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Topic: Level shifter - suitable through hole MOSFET? (Read 2850 times) previous topic - next topic

djc

Hello.

I'm trying to make a bi-directional level-shifter for a 3.3v SPI chip.

Following the principles here (http://www.kip.uni-heidelberg.de/lhcb/Publications/external/AN97055.pdf). Suitable MOSFETs are inter alia BSS123 (http://i2c2p.twibright.com/datasheet/BSS123.pdf) but in SOT23, I can't see the thing, let alone solder it!

Would BS170 (http://www.onsemi.com/pub/Collateral/BS170-D.PDF) be a suitable replacement?

I would be very pleased if, as well as a 'yes' or 'no', you might explain how it's possible to make this kind of judgement.

Thanks.

BenF

I've used the BSS123 in a number of projects as a bi-directional level shifter for I2C between 3V3 and 5V. This MOSFET is available in a TO-92 package.

http://www.datasheetcatalog.com/datasheets_pdf/B/S/S/1/BSS123.shtml


SPI however is typically unidirectional and so a simple voltage divider in the 5V to 3V3 direction is all that's needed. Are you sure a bi-directional level shifter is what you need?

BenF


DISCLAIMER: I mentioned stuff from my own Shop...  But I like finding solutions  :)

That may be so (to ... finding solutions), but a power MOSFET board is no solution for a SPI level shifter. There is just no relevance to the topic whatsoever and so it's hard to see this as anything else than SPAM.

What do you think?

Terry King

Wow! Very Sorry! Somehow got this in the wrong post! Outta Here!!!
Regards, Terry King  ..On the Red Sea at KAUST.edu.sa
terry@yourduino.com  LEARN! DO! (Arduino Boards, Sensors, Parts @ http://yourduino.com

davekw7x

#4
Mar 26, 2011, 03:46 pm Last Edit: Mar 27, 2011, 12:23 am by davekw7x Reason: 1
djc:
2N7000 devices are readily available in T0-92 packages from my usual small-quantity suppliers: Digikey, Mouser, Jameco.  (They do not seem to stock BSS123 in TO-92 packages.)

I have used 2N7000 devices for the (bidirectional) clock and data signals for 3.3 Volt I2C devices connected to 5 Volt Arduinos with circuits shown in AN97055 that you linked.

As far as I can tell, the BS170 appears to be suitable, but I don't have direct experience with it.  It appears that Mouser and Jameco stock them in TO-92 packages.


Regards,

Dave

MarkT

Quote
Would BS170 (http://www.onsemi.com/pub/Collateral/BS170-D.PDF) be a suitable replacement?


No.  It needs 10V gate drive - you need a FET with a Ron specified for 3.3V. 
[ I won't respond to messages, use the forum please ]

davekw7x


... needs 10V gate drive
Yeah, if you want it to pass 1.8 Amps.

The data sheet shows a threshold turn-on voltage of 2-3 volts for small values of current.  (Same as 2N7000, which I have used).  The BSS123 mentioned in the Original Post is a little better (2 Volt threshold for small currents), but any of these should be OK for 5 volt logic.

Regards,

Dave

MarkT

Threshold currents can be 1uA which is not enough.  Also there is considerable device-too-device variation and temperature dependence of the threshold value.

A power MOSFET is not the right solution, its input capacitance is way too high for logic signals.
[ I won't respond to messages, use the forum please ]

davekw7x


...not the right solution...
Fair enough.  I concede the point.

Can you give a specific recommendation?  (Available in small quantities from "convenient" distributors in TO-92 or other through-hole packages?)


Regards,

Dave

MarkT

Nearly all small signal MOSFETs are surface mount these days by the look of it - coudn't see anything at Farnell.
[ I won't respond to messages, use the forum please ]

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