Author Topic: A quick question on interpreting Absolute maximum rating in datasheets  (Read 1170 times)

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Offline Cicada

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Hello

Look at the attached schematic Question.pdf.
I am driving a MAX3485 (power from 3.3V rail) with a output from an Atmel ATMEGA8535 (power from a 5V rail). NO Current limiting resistor in series! My mistake.

The circuit has being operational for a while now but I am concerned about long term stresses on the MAX3485 input.

The attached datasheet for the MAX3485 shows in the "Absolute maximum ratings" section that the device can be powered with a rail up to 7V with maximum input voltages on the DI pin of up to 7V. Usually the maximum input voltage on an input pin (such as DI) is specified as VCC+?V as can be seen in the datasheet for the SN65HVD178 attached.

Do the MAX3845 use a different protection mechanism on its input pins that permits a 7V input on its input pins even if it is power from a lower voltage rail? The SN65HVD178 datasheet is clear about the relationship between supply rail and maximum input voltage on a input with its VCC+0.3V spec. The datasheet for the  MAX3485 is certainly not. I think this might be a gap in the datasheet specifications.

Any comments are welcomed.

Cicada
 

Offline Andy Watson

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Quote
Features
 Interoperable with +5V Logic
I would interpret that as meaning the the inputs are, by design, 5 volt tolerant.
 
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Offline MosherIV

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Hi

Quote
The attached datasheet for the MAX3485 shows in the "Absolute maximum ratings" section that the device can be powered with a rail up to 7V with maximum input voltages on the DI pin of up to 7V. Usually the maximum input voltage on an input pin (such as DI) is specified as VCC+?V
Data sheets do use both ways to specify the absolute parameters for devices.

Quote
The circuit has being operational for a while now but I am concerned about long term stresses on the MAX3485 input.
To answer your implied question " am I ok driving the Max3485, with supply of 3.3V, with 5V" - Yes
The data sheet specifically states that the DI pins can with stand a Maximum of 7V, regardless of the supply voltage.
 
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Offline Cicada

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Good.

I think it settles it then. I accept that the MAX3485 datasheet means what it says. That the input pins can be driven with voltages that are within the 7V limit regardless of what the supply voltage to the devices is.

Thanks for your answers MosherIV and Andy Watson.
 


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