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Level Shifting 5V->3.3V

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bmcculla

New Member
I have a 5V MCU that needs to be connected to a 3.3V Bluetooth device through a UART (115kbps+). The Bluetooth device isn't 5V tolerant. Is there a good way to shift the 5V signal to a 3.3V signal? I need a solution that will be as small as possible though preferably still hand solderable (i.e no BGA or CSP packages). I need the 5V supply to sample 5V Accelerometers so I can't just run the MCU on 3.3V.

Also, is there any reason not to connect 3.3V outputs to 5V inputs? The high and low switching voltages will work together.

The MCU is a Cypress PSoC CY8C29 and the bluetooth module is a ConnectBlue OEMSPA13i.

Thanks
 

bmcculla

New Member
I was hoping for something a bit more integrated. It turns out I'm going to need a couple more lines so I'd like to avoid discrete resistors if possible.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
bmcculla said:
I was hoping for something a bit more integrated. It turns out I'm going to need a couple more lines so I'd like to avoid discrete resistors if possible.

I don't see how it could get much smaller (and cheaper!) than just resistors?. If size is that important, a chip isn't going to fit anyway, and you could always use surface-mount resistors!.
 

Roff

Well-Known Member
I Googled "5v 3.3v translation" and got hits for TI and IDT, and possibly more.
 

John Sorensen

New Member
There's lots of ways to skin that cat. You're going to have to weigh the advantages and disadvantages of each yourself. A resistor divider may work up to certain speeds. As for me, I like the 'HC4050.

j.
 

bmcculla

New Member
Thanks John, the HC4050 looks just right.

Nigel - I'm already using 0603 resistors and I'm about at the limit of my eyesight :wink: .
 

laroche73

New Member
5V <=> 3V

Nigel - I'm already using 0603 resistors and I'm about at the limit of my eyesight .
I'm using 0402's and 0201's (a 0.1uF cap in a 0201 footprint is an impressive bit of engineering and hard to find on the floor) and need a skilled technician along with a good microscope. Conductive epoxy and solder paste are useful at those sizes.

Another point in this thread might be whether the interface needs to be bi-directional. TI's CBT series include buffers that work as bi-directional level translators, as long as a silicon diode (~ 0.7 Fvd) is inserted in series with the 5V supply. I remember seeing a few that had the diode built-in (check Digi-key). It's a simple idea that was pioneered by companies like Quality Semiconductor (they put out a very nice app. notebook on the subject).

Each gate is a low-on resistance FET that can't drive any significant current beyond ~ one diode drop below Vcc, so inserting another diode in series limits the maximum Vout to around 3.6 V with a 5 V supply. It works in both directions, 5 V logic outputs are current-limited, and 3.3 V CMOS logic outputs satisfy the TTL minimum for a logic high.
 

bmcculla

New Member
A couple design revs later i just discovered that I will need a bidirectional line - I think this forum just set a new benchmark for quick responces: questions answered before you post them. I'll be sure to check out that TI part.

I'll also give some smaller resistors and caps a try. Do you have any advice on technique for soldering them?
 

Optikon

New Member
Re: 5V <=> 3V

laroche73 said:
Nigel - I'm already using 0603 resistors and I'm about at the limit of my eyesight .
I'm using 0402's and 0201's (a 0.1uF cap in a 0201 footprint is an impressive bit of engineering and hard to find on the floor) and need a skilled technician along with a good microscope. Conductive epoxy and solder paste are useful at those sizes.
.

Yes, laroche, I'd be interested to hear more about your experience with hand working 402's & 201's. What kind of other techniques and suggestions do you have on these?
 
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