PDA

View Full Version : Maximum Input Current



soshimo
01-14-2009, 02:58 AM
I need to interface with a 5v device and I wanted to make sure I picked the right value for the current limiting resister. If I'm dropping 1.3v across the resistor (parasitic catch diodes clamp input voltage to 3.7v) then if I choose 1k my input current will be 1.3ma. Will that be okay for the prop or should I go for say 4.7k which will give me ~270ua?

TIA

Beau Schwabe (Parallax)
01-14-2009, 03:36 AM
soshimo,

1.3ma should be fine.· The diodes are actually intentional and part of the drive transistors structure.·· In comparison, the number of fingers used when the transistor is "ON" is small compared to the number of transistor fingers who's gates are tied to the source that function as the diode.· Personally I would choose a resistor value that will yield the least amount of current waste.

Attached is a screenshot of the driver section of one of the I/O's.· Both drive transistors have 20 fingers.· The PMOS (Far Left)·has 10 active fingers used when the transistor is "ON", while the NMOS (Far Right)·has 6 active fingers used when the transistor is "ON".· This means, for the PMOS there are 10 fingers used for the diode that have as much drive strength as the active transistor would·when used as an output, where as the NMOS uses 14 fingers for the diode and has nearly twice the drive strength as the active transistor would when used as an output.

The main drawback when using the reverse biased diodes like this is that you run the risk of injecting noise onto the substrate which can cause problems.· Another reason to go with as little current as possible.




▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 1/13/2009 8:42:08 PM GMT

Paul Baker
01-14-2009, 03:53 AM
I hate contradicting Beau, upmost respect to you friend, but when I wrote the datasheet Chip and I discussed this value. His original suggestion was 1 mA, and after discussing it with him·a bit further we decided to list the absolute maximum to be +/- 500uA (page 26, Table 18 of the dataheet). Does this mean it will blow up if you inject more current? No, but we cannot be absolutely certain that constantly biasing the ESD diode beyond 500uA won't damage the chip after months or years of operation, especially if ESD events do occur on the pin. IOW, you can·follow Beau's advice since he understands what's happening·on the silicon,·just be aware that it is beyond the stated maximum in the datasheet and·if you do damage the chip Parallax won't replace it.·

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Paul Baker (mailto:pbaker@parallax.com)


Post Edited (Paul Baker) : 1/13/2009 9:16:08 PM GMT

Beau Schwabe (Parallax)
01-14-2009, 04:26 AM
Paul Baker,

Your not contradicting me really. There is a reason you want to keep the current as low as possible to these diodes because they are against the normal flow, remember the diodes will only conduct when the voltage is reverse biased across them... like what a 5V source to an Input pin·would do. I was trying to convey that the diodes will survive, but that it's still important to keep currents as low as possible with respect to them.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 1/13/2009 9:40:10 PM GMT