What kind of FET transistor do I need here? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: January 14, 2014, 10:19:22 pm
content: I am reading this portion of the Atmel LIN app notehttp://www.atmel.com/images/doc1637.pdfIt explains how to use a single fet instead of a linbus transceiver such at ata6663. The linbus uses a single bidirection wire that is pulled up to +12V and can be pulled down by either a remote node or by the local node (a 5V microcontoller on the left) and should also be sensed by both on both sides to detect collisions.What kind of a fet do I need here? I am familer with N and P chan but am not sure what symbol is shown. Any recommendation for actual component? The linbus input should sustain up to +40V.

author: Dave
author_rank: Super Contributor
timestamp: January 15, 2014, 03:20:33 am
content: An N-channel MOSFET would work there. Connect the gate to 5V, drain to LIN bus and the source to your MCU.When the MCU pulls the pin low, the G-S voltage of the FET becomes 5V and the channel opens, pulling the LIN bus low.When the bus is pulled low, the body diode of the FET pulls MCU's pin to about 0.6V, which is low enough to be detected.

author: zapta
author_rank: Super Contributor
timestamp: January 15, 2014, 03:54:49 am
content: Thanks Dave, I think I understand it now.Will give it a try, in LTSpice and then on a breadboard.

author: zapta
author_rank: Super Contributor
timestamp: January 15, 2014, 04:30:17 am
content: Here is the simulation (in course granularity steps). It seems to work. The threshold point is shifted (as percent of swing) but should work. Very nice.Circuit (12V to 5V direction)Transfer (12V to 5V):Transfer (5V to 12V):

author: Andreas
author_rank: Super Contributor
timestamp: January 15, 2014, 05:44:51 am
content: Hello,The LIN spec has its switching point at about 50% of the external supply voltage.so the single FET circuit is horror when you want to communicate with other devices relying on the LIN standard.I would use a LM393 (or LM29xx automotive grade) comparator instead.With best regardsAndreas

author: zapta
author_rank: Super Contributor
timestamp: January 15, 2014, 05:56:37 am
content: Quote from: Andreas on January 15, 2014, 05:44:51 amThanks Andreas. Yes 36V max voltage should be fine. But then I need to add glue resistors for reference and 5V output, an open collector transistor for TX and so on. In this case it's easier to just use a lin transceiver such as ata6663. It also has a timeout protection of the bus if the node asserts low too long. Other lin transceivers even have a 5V LDO to power the microcontroler. My current schematic with ata6663 is herehttps://github.com/zapta/lin-sniffer/blob/master/proto/eagle/proto.pdf?raw=true

author: ThomasDK
author_rank: Regular Contributor
timestamp: January 15, 2014, 01:14:31 pm
content: The FET approach works for I2C too, I've used it a couple of times for 3.3V -> 5V translation. I think I got the idea from an old NXP appnote

author: dannyf
author_rank: Super Contributor
timestamp: January 15, 2014, 02:22:34 pm
content: Yes, it is "stolen" from the nxp application note on i2c lever transfer: it uses the body diode in the mosfet.As such, similar things can be achieved with bjt + diode too.

author: Co6aka
author_rank: Unknown
timestamp: January 15, 2014, 04:52:43 pm
content: It's all a twist on the ubiquitous common-gate amplifier.

author: zapta
author_rank: Super Contributor
timestamp: January 15, 2014, 05:50:31 pm
content: Quote from: Co6aka on January 15, 2014, 04:52:43 pmIn the low to high direction. In the high to low it is the diode.The more I think about my problem, I think should go with one of these, lin interface + microcontroller + LDO in one chip. Shouldn't be too difficult to port my atmega328P code to this one.http://www.atmel.com/Images/Atmel-9111-LIN-Networking-ATA6612C-ATA6613C_Datasheet.pdf

author: dannyf
author_rank: Super Contributor
timestamp: January 15, 2014, 06:40:59 pm
content: QuoteMaybe semantics but in either direction, it is switching: in one direction, it is switching with a diode and in another it is "source switching".