A very basic IC supply question , I'm sure; but from small acorns...

[waggy55]

A basic IC supply question, but help would be much appreciated

Hi,

This is my first post, so I hope it's at least in the correct forum!

I could really do with some help, on a very basic question, as most of my electronics knowledge seems to have escaped me!

I am trying to supply the IC, which is a PAL16R8A (pictured) with 4V and 2mA.

Box 1 shows my first attempt. I used a current limiting resistor to limit the current to 2mA, which it does. However, the voltage drops considerably to 1.2V.

My second attempt, box 2, used a voltage divider (box 3) in a (vain) attempt to negate the resistance of the IC (which I assumed was causing the voltage drop to 1.2V, as without the IC the voltage is 4V (box 3). However, the IC was still only supplied with 1.2V.

If I decrease the 2K5 ohm resistance, the current will increase and the limit for the IC 5mA. So I'm stuck....there's no real help using the google search engine, and the electronics book I've ordered is a few days away yet!

Would anybody be able to give me some advice as to how I may supply the IC with 4V and no more than 5mA. Any help would be really (really) appreciated!

Thanks in advance!

 

[Norlin]

I don't think you really need to worry about current limiting. Just supply it with 4V and it'll draw the amount of current it needs. I think that the PAL16R8A is meant to run at 5V?

Oh and if you want to provide 4V from your 5V supply, just put a diode in there with a 1V drop.

 

[Vizier87]

Hi Waggy, welcome to the forum.
I've looked at the **broken link removed**, and the max supply is 7V. You don't need to drop the 5V.
Here's an extract:
**broken link removed**

 

[waggy55]

thank you both for your really quick replies!

I think my problems have stemmed from not really understanding the datasheet notation, (I was looking at a more complicated one, so thanks for that link) and my uni degree set me up with the theories, but I'm left me a bit high and dry when it comes to actually designing a circuit and reading datasheets!

I'll forget about limiting the current at the Vcc pin then (I was looking at a datasheet and mistaking Max DC Input Current for the Vcc I/P, rather than the I/P pins).

One quick question though, I was always under the impression that I shouldn't really supply an IC straight from Vcc, and there should be some sort of resistor between the chip and the Vcc rail - is that not the case then?

Thanks again for your help!

 

[mramos1]

From the posted sheet and Note 1, just run it at 5VDC. You should be fine.

 

[Norlin]

As long as the 5V is regulated, you should be fine

 

[Vizier87]

One quick question though, I was always under the impression that I shouldn't really supply an IC straight from Vcc, and there should be some sort of resistor between the chip and the Vcc rail - is that not the case then?
Thanks again for your help!

Nope, no need for resistors. The chip draws current as much as it needs. That is 'coz the regulator's doing the job limiting some parameters which renders most 5V electronics operable with it.

 

[waggy55]

Ah ok. Thanks a lot for your help, guys! Really appreciate it!