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I'm designing a dev board for 3.3V parts. On the dev board, I will have a header for an "FTDI Friend" USB-to-Serial converter.

The FTDI part can supply Vcc to the target board.

However, FTDI parts are available both in 3.3V and in 5V versions. I want to protect my board against someone accidentally using a 5V FTDI and feeding that into Vcc.

What's a cost-effective strategy for this?

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    \$\begingroup\$ Have a look at this article, particularly the section that begins, "Consider the 3.3V protection circuit". \$\endgroup\$ – David Schwartz Jan 27 at 1:47
  • \$\begingroup\$ LDO 3.3V regulator (some have dropouts in the tens of mV and are not expensive)? You might need something else to deal with the 5V UART signal (maybe just a series resistor is sufficient). \$\endgroup\$ – Spehro Pefhany Jan 27 at 7:40
  • \$\begingroup\$ Finding an LDO with an enough small dropout to be safe may take time. There are thousands of them so to speak. The problem is what happens when the voltage is just under Vout + Vdrop. How would the board react with 2.9V? I don't think an ideal LDO exists. You also need at least 500mA in this case. IMO it's best to have a dual circuit, one for 5V and another directly in 3.3V. \$\endgroup\$ – Fredled Jan 27 at 22:57
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Here is the type over voltage protection circuit I would use if it was essential to protect against a greater than 3.3V input. This circuit uses a low cost voltage comparator to compare a 2.5V reference voltage against a divided copy of the input voltage and then will turn off a P-channel MOSFET when the input voltage goes above 3.3V.

You can change the reference voltage part to some other similar part at another voltage if needed and the scale the voltage divider resistors accordingly. One example of a low cost reference is a TLV431. A low voltage Zener diode could also be used but the precision of the circuit would be lost and the voltage divider would have to be tweaked to allow for greater margin above the 3.3V level.

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  • \$\begingroup\$ Combining a zener with a mosfet risks having the gate in the intermediary state, when it's conducting but with some impedence and it can be very bad. I have build a ciurcuit using a zener with two mosfets, one triggering the other, a bit like a darlington. It reduced probability to reach an intermediary state enough to be safe. But it was for a much higher voltage. Not sure if it's apllicable for 3.3V. So, IMO the comparator is necessary. \$\endgroup\$ – Fredled Jan 27 at 22:28
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Ti has an over-voltage app-note utilizing the LMV431 (cheap voltage reference that can be used like a comparator) with a P-Channel MOSFET to block input when Vin is greater than your chosen threshold. The part is available unencrypted, so you could use a free tool like LTspice or Tina to tweak values.

You could probably delete a few of the zeners and resistors that limit Vgs on a 5V implementation, compared to the automotive (12V bus with 60V excursions) type situations the app-note was originally targeted at: TLV431 app note

It would probably cost 25-40 cents to do.

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  • \$\begingroup\$ Thank you! I'll look into that. My analog skills are a bit lacking so it'll take me some time to learn how it all works. \$\endgroup\$ – Nevo Jan 27 at 13:25
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By combining the regulator with a voltage comparator, a P-mosfet and a voltage regulator, you can have both the protection and keep it functional at 5V or at 3V. (For intermediary voltages data sheets for each part has to be examined carefully and additional or other components may be needed). Note: I didn't test the present circuit yet.

schematic

simulate this circuit – Schematic created using CircuitLab

When the voltage is under the voltage comparator threshold the gate of the P-MOSFET M1 is at 0V (or GND level) and the MOSFET conducts. The 3.3V regulator has no effect. Current flows normally. When the voltage is above the comparator threshold (e.g. 5V) its output turns high and the P-MOSFET stops conducting. Then the current goes through the voltage regulator and Vcc receives 3V.

The regulator must have a drop out voltage of less than 2V. R1 is necessary to bring current to the MOSFET gate with the BD48 serie because it's open drain. There are other variants in the market. This is only one non-exclusive example.

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