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johnl
Joined: 30 Sep 2003 Posts: 120
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Maximum Current Specs |
Posted: Thu Jun 18, 2015 2:28 pm |
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The data sheet for a PIC16F720 says:
Maximum current out of VSS pin ......................... 95 mA
Maximum current sunk by all ports, -40°C to +85°C for
industrial............ 200 mA
How can ports sink 200mA without exceeding 95mA out of Vss?
It's a similar scenario for sourcing and current into Vdd.
What am I missing? |
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asmboy
Joined: 20 Nov 2007 Posts: 2128 Location: albany ny
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Posted: Thu Jun 18, 2015 3:11 pm |
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i have always treated the LESSER value as steady state MAX DC value.
however for very short pulses - you can safely come closer to the HIGHER of the values. it is a fool's game to depend on any value greater than the MAX current into or out of any ONE pin-(+/-25ma) or to exceed the maximum core Vss Vdd specs on the sheet.
if you need lots of current -use an external transistor or mosfet to do the heavy lifting. that's what the pros do.
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johnl
Joined: 30 Sep 2003 Posts: 120
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Posted: Thu Jun 18, 2015 4:04 pm |
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I guess I'm questioning the logic of the specs. If they say you can't have more than 95 mA out of the Vss pin, then they should say you can't have more than 95 mA sunk by all ports. |
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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johnl
Joined: 30 Sep 2003 Posts: 120
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Posted: Thu Jun 18, 2015 7:15 pm |
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Yes, it does look like an error, though there doesn't seem to be an errata sheet that mentions it.
I have a design that will sink 85mA into the the C port, but I'd feel better if I knew the 95 mA spec out of the Vss pin is supposed to be higher.
I'll write to Microchip. |
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dyeatman
Joined: 06 Sep 2003 Posts: 1934 Location: Norman, OK
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Posted: Thu Jun 18, 2015 8:10 pm |
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My advice is to avoid sinking that kind of current with the PIC regardless.
You are begging for current transient issues. Why not use a FET driver
chip (ULNxxxx) instead? Cheap, much more stable and adds protection
for the PIC as a bonus. Not to mention heating issues. _________________ Google and Forum Search are some of your best tools!!!! |
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RF_Developer
Joined: 07 Feb 2011 Posts: 839
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Re: Maximum Current Specs |
Posted: Fri Jun 19, 2015 1:48 am |
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johnl wrote: |
What am I missing? |
Hmm, possibly its something to do with mulitple Vss pins. Each pin supplies a different region, probably meaning different IO ports, of the die. As there are several Vss pins, the total current sunk by all ports is the maxiumum currunt per Vss pin times the number of pins, assuming current is shared evenly.
For PICs with one Vss pin, I'd expect the two specs to be closely related, with the Vss current a bit higher than the total for the ports.
A problem is that I don't recall ever seeing any spec stating which ports are associated with which pin.
EDIT: I see from the data sheet, that the PIC16F720 has only one set of power pins.... |
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temtronic
Joined: 01 Jul 2010 Posts: 9246 Location: Greensville,Ontario
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Posted: Fri Jun 19, 2015 4:57 am |
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I have to 'second' dyeatman about using a ULNdriver chip ! Something rarely discussed is overheating of PICs and well, bad things happen when they get hot. If one region of the PIC is hotter ten the clock can go wrong, ADC goes whacky, I/O latches up, etc. There is a 'max temp, heat disp' spec you need to be WELL under in order to be sure the PIC is 'happy' and will outlive you and I.
It's well worth the extra buck per PCB to add the ULN,even add a socket for the 'high powered' driver. My energy control boards (4000 series, pre PIC) had every chip in sockets.Though it cost me a penny a pin,repair time to swap out a bad chip was FAST and easy unlike desoldering,cleaning, install,cleaning,etc.
Also be sure the PCB traces for the 'high power' section are wide enough to handle the current! I see too many designs with ultra thin traces that need to pump 1/2 amp through ! Those usually become unknown 'onboard fuses' !!
Jay |
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johnl
Joined: 30 Sep 2003 Posts: 120
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Posted: Fri Jun 19, 2015 9:17 am |
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This application needs to be cheap. Sinking 12 mA into each of 7 I/O pins doesn't seem to be asking too much, given the fact that each can handle 25 mA and the feature sheet boasts direct LED drive.
