The MM5451 has been described as outdated, but it seems pretty neat to me, having 35 discrete constant current sinks on one (large) chip. My question is, can it be used to drive a matrix, which I guess is a bit different than driving a single LED. I realise it doesn't do PWM on a per pin basis, but for example I have a 16x16 common anode red matrix - could that be driven by one MM5451, rather than four(I think it would take?) 7219s ?
Sure - use 16 pins to sink current from common cathodes. Use 16 pins to sink the base of PNP transistors, or the gate of P-channel MOSFETs, to source current into the common anodes. You'll have to write the multiplexing code to sink the cathodes, drive an anode on/off, sink the next cathodes, drive the next anode on/off, etc.