Need Analog/digital Switch for CAN bus

Dear Govind,

Strictly, there's few analog switches which are fully qualified for the CAN transceiver SN65HVD232 because as the parameter of SN65HVD232 says,

1. Voltage at any bus terminal can be -2.5~7.5V-- That is to say the analog switch should be dual powered , as high as 7.5V and as low as -2.5V.

2. Output current as high as 48mA, double direction-- That is to say the analog switch has the ability of high On current and low on-resistance.

3. 1Mbps data rate-- That means the analog switch should be high speed switch. Usually high speed means low On current.

4. Input current 30uA-- That is to say when analog switch shut off, the Off resistance should be much larger than 3.3V/30uA

5. withstand common-mode transients of +/- 25V

6.ESD HBM 16kV, CDM 1kV

There's nearly no analog switch which has all of the six features listed above. So there must be a device which is best suitable, even though not satisfied, for your application. You can measure each of the features listed above on your bus. For example, voltage on the bus is always within 0V and 3.3V, you can use a 3.3V powered analog switch other than an analog switch of over -2.5~7.5V. Of course, as a test equipment, protection is necessary.

Still confused? So suggest you use TS3DS10224. It is a bidirectional analog switch, which may be useful in the CAN bus. You can refer to the datasheet to confirm whether it is suitable for your application.

Thanks!

Vio Liu

Dear Vio Liu,

Thank you very much for clear explanation. Based on your inputs, i have gone through data sheets of few more switches. I found one Digital switch: SN74CBT3306C which is having same specs but low ON resistance (For 5V supply), little higher Switch current (128mA Max) and lower ON capacitance (6pF). Only concern i have is: it is FET Switch, which is different from analog switch. Will it be OK for CAN Bus...??
And bandwidth of the switch is not mentioned in datasheet. Only confidence i have is, one application area mentioned as USB interface, so 1Mbps should be Ok (??).
Can i use this switch..?? Kindly suggest.

Regards,
Govind.

Dear Govind,

Sorry for replying a little late.

SN74CBT3306, or bus switches, usually have better features than analog switches. If you have a glance at the analog switch catalog and the bus switch catalog, you can find that bus switches usually have lower ON resistance, higher current, lower ON capacitance, also, lower price.

That is because analog switches are a little different from bus switches. Bus switches are designed to isolate some devices from the bus with a signal so that the bus will have lower capacitance and of course, bus access time and noise can be reduced for the other devices on the bus.

You can see the internal circuit in the datasheet of 3306 as following:

We can see that there's only one FET. Usually it is a N-FET. So something should be taken into consideration with only one FET---If the input signal rises, the Vgs of the FET will become smaller. Once Vgs is smaller than Vt, the signal cannot come to the other end. That's the difference from analog switch. For an analog switch, there would be a P FET for this situation so that higher voltage can get through. P-FET has lower performance with the same price, so that's why bus switch has better performance in some aspects.

Here's a parameter from analog switch TS5A9411. You can see the full range of signal can go through.

As for your application. I think the device is just OK for CAN switch because there's no such problem. Usually the signal of CAN is within 0V~3V, and it is lower than the power supply of 3306, 5V. But there's still two points you need to consider:

1. The unexpected common mode voltage. If common mode voltage changes, probably the switch would work. 

2. SN74CBT3306C is a SPST switch, though it has two channels. Better to find a dual SPDT switch, and it would be more suitable for your application.

Hope it helps.

Thank you!