[Noisebridge-discuss] [dorkbotsf-blabber] 3.3v serial-parallel shift register with open-collector output?

[John Adams]

I used to do this with D flip-flops and NPN transistors. Driving FETs
directly off of TTL outputs seems like a bad idea to me. At least with the
transistors you'll be able to drive far more current.

-j


On Wed, Jun 20, 2012 at 1:50 PM, Anders Nelson <anders.k.nelson at gmail.com>wrote:

> Hi Tymm,
>
> Thanks so much for the detailed input!
>
> Yeah it seems like the 74AHC595 from NXP responds to a VinH of 2.1v at 3v
> supply voltage for a ratio of 0.7. If I apply that ratio to 4.2v I get a
> VinH of 2.94v which ought to work when I throw 3.3v at it. Provided of
> course my supply is stable enough... More caps!
>
> =]
>
> --
> Anders Nelson
>
> +1 (517) 775-6129
>
> www.erogear.com
>
>
> On Wed, Jun 20, 2012 at 1:24 PM, Tymm Twillman <tymmothy at gmail.com> wrote:
>
>> In similar situations I've generally thrown a cheap npn transistor on
>> the uC outputs & switched that with the GPIO.  Tristating should be
>> relatively ok, but the npn transistor will generally give you more
>> current -> faster transitions (dumping the charge on the fet's gate
>> more quickly).  Hmm; at 250ns... if things have to actually switch
>> that quickly you may have issues getting enough charge through the
>> pull-up resistor on the fet's gate to turn it off in time.  if it's
>> really important to meet timing, you'll want to calculate the
>> worst-case timing based on the capacitance of the fet's gate, the gate
>> threshold voltage and the resistance to the high rail... capacitance
>> is much higher generally with larger fets
>>
>> Tristating, depending on the microcontroller, should be plenty fast on
>> the control side of things if you are able to set all the pins on the
>> port at once and can pre-compute the new values... though there's
>> still potentially the above fet switching issue... also with almost
>> any uC you'll be limited to somewhere 40ma or less, which may also
>> extend the fet switching time.
>>
>> There are assorted serial interfaced driver chips that one can use,
>> using fets or bipolar outputs... tpic6b595 and others... though
>> they'll generally be quite a bit more expensive than driving via gpio.
>>
>> other approach would be to run e.g. a non-open-collector 74hc or
>> similar shift register at 4.2v -- worth checking data sheets but in
>> most cases your 3.3v signaling should work fine as an input, and the
>> full swing on the output can help switch the fets more quickly.
>> 74ac's will also give more drive current, which can also help speed
>> switching times.
>>
>>
>> On Wed, Jun 20, 2012 at 12:57 PM, Anders Nelson
>> <anders.k.nelson at gmail.com> wrote:
>> > Heyoo,
>> >
>> > I've become rather exasperated in my search for an 8-bit serial-parallel
>> > shift register with open collector outputs. See, I'm trying to switch
>> the
>> > gates of some P-FETs that act as high-side current switches for the
>> rows in
>> > a scanned LED matrix. The LED bus voltage is 4.2v and all the ICs run on
>> > 3.3v.
>> >
>> > I found the SN74LS596, but it's an ancient part that is not only
>> enormous
>> > but runs at 4.5v-5.5v which won't work in my system.
>> >
>> > I could use a microcontroller and just toggle the tri-state status of
>> the
>> > pins but I worry about how fast the device will respond - It will have
>> to
>> > blast out the I/O change within a blanking cycle lasting 250ns. That
>> > requires between 4 and 8 MIPS which is not too powerful but this is a
>> > low-cost product. =/
>> >
>> > Any suggestions are most appreciated!
>> > --
>> > Anders Nelson
>> >
>> > +1 (517) 775-6129
>> >
>> > www.erogear.com
>> >
>> >
>> > ----------
>> >
>> > dorkbot: people doing strange things with electricity
>> > http://dorkbot.org/dorkbotsf/
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>
>
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