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Maximum Current Specs
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asmboy



Joined: 20 Nov 2007
Posts: 2128
Location: albany ny

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PostPosted: Fri Jun 19, 2015 4:54 pm     Reply with quote
Quote:

To what margin do you guys normally design?

10 to 15% of max current @ rated Vdd or less is my target , and half the maximum clock frequency if the design does not need to "go faster". In a 5v nominal design I always use a 240 ohm resistor(OR MORE) to couple to the base/gate of an external transistor/mosfet/driverIC to more or less insure against possibly sourcing current greater than +20ma into a short.
Watchdog recovery code in release versions.
Thermal stress in an incubator and for some designs "freezer to full power" if warranted.

My work is focused on portable RF, medical instrumentation and specialized military emergency light products, and cheaping out on any aspect of reliability is anathema to me. It only takes one crappy design to waste years of good work-reputation, IMHO . I've been designing circuits for almost 40 years and my customers wouldn't work with anybody else if they can help it.
-dp-
johnl



Joined: 30 Sep 2003
Posts: 120

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PostPosted: Fri Jun 26, 2015 8:43 am     Reply with quote
Still waiting to hear back from Microchip, but in the meantime I have taken the consensus opinion of the group and have added a driver to the design.

Thank you all for responding!
asmboy



Joined: 20 Nov 2007
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Location: albany ny

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PostPosted: Fri Jun 26, 2015 10:33 am     Reply with quote
i should have added:
Unless space is incredibly tight -
single transistors/mosfets can be very cheap to add as drivers.

The MMBT3904 up to to 28v @ 100ma
or MMBT5551 up to 150V
driven by a 4.7k base resistor

2 cents USD is a typical US production price for the 3904.
A business associate buys these at 3 for a penny on ebay as they are ubiquitous in the surplus market as "new reels" and has never been burned to my knowledge.

The charge to "pick & place" is typically greater than the cost of the part !!!

or for really low pic pin loading
MMBT7002 up to 48v @ 100 ma MOSFET

and of course ever higher voltage and current "logic level" mosfets for really heavy loads.
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