One Circuit, Three Different Voltage Levels

[rsocor01]

Hi all,

I need some help here . I have a project with a PIC18F4550 that uses USB interface to communicate with a PC. Also, the PIC interfaces with a color GLCD and an SD card. I'm planning to use a rechargeable battery. I have three different voltage level requirements for this project. The voltage levels that I need are as follows

• 3.0V for the GLCD Vin and I/O ports, and also for the SD card.
• 5.0V for the USB interface.
• 12.0V for the GLCD backlighting. The backlight voltage feed uses a 220 Ohms resistor in series.

I would like to keep the circuit as simple as posible with a minimum number of components. That is why I would prefer not to use two voltage regulators, 3V and 5V.

What is the best way to do this? Can I run the PIC at 3.0V and use a level converter for the USB interface on D- and D+? How do I go up to 12.0V for the GLCD backlight? What chips can I use?

Thank you for your help.

Robert

[Archangel]

Hi Robert,
Several options available here, depending upon board real estate, cost, power consumption . . . Simple, power hungry way is a voltage divider down from a regulated 12v supply.

Or you could set up HPWM as a charge pump to get 12V from 5 v

Or probably the easiest, and cheapest is separate regulators, or hybrid with one of the above choices.

Using battery power, I think I would use switching regulators for each voltage, and only power them up as needed.

[mackrackit]

Might look into not needing the 5 volts if you are just sending data on the USB and not planning to power a device.

[aratti]

Can you replace the 220 Ohms resistor on the GLCD?

If yes, then you can experiment to turn on the backlight with 3 or 5 volts, if it works you have solved half of your problem.

Al.

[rsocor01]

Thank you all for your response.

mackrackit,

I'm not powering any devices with the USB host 5 volts, but I'm almost 100% sure that the signal comming in and out of D- and D+ on the PIC18F4550 needs to be 5V. That's why a voltage level converter is needed, but I don't know which chip is appropiate.

Aratti,

The GLCD backlight voltage must be around 9.3 Volts. But, the datasheet recomends to use 12V with a current limiting resistor of around 220 Ohms.

Joe S,

The circuit is going to be fed from a rechargeable battery, and a voltage divider will drain a battery very quickly. The battery that I will use is probably a 6.0V lithium battery. I read somewhere that there are some LED driver chips that give you 12V from a 5V or 3V voltage input. But, if this is true I don't what chips these are.

Again, thank you all for your ideas.

Robert

[mackrackit]

Check out figure 17-1 in the 4550 data sheet.
Just run the 4550 at 3.3 volts and save one regulator.

[rsocor01]

Check out figure 17-1 in the 4550 data sheet.
Just run the 4550 at 3.3 volts and save one regulator.

Nice! . So, the USB module works with 3.3V. Everything starts to make sence now. Thanks a lot. That saves me the use of an extra regulator and some voltage level converters.

Now, for the GLCD backlighting I'm planning to use something like this power booster DC/DC step-up converter. I just need to find the right chip for what I need. Has anybody used these DC/DC step-up converters before? Thanks again.

http://www.onsemi.com/pub_link/Collateral/CAT4238-D.PDF

Robert

[rsocor01]

mackrackit,

Thank you for guiding me in the right direction. I did some more research and found out that the internal USB regulator must be turned off. The capacitor at Vusb is not needed anymore and the internal USB transceiver and pull-up resistors can be used in the configuration I'm using. I just need to feed 3.3V at Vusb and Vdd.

So, I'm running my PIC18F4550 with Vdd=3.3V and the USB peripheral works! I was able to send and receive data from my computer to the PIC.

As for the GLCD backlight voltage, I have already ordered this chip and the required external components from mouser.com

http://www.onsemi.com/pub_link/Collateral/NCP5007-D.PDF

This chip is a backlight LED boost driver. The input voltage range is from 2.7 to 5.5 Volts, so it should work for my application.

Thank you all again for your help and ideas.

Robert

[Archangel]

mackrackit,

Thank you for guiding me in the right direction. I did some more research and found out that the internal USB regulator must be turned off. The capacitor at Vusb is not needed anymore and the internal USB transceiver and pull-up resistors can be used in the configuration I'm using. I just need to feed 3.3V at Vusb and Vdd.

So, I'm running my PIC18F4550 with Vdd=3.3V and the USB peripheral works! I was able to send and receive data from my computer to the PIC.

As for the GLCD backlight voltage, I have already ordered this chip and the required external components from mouser.com

http://www.onsemi.com/pub_link/Collateral/NCP5007-D.PDF

This chip is a backlight LED boost driver. The input voltage range is from 2.7 to 5.5 Volts, so it should work for my application.

Thank you all again for your help and ideas.

Robert

I think you only read the first line of my post, where I said simple POWER HUNGRY way. You can set a Hardware PWM on your pic and not need that chip you ordered from Mouser, I have used simple 12F chips to do that with pins to spare without using HPWM, so How much easier is using it ? Your money, spend it your way. Cost extra real estate on PCB too.

[rsocor01]

Joe,

Thank you for your comments. Yes, I read the part were you talk about HPWM in your post. That would work if I needed to obtain 3.3V from a PIC running with Vdd=5.0V. But in my case I have a 6.0V rechargeable battery and a 3.3V voltage regulator. Somehow, I need to obtain 12V to feed the GLCD backlight.

Please, take a look at the attached drawing. In the left lower part it says the power requirements for the backligh. It says "Backlight Vf=9.9V, 12.5 mA per side". However, the datasheet recommends to use 12V with a series current limiting resistor. If you don't feed 12V to the GLCD it will not work. Now, do you see why I had to buy a backlight LED boost driver? They cost about $0.70 a piece.


Robert