Aspects of the disclosure provide for a circuit. In some examples, the circuit includes a Zener diode, a first current source, a first n-type field effect transistor (FET), a first inverter circuit, and a second current source. The Zener diode has a cathode coupled to a first node and an anode coupled to a second node. The first current source has a first terminal coupled to the second node and a second terminal coupled to a ground terminal. The first n-type FET has a gate terminal coupled to the second node, a source terminal coupled to the ground terminal, and a drain terminal coupled to a third node. The first inverter circuit has an input coupled to the third node and an output coupled to a fourth node. The second current source has a first terminal coupled to a fifth node and a second terminal coupled to the third node.