On 04/03/11 21:48, Dwayne Reid wrote:
> This unit actually has 3.3V, 1.8V, 1.25V rails for the TMS320xx DSPs
> and all of the codecs.  But these shift registers are being used to
> control analog gate packages that want to see a 5V control signal.

Quick back of the envelope calculation based on current Farnell prices...

Xilinx XC9572-15PCG44C   -- =A35.40 each 1-off, =A33.90 each 10-off.
Xilinx XC9572XL-10PCG44C -- =A32.11 each 1-off, =A31.86 each 10-off.
STMicroelectronics 74LVX04MTR -- 18p each 1-off, 12.2p each 10-off.

So we have two solutions. Let's say we're making ten of these things:
   XC9572 alone         -- =A33.90 each
   XC9572XL and 74LVX04 -- =A31.97 each

Take 5V from your main power rail, run the LVX04 from that. Run 3V3 from=20
your DSP supply and run the 9572XL from that. Inputs to the 9572XL go=20
through 33-ohm current limiting resistors, outputs go through the LVX04=20
(which boosts the voltage to 5V).

The 3V3 solution reduces cost by almost half, and stock availability is=20
MUCH better. It uses more board area, but if you're installing a=20
bodge-board that probably isn't a major issue (it's maybe an extra=20
square inch for the LVX04).

--=20
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/

--=20
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