Practical current load on a 18F4520

Hi,

Am looking to constantly sink some 10ma leds directly from the ports of this chip, without the need of additional driver chips/ transistors

The specs of the chip show that 200ma is the max that can be drawn from all the ports.

Any suggestion as to the maximum load its prudent / safe to Sink on a continuous basis without shortening the chips life dramatically.

Want to avoid pulsing the leds to save current as that could really complicate the rest of the program.

thanks


Maximum current out of VSS pin ...........................................................................................................................300 mA
Maximum current into VDD pin ..............................................................................................................................250 mA
Maximum output current sunk by any I/O pin..........................................................................................................25 mA
Maximum output current sourced by any I/O pin ....................................................................................................25 mA
Maximum current sunk by all ports .......................................................................................................................200mA
Maximum current sourced by all ports ..................................................................................................................200 mA

In my experience with PIC18F chips (I use the PIC18F14K50 a lot), I've not had a problem sourcing LEDs within 10mA of the current specified. I think the PIC I often use has a capability of delivering 90-100mA total. I have sourced 60-80mA without any problems before, so if your chip specifies 200mA, I would source 160mA without worrying.

Then again, if this is a high-reliability long-term critical, or battery operated design....I'd pulse them. Or just drive them through some 2222s.

Also, don't underestimate the technique of putting your finger on the PIC and feeling how hot it gets during normal operation!

The datasheet indicates V_{OL} ~ 0.3 V when sinking 10 mA. This corresponds to ~3mW dissapated at each of the IO's per attached LED.
The temperature rise (above ambient) experienced by the silicon can be estimated by multiplying the total package power dissapation (which we'll assume is dominated by the LED drivers) by the thermal resistance \theta_{ja} which is device/package dependant.

(Note: all of this and much more is contained in the neat National Semiconductor article: **broken link removed**)

By way of illustration though, if we assumed a 40 pin DIP package for your PIC then \theta_{ja} might be ~50 degrees C/W. The die would thus rise ~0.2 degrees C per 3mW dissapated in the pull-down transisor in the IO stage. I wholeheartedly recommend the sanity check proposed by milesguidon too - with the additional rule of thumb that if you can't keep your finger on the device, then it's hotter than ~60 degrees (and the die is hotter still)!

There are all sorts of reliability models predicting different relationships between temperature and failure rate for various devices, but suffice to say cooler is better

Hi,

Thanks guys,

Been testing it overnight with 14 leds on 14 ports = 140 ma ( meter checked )

Have a temp sensor sealed on top of the chip and very surprised to see that the temp only rised 1c over the ambient.
Double checked by feeling the chip , still cool !