Converting 3V to 5V for driving a LCD with an ARM processor kit

[PS23]

Hello,

I am trying to drive a LCD with an ARM procesor kit. The output of ARM7 ports is 3V and the LCD requires 4.5-5V to operate. Can anyone tell me any device that could achieve a 3V to 5V converstion accurately. It could be a regulator since I might be using the LCD with a serial-to-parallel covertor.

Thanks in advance.

 

[BABESH]

Converting 3V to 5V

due,
look at 74LVC4245A 3.3V to 5V level shifter.

--babesh

 

[PS23]

Re: Converting 3V to 5V

Thanks, it helped...

 

[PS23]

Re: Converting 3V to 5V-- need help again

Hi,
I looked into the datasheet of 74LVC4245 and it seemed like the apt IC, but all of the vendors offer this product in SSOP packaging. Since I am developing my project on a general purpose PCB, it will be very difficult for me to solder an SSOP IC. Do you know any of the vendor that may be providing a standard DIP package or any other 3V-5V buffer? Could some optocouplers help?
Thanks..

 

[betwixt]

Re: Converting 3V to 5V

You can convert 3v to a higher voltage with this simple circuit if it is only for prototypes, you should really look at the earlier suggestion for production runs.

1. Use a small NPN transistor
2. Connect the emitter to the 3V circuit (the one you want to convert FROM)
3. connect the base to the 3V supply through a resistor of 10KΩ.
4. connect the collector to the 5V supply through another 10KΩ resistor.
5. take the signal for the LCD from the collector.

How it works:

The transistor base is held at 3V potential, if the input (emitter pin) is logic 1 there will be no voltage across the B-E junction and the transistor will not conduct. The collector will therefore be pulled to 5V by it's resistor. If the input is logic 0, current flows in the B-E junction and the transistor conducts. As it's emitter is low, the collector voltage will drop to (Ve +VCEsat) which should be low enough to bee seen as a logic 0 at the collector.

Crude but for testing purposes, it works!

Brian.

 

[PS23]

Re: Converting 3V to 5V

Thanks Brian,
I havent tried it yet, but I am pretty sure it'll work. Would you recommend any specific tansisitor or any vendor's would do?

 

[betwixt]

Re: Converting 3V to 5V

Almost any small signal transistor will work. The current flowing through it will be less than 2mA and the most voltage it has across any junction is 2V. I use PN2222 (T0-92 plastic packaged 2N2222) or BC108 types.

Brian.

 

[nancyn31]

Converting 3V to 5V

use modulation method

Added after 2 minutes:

use modulation method to get 5V apply 1kz signal at 300mV and carrier of 500hz same at 300mV.

Added after 54 seconds:

Almost any small signal transistor will work. The current flowing through it will be less than 2mA and the most voltage it has across any junction is 2V. I use PN2222 (T0-92 plastic packaged 2N2222) or BC108 types.

 

[PS23]

Re: Converting 3V to 5V

Thanks both of you,
Using the modulation method would make it pretty complicated. Thanks anyways nancy.

 

[ssankurathri]

Re: Converting 3V to 5V

hi

i have a small question on this. I know the transistor circuit is pretty good to work on, but my doubt is can we use a simple pull-up resistor to pull it up to 5V?

 

[PS23]

Re: Converting 3V to 5V

Hey,
We cannot use the pull-up resistors because the voltage source here would be the output ports of my ARM processor, i.e. at 3V. Pull-up resistors cannot pull the voltage up until they have an appropriate source connected to it. Ever heard of a resistor generating 2V by itself?

 

[ssankurathri]

Re: Converting 3V to 5V

hi
when we pull up the output port of ARM to 5V using a resistor, will it not solve the purpose?
if output is logic 0, it will send zero output.
if output is logic 1 (i.e., 3V on the output port), it will be pulled up to 5v.

if not pull up resistor, we can use a buffer to drive from 3.3v to 5V.

 

[betwixt]

Re: Converting 3V to 5V

It WILL work - trust me!

Even if your 5V is from a pull-up internal to the ARM it should work, it is the fact that the transistor is turned on or off from the 3V source that matters. In theory, you can pull any load to a low state using this method but you must remember that the driving circuit (the 3V one) carries the load current when it is in a low state.

For logic circuits this isn't usually a problem but if you try to convert 'heavy' loads it may be too much for the 3V driver to sink.

I think nancyn31 has been smoking 'herbs' - what nonsense!

Brian.

 

[ssankurathri]

Re: Converting 3V to 5V

hi bet,

i think the 3V driver should be taken care of if we use a pull-up resistor.
when the output is logic '0', the 3V ARM should be able to sink the current coming from the 5V through pull-up resistor. if we use high value resistor and make sure that the sinking current in to the ARM is minimum, then i don't see any reason why a pull-up resistor is not used.

correct me if am wrong.

 

[betwixt]

Re: Converting 3V to 5V

You are correct.

You can use pull-up resistors on the input or output or both. Obviously the input pull-up goes to the 3V supply and the output pull-up to the 5V supply. As long as the driving logic goes close to 0V for low and close to 3V for high, the output will switch correctly.

Brian.

 

[ssankurathri]

Re: Converting 3V to 5V

thanks betwixt.

i was confused when someone told that we shouldn't use pull-up resistor for this purpose. It may not be the robust way of doing, but it will serve the purpose.