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.
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:
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?