5v to 3.3v conversion???

[picforall007]

I have a ARM controller whose supply voltage is 3.6v, but i don't have any 3.3v regulator IC. My question is can I use two diodes in series with a 5v regulated output to reduce the voltage into 3.8v (5-0.6x2 = 3.8v).


Thanks.

 

[john blue]

Sure. Why not..

 

[betwixt]

That's assuming the ARM is happy with 3.8V, some devices I use are designed for 3.3V but have an absolute maximum specified at 3.6V. If you use diodes and there is any risk of the current consumption dropping very low (I suggest 1mA is a good threshold), connect a leak resistor across the 3.8V to establish a minimum current through the diodes.
Brian.

 

[picforall007]

thanks.
Can i use 1k as leak resistor (ie., 3.8mA minimum current through diodes)

 

[keith1200rs]

Just use a regulator. It is cheaper than blowing up processors by trying to use diodes. If you don't need much current then there are tiny devices available.

Keith.

 

[millwood]

My question is can I use two diodes...

My question is "have you read the @#$@# datasheet?" Maybe the people who designed your chips can help you on this question?

 

[picforall007]

Just use a regulator. It is cheaper than blowing up processors by trying to use diodes. If you don't need much current then there are tiny devices available.

Keith.

Actually I need both 5v and 3.8v in the single board thats why I decided to use only one regulator instead of using two.
I saw an article about 3v supplies (page 12)
https://www.newark.com/pdfs/techarticles/microchip/3_3vto5vAnalogTipsnTricksBrchr.pdf

Thanks.

 

[keith1200rs]

If you are dropping say 6V down to something that can tolerate say 3V to 5.5V then yes, that may be an option. If you are using a device specified at 3.3V +/-10% it isn't.

Keith.

 

[millwood]

the problem with diode- and resistor-based approaches is that their voltage drop fluctuates with the current going through the diode / resistor. so when your mcu is consuming very little current (typically a few ma but it varies), you drop very little voltage on the diodes and the Vdd on your mcu goes above the absolute maximum -> here goes your chip.

or if your chip drops lots of current (like lighting up a few LEDs, and such current can be over 100ma), the Vdd drops so low that a logic high output from the mcu can appear to be logic low for the next device.

Again, the only definitive way to answer your question is to check the datasheet.

 

[picforall007]

thanks millwood and keith1200rs

If you are dropping say 6V down to something that can tolerate say 3V to 5.5V then yes, that may be an option. If you are using a device specified at 3.3V +/-10% it isn't.

Keith.

But then why company like microchip supporting this type of supplies.

The operating voltage is 2.3V to 3.8v.

 

[keith1200rs]

If your ARM chip will work from 2.3V to 3.8V then you may be OK if you target the typical values to be in the middle of that range, say 3V. A lot of the Microchip devices have a wide operating range. Some don't - only 10%. Personally I feel regulators are so cheap that it isn't worth using 3 diodes and a resistor instead of a single regulator, particularly when you take placement cost in to account.

Keith.

 

[picforall007]

Thank you sir.
I am trying to stick with this method because I can't replace diodes from my PCB.

 

[srikanthsamaga]

If your ARM chip will work from 2.3V to 3.8V then you may be OK if you target the typical values to be in the middle of that range, say 3V. A lot of the Microchip devices have a wide operating range. Some don't - only 10%. Personally I feel regulators are so cheap that it isn't worth using 3 diodes and a resistor instead of a single regulator, particularly when you take placement cost in to account.

Keith.

Can you name some regulators which is cheaper and easily available in retail.

 

[keith1200rs]

It is difficult to give specific part numbers without a specification, but have a look at Maxim and Micrel. MAX8511 is fixed 3.3V output at 120mA in SC70 package. If you have the space then the 78xx or 78Lxx series regulators are usually very cheap.

Keith.

Also, look at some of the National Semiconductor ones - LP2980, LP2982

 

[millwood]

But then why company like microchip supporting this type of supplies.

1. they don't know better?
2. the parts are different?
3. the designs are different?
4. the applications are different?
.....

just because someone is doing it doesn't mean you should do it.

what you should really do is to take a look at the @#$# datasheet and see, most definitively, under what voltage your parts can live and look at your design and how that fits with your supply voltage choices before settling on an approach.

 

[srikanthsamaga]

It is difficult to give specific part numbers without a specification, but have a look at Maxim and Micrel. MAX8511 is fixed 3.3V output at 120mA in SC70 package. If you have the space then the 78xx or 78Lxx series regulators are usually very cheap.

Keith.

Also, look at some of the National Semiconductor ones - LP2980, LP2982

Thanks,
But in my location 78xx series starts with 7805, 7803 is hardly available.
Thanks anyways, I just asked casually, now I dont have any use of 3.3v since i am working with DIP 8 bit MCUs which works at 5v.