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Author Topic: Level shifter - suitable through hole MOSFET?  (Read 2444 times)
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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.
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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?
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DISCLAIMER: I mentioned stuff from my own Shop...  But I like finding solutions  smiley
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?
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Wow! Very Sorry! Somehow got this in the wrong post! Outta Here!!!
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Regards, Terry King  ..On the Red Sea at KAUST.edu.sa
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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
« Last Edit: March 26, 2011, 06:23:18 pm by davekw7x » Logged

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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. 
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... 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
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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.
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...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
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Nearly all small signal MOSFETs are surface mount these days by the look of it - coudn't see anything at Farnell.
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