Power Supply value problem 18f46k20

bolubeyi · Joined: 27 Apr 2012 Posts: 15 Location: TURKEY

Hi,i have big problem because pic power supply value is bigger than write on datasheet.I use pic 18f46k20 and write on datasheet “Operating Voltage Range: 1.8V to 3.6V” but I made the big mistake and my pcb power supply is 5v(i dont have to change the pcb).i connect 5v to 18f46k20 and processor is running now but later will be any problem ?and my adc ,pwm How many volts makes/read?or any advice?

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

The absolute maximum rating for the 18F46K20 4.5V

Microchip includes this note in the data sheet

[NOTICE: Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the
device. This is a stress rating only and functional operation of the device at those or any other conditions above those
indicated in the operation listings of this specification is not implied. Exposure to maximum rating conditions for
extended periods may affect device reliability.]

So, what do you think is going to happen, if you continue to run at 5V?

Mike

Ttelmah · Joined: 11 Mar 2010 Posts: 19496

Drop the voltage.
Just cut the power line where it comes into the board, and add a two or three diodes in series. Will give about 1.2 to 1.8v drop.
The data sheet gives the _absolute maximum_ the chip can handle as 4.5v, with max recommended as 3.6. This is to work over the full range of other specs (temperature, load etc..). It may well work for a few days at 5v, but will probably fail when the weather gets hot, or you try to put reasonable loads on the pins.
There are 5v chips that are pretty near equivalent. If you can't drop the voltage, change to one of these.
It _will_ fail sometime.

Best Wishes

temtronic · Posted: Fri Feb 01, 2013 8:41 am

or..you can use a red LED instead of 3 1N4001s, providing total current drawn is less than 30ma.
The red LED drops the voltage by about 1.6V and provides a 'free' power on indicator.

hth
jay

dbotkin · Joined: 08 Sep 2003 Posts: 197 Location: Omaha NE USA

Or... if you can get by without the extra features... use an 18F4620.

Too bad you're in Turkey. I have a batch of 18F46K20s I bought by mistake... and will never use. I'd love to sell them to someone who needs them, but shipping and taxes would make them pretty expensive for you.

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

if you use any of these voltage dropping methods BE SURE to place a capacitor of at least 10uf between the PIC Vss Vdd pins to keep the PSU "Z" low , and insure stability.

A L S O .......

if you have code that uses Vdd as an ADC reference, for example, this approach to powering the pic will not work , as the forward drop across ANY of the diode types mentioned will vary with net pic current demand.

if you need to fix it right , use an LDO 3.3 volt regulator