I2C switch / output expander with Open Drain

Question

I'm looking for an I2C expander or switch that can put output to open drain.

Should be

- 5V tolerant

- at least one output that can be driven low and high-Z

- if possible SOIC package

- no isolation needed

- a few mA is enough.

- does not need to be bidirectional. Output only would be ok.

Is there a good candidate?

rachaelp · Accepted Answer

I've used TI port expanders from the PCA95xx range in the past and they seem quite good. How about this one? http://www.ti.com/lit/ds/symlink/pca9557.pdf

P0 is open drain, P1 through P7 are push-pull.

Best Regards,

Rachael

hlipka · Answer

Microchip has the MCP23009 (8 bits) and MCP23018 (16 bits). If you need higher voltages, just use the regular versions and an open drain driver (e.g. the venerable ULN2803, or something other from Microchip while we are at it...)

Robert Peter Oakes · Answer

I dont think anything more than we already have, all the op-amps have suitably high resistors to protect them and a diode off board should be across the sense resistor, maybe two could be there (Back to back), the FET already has one and as already suggested, a fuse should also help. we just dont want too much in the power path in order to still handle high currents with low voltage

rachaelp · Answer

Or the NXP equivalent from here? PCA9557D NXP, I/O Expander, 8bit, 400 kHz, I2C, SMBus, 2.3 V, 5.5 V, SOIC | Farnell element14

Robert Peter Oakes · Answer

btw, just simply leaving it disconnected will allow the system to work, it won't drive the output on if either of the other two connections are still low

jc2048 · Answer

Erm, what's this for exactly? If you're pulling down the gate of the output MOSFET, it needs to tolerate more than 5V. The nice power MOSFETs that Peter found and wants to use are not logic-level devices and the gate will be working up at 6 or 7V.

A small MOSFET controlled by one of the regular outputs of your expander would be better.

jc2048 · Answer

Don't make it too small - I can just about manage Small-Outline-Transistor-number-23 (SOT23) packages. Either a MOSFET or a BJT would do. BSS138 has a low enough threshold voltage to work well with a 3.3V gate drive. Perhaps include a pull-up resistor on the gate so that it is held on at reset (disabling the load) until the processsor is ready to start controlling everything. Looking at that circuit fragment, you might want to lay out your board so that it can be built without the 6V zener there. (Although you could just link across the pads, it would be neater, and easier to document, with a parallel placement for a zero-ohm link as an alternative build.) The circuit won't work if you leave the zener in there with the MOSFETs Peter wants to use. (That's not quite true, because the gate voltage slowly drifts up to the threshold on the zener leakage, but it is true to say the circuit won't operate the way you'd like it to.)

jc2048 · Answer

Yes, use one of the push-pull outputs.

I2C switch / output expander with Open Drain

Top Replies