R
Ricky
Guest
On Monday, April 10, 2023 at 12:33:08â¯PM UTC-4, piglet wrote:
The spec on most chips is Vdd+0.3V abs max. The 4.2V is at 6 mA.
1 mA would be fine, if I believed that\'s the number it would be. What point will the RC delay be significant? If this part actually is only passing 1 mA, it isn\'t doing much regulating of the current. A small change in either power supply voltage and the current will easily double or triple, or more.
Every approach I looked at has issues like this, which make it a bit awkward, or even potentially unreliable. I know people talk about injecting current into the IO pins and into the power supplies this way, but a bit too much current and it goes into latchup... fatally.
There\'s just not enough utility in the single resistor approach to justify using it. The alternative is a single IC (a very small one) for the two lines. I started out looking for a two channel version of the 10 bit chip I had been using. It\'s not an easy thing to look for on Digikey or Mouser. This voltage conversion switches don\'t have a category of their own. So I got frustrated and started looking for a short cut. Not finding any of those that I liked, I searched for a digital buffer type part and they are small, cheap and plentiful. That\'s the route I will be going. The only thing that bugs me is adding another line item to the BOM, but the 200 ohm resistor does that as well.
Thanks for your suggestions.
--
Rick C.
---- Get 1,000 miles of free Supercharging
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On 10/04/2023 3:32 pm, Ricky wrote:
On Monday, April 10, 2023 at 8:03:54â¯AM UTC-4, piglet wrote:
On 09/04/2023 15:44, John Larkin wrote:
On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichp...@hotmail.com
wrote:
On 09/04/2023 02:22, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.
So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.
Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.
I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.
Any thoughts?
If the 3.3V input has a 5V tolerant ESD structure then of course no
level shift is needed. If the 3.3V supply rail is stiff enough to accept
a few mA injection then a series resistor might be all you need. For
example 200 ohms would limit ESD diode current to 5mA and assuming 10pF
input capacity would only slowdown 2ns,
piglet
200 ohms was my suggestion!
Great minds think alike!
Too bad they come up with the wrong answer. What will be the current when the TTL output is high? Does anyone consider the requirements?
Parameter TEST CONDITIONS MIN TYP(1) MAX UNIT
VOH High-level output voltage VID = 200 mV, IOH = â6 mA 3.8 4.2 V
We\'ll see if that makes it through the usenet filter. Likely not.
I don\'t get your point? If the TTL Voh is 4.2V then the current in 200R
would be 1mA (assuming 0.7V drop for the input clamp diode to 3.3Vdd)
If even 1mA is too much then one could increase the resistor until RC
delay becomes significant.
The spec on most chips is Vdd+0.3V abs max. The 4.2V is at 6 mA.
1 mA would be fine, if I believed that\'s the number it would be. What point will the RC delay be significant? If this part actually is only passing 1 mA, it isn\'t doing much regulating of the current. A small change in either power supply voltage and the current will easily double or triple, or more.
Every approach I looked at has issues like this, which make it a bit awkward, or even potentially unreliable. I know people talk about injecting current into the IO pins and into the power supplies this way, but a bit too much current and it goes into latchup... fatally.
There\'s just not enough utility in the single resistor approach to justify using it. The alternative is a single IC (a very small one) for the two lines. I started out looking for a two channel version of the 10 bit chip I had been using. It\'s not an easy thing to look for on Digikey or Mouser. This voltage conversion switches don\'t have a category of their own. So I got frustrated and started looking for a short cut. Not finding any of those that I liked, I searched for a digital buffer type part and they are small, cheap and plentiful. That\'s the route I will be going. The only thing that bugs me is adding another line item to the BOM, but the 200 ohm resistor does that as well.
Thanks for your suggestions.
--
Rick C.
---- Get 1,000 miles of free Supercharging
---- Tesla referral code - https://ts.la/richard11209