DAC8760: Possible to protect it from accidental supply connection?

Gabriele,

The answer to this depends on how you have the device set up in the circuit. First, will the AVDD be at one of these power supplies? Will it be at 12V or 24V? If it is at 12V, would a 24V connection be possible?

If the device is only used in current output mode, then you may be able just to use a diode to source the current. If the output is pulled high then the diode will be reverse biased, blocking any input current. If the voltage output is used, then the protection would need to source a sustained current through ESD protection diodes and some series resistance to lower the current.

How do you intend to use the device? Can you show a basic schematic?

Joseph Wu

Thanks for your answer, Joseph.

You can see the circuit in the picture below.

VDD is the power supply taking after reverse polarity protection with 1N4148. VDD is in the range of 10/32V (+12V/+24V system).

I know there are not any protection to the output AOUT. I'm working to redraw the circuit, because that product is obsolete.

Meantime I would like to protect the output AOUT with additional components outside of our board.

I can add on the TVS diodes between AOUT line and GND line across the cable. However, if anyone incorrecly connect power supply directly on AOUT, the outputs IOUT and/or VOUT of the DAC are damaged. How can I protect that output from accidental connection until I have redraw the board?

Gabriele.

Gabriele,

Offhand, I don't have a good solution for you. Most over-voltage solutions that I know of use two combined methods to protect the device. First there is something holds the input voltage. This would be a TVS that turns on at a particular voltage or a forward based diode that connects to supply that prevents the input voltage going far above the supply voltage. Second, there would be series resistance that reduces the amount of current going into the diode or TVS. The use of both methods shunts current away from the device and protects the circuit when there is an over-voltage event.

If the over-voltage is not sustained or if the current is limited, then you might not even need the resistance. However if the supply current limit is very high, then you would need reduce the current by inserting a large resistance.

Adding a large resistance might be ok for a current output with large enough headroom on the supply, but this would likely be a problem for voltage outputs where the voltage drop in the circuit would cause a large output error.

Note that varistors may be helpful for overvoltage protection and I've seen them used for protection in a variety of circuits. These are variable resistors where the resistance start off low, but increase in value when the through current increases. They can be used as the resistive element that limits the current, but the thermistor must be fast enough to prevent damage from occurring.

Recently, I have seen an active protection device for these types of parts. There is the TPS26611, which is a 50V, 4-20mA current loop protector with input/output miswiring protection. This is a rather new device used for current loop protection. I haven't really used this device yet, so you may need to post in another form for information.

Joseph Wu

Hi guys.

I think about another solution to protect the DAC8760 voltage and current output from customer miswiring.

The problem is that the customer does not want to apply the connector at our sensor's cable. Therefore it is possible to mistakenly connect the cable.

The cable has three terminal: positive power supply +VBATT (from 10V to 32V), GND and DAC8760 voltage&current output.

The board is protected from power supply reverse polarity by the series diode apply on +VBATT.

Now the problem is: how can I protect the DAC output from customer miswiring in all possibile combinations?

I think is a good idea to introduce an analog switch after the DAC output (TMUX7219). I would connect 0-10V out to S1, 4-20mA out to S2 and D to the board output. So I place the TVS to the board output to protect the TMUX7219 from the ESD events.

My question is: what happens if the customer apply, for example, +VBATT to the board output (pin D of TMUX7219) and, for example, output cable to +VBATT pin board? And in the other cases?

What happens if the TMUX7219 is not powered and, for example, a voltage is applied to the output? Is it possible the voltage pass through the analog switch and is applied to the DAC8760 output.

My second question is: is it mandatory to use a 15 Ohm resistor in series to the DAC ouputs? I don't know why the datasheet report that resistor value. Do you know a formula to calculate this value? Is it possible to change that value and increase it from 15Ohm to 500-600Ohm?

BR,

Gabriele.

Gabriele,

The 15Ω resistor comes from the protection that was put together for a TI Design. There is a link below for the TI Design folder, and a link for the writeup of the TI design.

www.ti.com/.../TIPD153
www.ti.com/.../tidu011

The 15Ω resistors help with protection for EMI/EMC, ESD, and Electrically Fast Transient events. In short transients, they can help reduce the instantaneous current going to the device. In other cases, it helps filter the effects of EMI/EMC. We feel that this configuration is important for device protection. Larger resistors may change the headroom of the output or change the gain error (if placed at +VSENSE).

There is another design that has some more robust version of protection, but I don't think it's robust for continuous overvoltage protection either. Just in case, I'll put up the same for this TI design.

www.ti.com/.../TIDA-00559
www.ti.com/.../tidubk2

As for the TMUX7219, I'm not very familiar with the device. Based on my reading of the datasheet, overvoltages will not latch up the mux based on it's silicon on insulator process, but there will still likely be some sort of diode to supply that's able to shunt the current from an overvoltage event. However, this depends on the supply of the TMUX7219 and it's relation to the SUPPLY of the DAC8760. Again, this isn't always helpful for continuous overvoltage protection. You would still need to use some sort of TVS to clamp the overvoltage and you'd need series resistance to reduce the current going into the device. I would post in the Switches and Multiplexers Forum for more details about the TMUX7219 performance in overvoltage conditions.

Joseph Wu