If you have any resources for static start up

Question

< U01819B63HP> If you have any resources for static start up circuits please publish them

Luis Henrique Rodovalho · Answer

Luis Henrique Rodovalho · Answer

This is the start up You should connect the first NMOS gate terminal to the input of your amp inverting input and the drain terminal to the capacitor Cs The second NMOS should have its gate input connected to the capacitor and its drain and source terminals should be connected to the PMOS current mirror gate which is itself connected to the amp output and to the amp inverting input This make a positive feedback See these simulation results In the start the capacitor voltage Vz goes to VDD then the startup starts the circuit and Vz changes to 0 turning the start up circuit off The DC simulation works without nodeset directives You can see this code in <https github com lhrodovalho sky130 ldo>

Stefan Schippers · Answer

< Mustafa Ahmad> The specific bandgap startup circuit depends on the circuit topology This is an example the unwanted equilibrium state is when `V+ = V = Vbgap` no current flowing in the resistors and in the bipolar transistors all nodes are at a low voltage may be 0 3 0 4V depending on the Differential amplifier offset circuit may remain stuck in this state The MX MOS put in parallel with the left differential input MOS forces the DRIVE node low when Vbgap = V+ = V so the Vbgap node will be pulled high After the bandgap settles to its correct operating point Vbgap=~1 25 while V+ = V = ~0 65V so the MX transistor will be in the OFF state not disturbing the differential amplifier If for some reason the bandgap switches off again the MX will again pull the Vbgap high Note that this is a very old design on a 5V technology For 1 8V only different solutions must be adopted See the `sky130 tests tb bandgap sch` example circuit provided in the xschem sky130 that has the same startup mechanism and also includes dynamic opamp offset cancellation

Stefan Schippers · Answer

< Mustafa Ahmad>