open-source-silicon.dev

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

<https://www.researchgate.net/publication/224647766_Fast_startup_CMOS_current_references>

image.png

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>

1.png

<@U02G2L5VBRC> 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.