voltage divider and current for 23k256

Hello everyone,

I am building a small circuit with an extra SRAM, for the arduino.
I am using two 23k256 and a TXB0108 logic level shifter. I am thinking of powering the chips simply by voltage divider from 5 volts, the whole circuit is using.
I wonder how much current should I leave for the chips?
I went through data sheets of both chips, and all I could find that 23k256 needs 1mA and TXB0108, barelly uses any current - 1uA. So in total I understand I need 3mA, but should I leave a bit more? as using standart 330/680 resistors I could supply 10mA, or would it be way to much and would fry the chips.
Any comments welcome.

Cheers

I am thinking of powering the chips simply by voltage divider from 5 volts,

Stop thinking that, and use a regulator or DC-DC converter.

do you know any voltage regulators, which could output 5V and 3V3 from one chip? I could not find any and I am trying to save space on PCB, so though of using the voltage divider.

smt v.regs are pretty small, especially since you don't need a lot
of current.

so though of using the voltage divider.

Please, you really need to stop thinking like that.
It won't work.

AWOL:
Please, you really need to stop thinking like that.
It won't work.

You should explain why :slight_smile:

I don't need to, Mr Ohm passed a law. 8)

Oh right, so a voltage divider would also divide the current, that's it?

No, just think about the impedance of the output of the divider.

hi,

i know, that you can't use voltage divider for low impendance circuit, because the current will just go to ground throught the circuit.
but is this the case? (just for learning purposes) :slight_smile:

thanx anyway, will use voltage regulators.

I am thinking of powering the chips simply by voltage divider from 5 volts,

A few possibilities:

  1. See if you can power the chip at 5v. They may have maximum Vcc at 5v. Or they may be able to operate at 5v - you will need to see if you can take the changes.
  2. If you have to power it at a lower voltage, see if you can drop the 5v easier. Two silicon diodes in serial, plus a resistor to ground will provide about 1.4v drop; or a resistor to an LED (white LEDs for example are around 3v). or a zener.
    ...

At the low currents your chips need, a simple series resistor and a 3.3 V zener diode is all you need.

mauried:
At the low currents your chips need, a simple series resistor and a 3.3 V zener diode is all you need.

thanks for that. Zener diode sounds like a good option.

How much current should I pass throught the resistor for 3mA requirement? 5mA? 10mA to be safe?

Also, just to make sure, am I right with the resistor calculation:

R = (Vin - Vout) / Iout

Thanks

EDIT: well, small 50mA voltage regulator is half the price of the zener diode. will use the the voltage regulator.

Thanks everyone for the help. thread closed :slight_smile:

I've used a simple RED LED in cases like this. It will reduce 5V to something a 3.3V chip will be happy with. I also use a 680 ohm resistor to ensure the LED lights up. That way I get my pilot light (power on) too.

LEDREG.jpg

For this case a zener diode would work well.
Have a look here:
http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm

It provides a tool to calculate the required resistor value.

BillO:
I've used a simple RED LED in cases like this. It will reduce 5V to something a 3.3V chip will be happy with. I also use a 680 ohm resistor to ensure the LED lights up. That way I get my pilot light (power on) too.

Nice bit of thinking outside the box, provided you don't overload the LED by pulling too much current through it.

pluggy:
Nice bit of thinking outside the box, provided you don't overload the LED by pulling too much current through it.

Thanks. Yes, for tiny loads (IL < ~15ma), but with all the great lower power stuff these days, that can be quite a lot.

BillO:
I've used a simple RED LED in cases like this. It will reduce 5V to something a 3.3V chip will be happy with. I also use a 680 ohm resistor to ensure the LED lights up. That way I get my pilot light (power on) too.

thats a very good tip. thank you.
saved me some space on tiny pcb and three components less (no v regulator circuit + led works as power on indicator)
:slight_smile:

there should be a book with tips like these :slight_smile: