Have you purchased the relays yet? I notice there are 4.5V and 5V coil variants too, with correspondingly lower coil currents.
A bit of an off-the-wall solution would be to use pairs of 74ACT540 inverting and ACT541 non inverting buffers. Each of the data inputs would be be connected together, also the output enable pins. The relay coils would be then connected between respective pins of the two buffers.
This would provide 8 differential coil drives per pair of buffer packages, with output tri-states to allow pulse driving of the coils. If you check the 74ACT54x datasheets, they are rated for 24mA per output, with Absolute maximum rating of 50mA (both output and VCC/GND current per pin). 33mA is pushing the specs a bit, but things get better for 4.5 and 5V coil variants. The relays have a 'must operate' voltage of 80% of rated voltage.
I wouldn't worry about exceeding the maximum current rating of the buffers. The latching relays only need to be switched on for 3ms and the ICs should be able to withstand a much higher current, for that length of time. The buffers might not be able to drive 5V relays, off a 5V supply, due to the voltage drop, of the relatively high resistance outputs, but stand more of a chance with 3V relays.
Datasheet, Table 4, Note 6: Each output must be limited to 25 mA output current. Roughly the same as ACT logic. There is a difference between destructive limits and operation limits.
Regards, Dieter
Yes, I didn't claim that it was a perfect design, but I think it would work for a one-off.
The 4.5V relay pulls 23mA, and the 5V pulls 21mA, both at rated voltage of course, before the voltage drops of the ACT outputs. The bistable relay has a 'must operate' voltage of 75%.
It would take a little bit more fiddling with the specs and a calculator to double check but I think it's workable with 4.5V relays. With 5V supply, the guaranteed ACT tristate output voltage drops are ~0.75V (high) and ~0.44 (low) @24mA so about 1.2V total (typical at 25'C is more like 1V total). With the 4.5V relay you need a minimum of 3.4V to guarantee switching. leaving 400mV of slack for the full temperature range ACT spec (600mV typical at room temperature).
A little slim, I agree, but achievable. Bumping the supply up to 5.5V would make things more comfortable (approx nominal coil voltage). The ACT logic would be operating within spec, and at half absolute maximum rating.
The 3V relay would work too, the 33mA current would probably increase the voltage drop of the ACT outputs to bring the coil voltage to nearer 3V (the ACT spec sheet stops at 24mA). It
is definitely pushing the outputs harder though (outside nominal spec) - depending on how frequently the relays are going to get switched.
Switching the relays in blocks of 8 (even if closely timed) would drop the peak supply current to something more acceptable - 30 relays switching simultaneously at 33mA is almost an amp. 8 relays at 23mA is only 184mA.