Alternative to optocoupler in circuit design

Question

I have designed a circuit that drives a +/- 20 to 30 peak to peak external signal received as an input to 16 different outputs. Another requirement is that I have to be able to switch the state of these outputs in less than 100us (ideally 50us).

To meet these requirements, I am currently using a AQS221N2S low capacitance optocoupler array from Panasonic, precisely 4 of them. The actual circuit has been produced and tested and is functional for the specs.

However, I am now required to scale up the circuit to a 64 outputs, which brings me to reconsider my part choice for two reasons:

1. AQS221N2S are relatively expensive.
2. The AQS221N2S's turn on time meets the specs in the typical value (0.03ms) but not on the maximum value (0.2ms).

I cannot use relays or solid state relays as they are often slower and more expensive than the actual part.

The alternative I was considering was using an AC switch, such as the MAX14756, which is both faster and cheaper than the optocoupler solution I am using currently. Unfortunately, using an AC switch requires to add a bit of extra hardware since I will need to rectify the input AC signal to generate VDD and VSS necessary to the IC, which has the drawback of adding complexity to the design and more development time on my part.

Before I start redesigning the whole system, I was wondering if anyone had a suggestion of a part that would have the benefits of using an AC switch without the drawbacks of needing to rectify the input AC signal?

Edit: Here are some additional information about the circuit:

• The circuit is currently powered from a single Micro-USB 5V supply, then a 3.3V linear regulator is used to supply 3.3V to the circuit.
• The AC signal that I am driving ranges between 55 to 65 KHz
• The AC value given above, 20 to 30V, is a peak to peak value, it was a mistake from my part to identify it as VAC.
• Each input of the AQS221N2S are in series with a 390 Ohm resistor, giving a 5.4uA current flowing through and a switching speed below 50us according to the datasheet.

Answer 1

You've run into a common problem. For a large number of switched input-outputs, the common industry solution is analog switching/multiplexing. The Maxim part you picked is from that category. These devices offer no isolation, and you don't seem to need it. They offer some of the fastest t(on) t(off) achievable (some under 100nS), and are reasonably priced compared to the opto-isolated solid-state relays. They will need the Vdd and Vcc supplies that encompass your AC signal with 0.3V overhead.. If your Vcc and Vdd is +15 and -15, your AC signal cannot exceed +14.7 and -14.7 V respectively. The power requirements are usually quite low, in the fractions of uA. You will also find that the 8xSPST devices often need some type of serial communication interface, while direct control devices are limited to 4x SPST. For a system with 64 controlled outputs, a serial control might be a good idea, even if it's just using serial-in parallel-out shifters. If your outputs don't have to be on at the same time, there are 1x32 multiplexer devices as well.

I think for your application, developing the proper supply rails would be well-worth the benefit of using the analog multiplexers. They are designed for this exact purpose.

Thank you @tobalt for bringing it up, I have forgotten about the "Beyond the Rails"(TM) line from Maxim that has integrated charge pumps to operate from 3.3 or 5V while internally still achieving those higher voltages necessary to pass through your AC signal amplitude.

Edit2: in the comments after some additional discussion, the solution that was accepted as the answer was: MAX335, octal SPST, tie together all the COM(X) lines, and it has a built-in shift register for control. Just get it the correct rails, which can be literally one part such as RB-0515D.

Answer 2

The AQS221N2S are rated at low diode current of 5mA and perform far better at 20mA. It seems from the Ton vs If curve is inversely related so you can improve the speed at 20mA. The nominal Ton/If = 50us/mA so at 20mA the Ton= 2.5us nominal for the conditions specified. Thus you have 5% of your goal nominal.

You can adjust \$I_f\$ to suit the ambient range and device tolerance margin you need. But the speed is adequate with more drive If current.

https://www3.panasonic.biz/ac/e_download/control/relay/photomos/catalog/semi_eng_rf4a_aqs22_s_cr10.pdf

Answer 3

This is not a full answer but might solve part of your problem with optoswitches, and is too long for a comment.

A neat little trick to speed up optoswitches is: Place a capacitor in parallel to the series resistor.

• Dimension the resistor so the resulting current keeps the switch barely closed. That way the turn off time will be fastest.
• Then add a cap in parallel to the R to allow the full short circuit current of the driver during turn-on which will speed up turn on.