the chip. VCC is 3.3V

 

[Speakerguy]

Unfortunately no. You don't want any zeners on the D+ or D- lines. Those don't work at 5V, they have their own voltage standard. Keep the series resistors on them (and the one pull up), but eliminate anything else on those two lines.

For the 5V line, what you do with it depends in if you are using it for power or if you are using it to just detect the connection presence. Right now you have no current limiting and the zener will pull the bus voltage down and draw waaay too much current. Do you want to use the +5V usb line for power, or just to detect the connection of a USB cable?

 

[galed]

i guess the way i have it set up here it would be for detection only. I suppose i could get rid of that zener diode too and add a voltage regulator. I'd use the output of the regulator both for power and detecting a device

 

[Mike_2545]

Sparkfun makes a logic level converter for cheap...SparkFun Electronics - Logic Level Converter

 

[Speakerguy]

Mike, that's not a good solution here, it would be very bad in fact!

The D+ and D- lines are differential signaling lines that work at thresholds of 2.8 and .3V for high and low level signals (respectively). Any micro or peripheral with D+/D- input pins will be able to deal with whatever voltage shows up on them.

Galed,

A voltage regulator would be a good way for both detection and power. Just make sure it is an LDO type, as you only have 1.7V or so to deal with dropping from 5 to 3.3V.

 

[galed]

is there any issue with reverse loading a voltage converter? i.e. when there's no USB and it's running from another power source, there would be 3.3v applied to the output of the voltage regulator but nothing on the input.

Also, if using it for both power and detection, I need to isolate the pin on the micro from the rest of the output, right? Otherwise whenever there's VCC, the pin would see 3.3V and believe it was coming from USB even though it isn't... I see a switching-something in my future here but i'm not sure what

 

[Pommie]

Reverse loading a voltage regulator shouldn't cause any problems as it happens on any device with a large cap after the regulator.

May I ask which micro you are using?

Mike.

 

[galed]

the ATXMega256A3

 

[Speakerguy]

Reverse loading of a regulator is OK, so long as its Vin is floating and is not shorted to ground.

 

[galed]

would this work for both detection and power? VBUS is where the detection would be and VCC goes to, well, VCC...

edit: now with a mosfet. this should work right?

 

[galed]

bump

the circuit in the last post ok or will there be significant voltage drop across the mosfet?

 

[Vizier87]

There is a tip on getting 3.3 V from 5V from Microchip:

https://www.electro-tech-online.com/custompdfs/2009/12/3_3vto5vAnalogTipsnTricksBrchr.pdf

Go for it.