Aside from powering an LED display, the app is basically static, just checking a few switches and sourcing a few other LEDs. No time-sensitive peripherals are used and no A/D.
Is it wrong to assume it will be fine since total dissipation will be less than 60 or 70 mW? (maximum is 800 mW.)
The current out of the Vss pin is the only rating I'd be pushing and is my only concern.
I have a ticket into Microchip and will post their response.
Thanks for the responses. |
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asmboy
Joined: 20 Nov 2007 Posts: 2128 Location: albany ny
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Posted: Fri Jun 19, 2015 12:08 pm |
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Quote: |
The current out of the Vss pin is the only rating I'd be pushing |
I disagree.....
if using the internal oscillator -thermal variations WILL destabilize the frequency - AND potentially alter BOTH EUSART and timing as well as ADC performance too, and produce a guaranteed degradation of long term life of the part.
But if reliability and stability really don't matter - then by all means toast that puppy. |
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temtronic
Joined: 01 Jul 2010 Posts: 9246 Location: Greensville,Ontario
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Posted: Fri Jun 19, 2015 12:24 pm |
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re:
This application needs to be cheap.
Be careful !! saving a few pennies now may spell disaster in the field !! Just cause it 'worked fine on the bench' doesn't mean it will for the customer. BTDT. Had an RC osc setup worked fine until the temperature went up and humdidty set in. Timing went 'out the window'....sigh, had to drop xtal and 3 chips in to get the right frequency.BTW nobody made 24Hz xtals 20 years ago......
also.. you might consider low current LEDs,if you haven't yet. Some are great at 5ma !
Jay |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19552
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Posted: Fri Jun 19, 2015 12:28 pm |
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The 720, is very specifically a low power chip. The current spec given is for the full temperature range. Maximum source current is only 65mA over this range. Beware also, that the Voh specifications only go up to 3.5mA for the pins. They droop a _lot_ more as the current goes above this, result more dissipation....
The pull down drivers have lower droop, so reduce dissipation. Better to work this way if you can.
So the question becomes what is the maximum temperature range you need (remember it can be surprising the temperature that quite mundane circuitry will meet...).
Chips like the ULN2003, in SMD, come in at just $0.13 in sensible quantities.
Remember with high efficiency LED's, 2.5mA is a fairly typical current, so 'direct LED drive' does not imply high current capability. |
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johnl
Joined: 30 Sep 2003 Posts: 120
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Posted: Fri Jun 19, 2015 2:35 pm |
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The spec for the current out of the Vss pin might be an error, as PCM programmer mentioned. (I am waiting to hear from Microchip on that.) So for the purposes of this discussion, let's just look at the other limits.
The consensus seems to be very conservative here and I don't see why that is.
The chip is capable of sinking 25 mA from each I/O pin and I'm looking to sink 12 mA from 7 of them in a modest temperature range environment of 0 deg.C to 50 deg. C.
So the current sunk by all ports is 84mA -less than half of the 200 mA rating.
If Vol is maybe .8v at 12 mA, then the power is about 70mW.
Why are running a dissipation of one tenth the max power and output pin current of one half the max and total sink current less than one half the max causes for concern?
To what margin do you guys normally design? I thought 50% of max has been a useful rule of thumb. |
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dyeatman
Joined: 06 Sep 2003 Posts: 1934 Location: Norman, OK
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Posted: Fri Jun 19, 2015 3:08 pm |
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The reason for my caution is extensive experience. When you start
switching loads directly with the PIC you get into switching transients/EMF.
I have designed many systems (PIC and other) and I always design on the
conservative side to maintain reliability. _________________ Google and Forum Search are some of your best tools!!!! |
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temtronic
Joined: 01 Jul 2010 Posts: 9246 Location: Greensville,Ontario
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Posted: Fri Jun 19, 2015 4:23 pm |
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Have you tested in 'real life' yet? 0*C is easy, toss into a freezer.. 50*C gets to be more fun, though I have built small 'environmental test chambers' in the past. You might need the 'industrial' rated PICs not the cheaper 'commercial' ones but check the specs real carefully and do some honest testing BEFORE you ship out 10,000 units and have half come back because they 'don't work right'.
Jay |
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