Quad or hex FET in single SMT package? - Page 1
Forum section: electronics / projects

author: bson
author_rank: Unknown
timestamp: November 30, 2016, 12:19:47 am
content: There seems to be plenty of dual FET devices out there, but I can't for the life of me seem to find any quad or hex devices. Four or six 3.3V logic (CMOS) friendly enhancement-mode FETs with reasonably low Rds(on), say < 10 ohm, but even a little more shouldn't be a big deal. Threshold around 1 to 2V would be perfect. Something like four 2N7002's would be perfect! SOIC, SSOP, or SOT232-x. Even QFN! Common source is fine too - I'm putting these low side so the sources will all be grounded. Drain should accept 10-15V or so - they'll be used to switch latching S-R small-signal telco relays that run off 5-7V with coils in the hundreds of ohms, so 10-15V gives plenty of margin. They will be controlled by a 3.3V MSP430 G or F series uC (not sure which way to go, but a 'G2553 would probably do the job and in SSOP is nice and small). How hard can it be to find a suitable transistor array?

author: Audioguru
author_rank: Super Contributor
timestamp: November 30, 2016, 12:37:04 am
content: Why are you looking at the max threshold voltage of a 2N7002 which is 2.5V when it barely conducts (only 0.25mA)? Its minimum rating is with a 5V gate-source voltage when its cool max resistance is 7.5 ohms. Other Mosfets have lower gate-source voltage ratings.

author: bson
author_rank: Unknown
timestamp: November 30, 2016, 12:45:52 am
content: It was just an example.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2016, 01:04:10 am
content: Transistor arrays suck, as you've discovered...There's a short list of 2N2222 or 2N3904 or 2N7000 style independent arrays, and a couple oddball boutique parts, like the precision matched monolithic sets (almost all gone, these days), or ALD's new EEPROM(?)-trimmed arrays (certainly nothing you'd want to use here!).So...- Duals. They're not so bad.- Specific applications. Relay drivers abound; if you must have CMOS, there's a ULN2003LV, MAX4820, Idunno, all sorts of thingys.- LED drivers, if you can disable PWM/dimming (or heck, use PWM to save power, it's _free_!).- General applications. Lots of general purpose load switches out there. Many with way more features than you need (overvoltage and overcurrent protection -- want to put the relays on a far-away board, and don't want to risk destruction in case of the cable shorting out?).- Get smaller relays, that can be driven directly from the MCU(!). Possibly latching, so they don't cook the poor thing.- I think there's a few "high voltage" CMOS logic gates too, which would be the ULN2003LV with weaker outputs and no clamp diode built in. (Notably they didn't make a "74HC06", whereas the original (7406) handled up to 15V. The 74HC07 (buffer, not inverter) has output clamp diodes to VCC.)- And if you don't mind old fashioned bipolar, the 7406 is still available!Most of these things are little more than individual transistors packed together with a common ground, but they don't sell them as arrays, because who the hell uses transistors anymore, anyway?Tim

author: edavid
author_rank: Super Contributor
timestamp: November 30, 2016, 01:06:13 am
content: You don't see many FET arrays because they don't really save space.However, there are various intelligent switches. One that comes to mind is the STP04:http://www.st.com/content/ccc/resource/technical/document/datasheet/54/4a/1d/50/03/0e/47/e7/CD00177556.pdf/files/CD00177556.pdf/jcr:content/translations/en.CD00177556.pdfIt has a constant current feature that you don't need, but it doesn't really hurt anything, and it's still pretty cheap.There is also an STP08... you guessed it, 8 channel.

author: bson
author_rank: Unknown
timestamp: November 30, 2016, 11:38:50 pm
content: Some reasonably good ideas, but after thinking this over I think I'll just go with dual transistors in SO-8... Ordered 10 TD9944's for a starting point, easily replaced with something else if I want to shave cost further. Cheap and simple. The LED drivers are interesting, but I'm a bit concerned over the constant current sinks combined with inductive loads; I'd have to do quite a bit of qualification and testing to make sure they don't suddenly go pop months or years down the road. Re: 7406, I'mamazedto see these are still made - in SO no less! Wow. Just wow.The real problem with a simple buffer is I can't easily guarantee it doesn't come up in a bad state; I'd rather make sure all relays automatically reset on power-on as part of the reset hold. I could also use something like a PCF8574 which can easily switch small relays (30-60mA for 3-5V operation) but then have to deal with I2C headaches - while the PCF8574 is as reliable as it gets (no registers or configuration, just write 8 bits of data and it shows up on the pins, read 8 bits and it samples the pins) there's still the potential of I2C master side lockups, or if I make it just do dumb writes ignoring any responses (single device, single bus) I could end up with the slave silently going off-line.The Omron G6K2S dual coil latching DPDT relays are only a couple of dollars ea in even modest quantity (~$4 in singles from Mouser), and my experience is they're excellent in terms of reliability and fidelity within a modest wideband (not truly modern RF of course), and I'd rather not design around 5-10 fancy $20 relays if there's an option not to. (I'm not a penny pincher but that just strikes me as expense without any perceivable benefit.) I'll just suck up the board space loss to a couple of extra SO-8s...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 01, 2016, 07:01:13 pm
content: Quote from: bson on November 30, 2016, 11:38:50 pmMmm, surely you mean latches, not buffers?Speaking of, if you need more IO, increasing fanout with bus latches or shift registers (SPI) is a time-honored method. Less internal state than an I2C or SPI chip with registers and stuff.Default values are easily assured with power-on reset conditions. Or for buffers and inverters (including bare transistors), the output state always reflects the input state, so all you need is a pull-down (or up) resistor to set the default state until the MCU wakes up. (MCU pins are usually tristate until asserted, but do check the datasheet to be sure!)If you want more crash-abatement insurance, you can also run a watchdog timer on the MCU, and produce a heartbeat signal on a pin -- a pin toggled by software control only. If the software freezes, the watchdog times out and reboots the CPU; meanwhile, a missing-pulse detector monitors the heartbeat, so if it stops toggling, all hardware outputs are disabled (so you need a bunch of AND gates for all the pins, but that's the price you pay to be sure of things).The transistor choice (TD9944) sounds fine, but I'm curious why the extreme voltage rating?Tim