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I need to design a circuit that can take a digital output from an MCU (3.3V) and amplify it to a selectable output voltage. The output voltage should be configurable between 3.3V, 5V, 12V, and 24V.

Additionally, the output must be able to operate in both push-pull mode and open-drain mode.

My current idea is to use two NMOS transistors with a gate driver. In push-pull mode, both the high and low inputs of the driver would be used, while in open-drain mode only the high or only the low side would be driven. The output voltage would be selected by switching between the supply rails (3.3V, 5V, 12V, 24V).

Would this approach work? What are better ways to achieve this?

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    \$\begingroup\$ What current range (min/max, +/-)? What transition time / edge rate? \$\endgroup\$ Commented Oct 2 at 15:33
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    \$\begingroup\$ Do you already have all those rails available? Or do you mean to DC-DC the needed rail, as well? \$\endgroup\$ Commented Oct 2 at 16:04
  • \$\begingroup\$ I have those rails already available. The system should be able to deliver the following outputs: 0-3.3 V @ 100 mA, 0-5 V @ 100 mA, 0-12 V @ 50 mA, and 0-24 V @ 20 mA. \$\endgroup\$ Commented Oct 7 at 12:13

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You could use half a TMUX4052 operating from 24V/0V and a single n-channel MOSFET. Plus a dual NAND gate.

Use half the mux to select from 3.3/5/12/24V (break before make switching) with the two logic-level address inputs.

schematic

simulate this circuit – Schematic created using CircuitLab

So you have four 3.3V logic level inputs (assuming the NAND is powered from 3.3V), A0 & A1 (high side voltage select), Input and /OD (low for open-drain).

A practical design would also have to consider ESD, transient conditions such as start-up and shut-down, and short-circuit protection.

Also whether it is wise to have something that can source 24V if misconfigured or during transient conditions connected to the user's 3.3V logic.

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If your current requirements are reasonable, you could use an op-amp to amplify the MCU output. Here's an approach that uses a selectable gain (simulate it here):

enter image description here

Notes:

This circuit literally amplifies the input signal, so it is sensitive to the input swing.

The voltage select works 'thermometer' style so only one signal changes at a time. That is, 000 = 3.3V, 001 = 5V, 011 = 12V, 111 =24V.

Adding discrete FETs allows control of high and low-side drive separately (simulate it here):

enter image description here

In this case it's not sensitive to the signal level. If you need more high-drive current, you could consider a linear regulator in place of the op-amp, or add a pass transistor to boost the output.

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  • \$\begingroup\$ The system should deliver the following outputs: 0-3.3 V @ 100 mA, 0-5 V @ 100 mA, 0-12 V @ 50 mA, and 0-24 V @ 20 mA. \$\endgroup\$ Commented Oct 7 at 12:12

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