Author Topic: Supplying MAX485 with 5.5V  (Read 939 times)

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Offline SharkTopic starter

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Supplying MAX485 with 5.5V
« on: June 23, 2019, 06:52:18 am »
Hi all,

I have a question regarding the MAX485 chip.
I found myself in a situation where I have a PCB board finished, with bad luck of supplying 5.5V to a MAX485.

Why bad luck? Because it's supposed to get 5.0V +/- 5% ==> 4.75-5.25V.
Although when I look at the absolute maximum ratings, it says Vcc max 12V.

Hence I am confused :) Which of the two is it?
Please help me with interpreting this properly and if I should be worried about the 5.5V supply long-term.
I'm also attaching three cut-outs from the manufacturer's datasheet.

Thanks!
 

Online Siwastaja

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Re: Supplying MAX485 with 5.5V
« Reply #1 on: June 23, 2019, 09:10:43 am »
Manufacturers need to test and specify all the gazillion parameters in certain conditions; otherwise they would be all complex, multidimensional curves. This is tested at 5V.

With no "recommended Vcc" section, you need to guess how much leeway you need to add to the 12V abs.max rating. You can read between the lines (from the pinout listing) that the recommended range seems 4.75..5.25V.

With no data about the other parameters at anything else but 4.75..5.25V, you need to guess how much the values drift when you run it outside this range.

It's very unlikely it would blow up due to overvoltage, you have a massive margin before the abs.max 12V (many digital ICs intended for 5V operation have absolute maximum between 6 and 7V). But they guarantee absolutely nothing about the functional parameters such as current consumption, timing...
« Last Edit: June 23, 2019, 09:12:36 am by Siwastaja »
 
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Offline David Hess

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Re: Supplying MAX485 with 5.5V
« Reply #2 on: June 23, 2019, 07:32:45 pm »
Because of the high specified absolute maximum voltage, the MAX485 will work fine on 5.5 or even 6 volts.  The output levels will be slightly higher but that will not matter for an RS-422/RS-485 application.  The inputs are designed to be TTL compatible and should remain so.

It's very unlikely it would blow up due to overvoltage, you have a massive margin before the abs.max 12V (many digital ICs intended for 5V operation have absolute maximum between 6 and 7V). But they guarantee absolutely nothing about the functional parameters such as current consumption, timing...

5 volts was a good "high voltage" standard for bipolar logic because it comfortably fits within the 5 volt reverse breakdown voltage of common bipolar transistor base-emitter junctions.  A higher supply voltage would have required a slower process or extra design considerations which are both present in higher voltage processes designed for analog operation.

There were some earlier higher voltage bipolar logic families though.
 
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Offline SharkTopic starter

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Re: Supplying MAX485 with 5.5V
« Reply #3 on: June 30, 2019, 11:10:38 am »
Thanks to both for answers :)
As I do not require the chip to meet any low power requirements, or that any levels/timings are strictly met, I will keep it as is, and improve the PCB for future iterations.
Cheers!
 


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