I2C/TWI controlled Darlington array?

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Has anyone ever seen something like this? I'm doing some Google searching right now, but there is a pile of bad info to sift though.

The reason I'm looking for one is to control relays already present in a car.

Besides, what a great way to increase output, just hang up to eight (assuming they have a0..a2 pins) of these, each with eight or more outputs per device!

Can somebody in the industry make me one?

"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools."
-- Douglas Adams

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It is not clear what you are asking but if you want to use serial spi to control relays look at the Texas Instruments TPIC2603. It will handle 6 devices per each chip and is setup to switch relays, injector solenoids , etc.

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What I want to do is simply add my own switch in parallel with existing switches to drive the necessary relays.
I assume a darlington pair will be able to do this, as they typically can handle 50 or so volts and a good amount of current.

that TPIC2603 might just do the trick, but I'm not a fan of SPI as it requires a /CS signal.

"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools."
-- Douglas Adams

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On Semi makes lots of part like you are looking for including Darlington arrays.. http://www.onsemi.com/PowerSolut...

You should identify the highest current requirement for the devices you want to control.

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What I'm asking for is simply this: a Darlington array controlled by an I2C interface instead of the usual parallel inputs.

This would make it possible for a single, small uC to control many heavy duty devices (DC/stepper motors, relays, lamps, etc.) without lots of parallel I/O.

On the other hand, I suspect modbus or canbus would be more appropriate as they allow much longer distances.

"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools."
-- Douglas Adams

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You will have to do your homework! I found this part with a quick search. You may want to consider using the parts at On Semi along with a part that does an I2C to 8-bit port. Otherwise you have to see if a part like the maxim below meets your needs or look for one that does..

MAX4820/MAX4821 integrated relay drivers. These devices offer eight independent channels with built-in kickback protection, and drive 3V or 5V single-coil nonlatching, or dual-coil latching relays. Each independent open-drain output features a 2 on-resistance and is guaranteed to sink 70mA (min) of coil current. Both devices consume less than 50µA (max) of quiescent current. These relay drivers are ideal for ATE, central office, and industrial equipment.