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[SOLVED] varying 5V to 3.3V logic level conversion

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JKJoy

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Hello,

I have a level converter which converts 5V to 3.3V using resistor divider.
But I need a precise 3.3V from 5V system, where the 5V supply may vary from 4.7V to 5.2V. I dont have any 3.3V supply (i had used TI's TXB series for 5V to 3.3V for another design). I tried with zener diode, but there was no accuracy.
Could somebody share any ideas please.
Thank you
 

You can use one of many available voltage regulator ICs. Try Mouse or Jameco or Digi-key for model numbers.
Zeners can be used but they are worse regulators than such ICs.
On the other hand, logic 5V and 3.3 V levels offer a window within which the systems work, so no tight regulation is often necessary.
Where your DC source generates lower than required voltage (like 4.7 V in a 5V system), you may need a DC/DC converter to get >5 V, then regulate.

- - - Updated - - -

In uncertain situations I would use comparators to get the nominal logic output from lower input. No regulated DC is needed for a comparator.
 

Hi,
In uncertain situations I would use comparators to get the nominal logic output from lower input. No regulated DC is needed for a comparator.
Click to expand...

true...
addditionally: to _generate_ precise output levels you may consider using analog swithces/multiplexers.


Klaus
 

Thanks for the Ideas,
jiripolivka said:
You can use one of many available voltage regulator ICs. Try Mouse or Jameco or Digi-key for model numbers.
Click to expand...

But can I use these voltage regulator ICs for high speed switching such as 2400bps or higher speed communication?


In uncertain situations I would use comparators to get the nominal logic output from lower input. No regulated DC is needed for a comparator.[/QUOTE said:
Since I dont have 3.3V supply, i will need to add another resistor divider at the output of comparator to make 3.3V from 5V, right?
Click to expand...
 

Hi,

Since I dont have 3.3V supply,
Click to expand...
The usual way is to use the suggested voltage regulators as power supply. Then you have a 3.3V supply.
You should not switch the voltage regulators, but supply comparators/switches or other circuits with the regulators output.

And then 2400bps is not high speed.

Klaus
 

KlausST said:
Hi,


The usual way is to use the suggested voltage regulators as power supply. Then you have a 3.3V supply.


Klaus
Click to expand...

Thanks. But I am trying to have as little parts as possible to make my design compact and small. That is why I am looking for a solution without adding another regulator.
 

Hi,

You can have a voltage regulator in SOT23, which is about the size of two 0805 resistors.
or even smaller: LP2985 in BGA is about 1.11 x 0.93 mm

You need a precise 3.3V. With voltage divider it is not possible because 5V is not that precise.
Even a zener is not that precise (although it needs about the same area than the small LDO).


For an LDO:
Usually you need an extra cap. But overall i think this is the better, precise and reliable solution.

Klaus
 
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    JKJoy

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KlausST said:
Hi,


For an LDO:
Usually you need an extra cap. But overall i think this is the better, precise and reliable solution.

Klaus
Click to expand...

Thanks Klaus. I need to purchase small package 3.3V LDO and also planning to translate 5V(not precise) to 3.3V with SN74LV1T34.
Thanks
 

Hi,

SN74LV1T34 seems to be a very specific device.

Any 74AHC1Gxx od 74LVC1Gxx will do. They are able to work with 3V3 supply while 5V input tolerant.
xx is for a lot of devices: AND, OR, BUFFER....



Klaus
 
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    JKJoy

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KlausST said:
Hi,
Any 74AHC1Gxx od 74LVC1Gxx will do. They are able to work with 3V3 supply while 5V input tolerant.
Klaus
Click to expand...

Thanks. I hope I can use this SN74AHC1G125.
 

Hi,

yes, SN74AHC1G125 is OK.


Klaus
 

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