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I have designed a DC/DC converter (based on Zeta technology) that will generate a split supply with a single buck controller and coupled inductor.
The controller's GND pin is the negative output. Thanks to the inductor & diodes configuration, the GND pin of the controller, called "HVM" will start from 0V and drift towards the negative regulation point.

Circuit topology

This controller has an EN pin with a threshold of 1.2V, and an internal pull-down of 1µA.

When tying EN to Vin, my circuit works perfectly.

I would like to drive the EN pin from a digital GPIO "Enable" (LVCMOS 1.8V)

Here are my design goals:

  • Low area is the main criteria.
  • Low cost is the second.
  • Do not use the 1.8V supply as reference. Only Vin, GND and HVM are available.
  • When "Enable" is low or unpowered, the circuit should not be active. (EN = HVM)
  • When "Enable" is high, the circuit should activate. (EN = 1.2V minimum above HVM)

To have low-cost & low area, I was planning on using BJT.

My first attempt with a single PNP transistor failed:

single PNP transistor level shifter

It works as soon as the power supply is started and HVM is sufficiently low. However, at cold start, the EN pin will not raise more than 0.6V above ground due to the diode between PNP's collector and base. So the circuit never starts.

Any ideas on how to design this with less than 4 transistors?

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  • \$\begingroup\$ If you don't mind a boutique part, it seems you have a very good answer to your question. If you don't want a boutique part, it still seems you have a good answer as you can build that with commonly available parts and it is less than your max of 4 BJTs. Do you have any remaining questions or trouble with the answer, as given? \$\endgroup\$
    – periblepsis
    Commented Jul 5 at 23:19
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    \$\begingroup\$ Yes, the answer from @Jens is exactly what I needed. \$\endgroup\$
    – PhM
    Commented Jul 6 at 13:04

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One tiny (2.2 x 2.2 mm) SOT363 IC DCX114YU from DIODES will solve this:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Hi, Exactly what I was struggling to draw! Thanks a lot. I will double check the specs (Vce max etc) and long term availability of these transistors using integrated bias resistors, vs a simple dual NPN-PNP couple + a bunch of 0201 resistors around. Thanks a lot ! \$\endgroup\$
    – PhM
    Commented Jul 6 at 13:03

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