Hi,

If you want to simulate the start-up behavior, you should try transient simulation. DC simulation only shows the sensitivity of the bandgap output to DC (or very low frequency) power supply variations, not the time-domain start-up.

You don't say what is the output when you do the DC sweep. It should look reasonable, at least when the start-up circuit is implemented.

Paul

Hi,

do you ramp the supply voltage? These circuits usually start up even without a dedicated start-up circuit, if you apply the full supply voltage at t=0 in your simulation, the operating point calculated by the simulator will usually correspond to this. Try starting your simulation with Vdd=0, then ramp it to its nominal value. Apply quite a fast ramp to see the start-up time of your circuit and not the rise time of your supply voltage.

You should also try to add a couple of estimated parasitic capacitances to the critical node, only then you should see a difference with/without the start-up circuit. As most of these circuits, when properly designed, start up by themselves based on leakage components, the goal of the start-up circuit is to make sure they do so over PVT variations, mismatch and so on, and also to do it in a convenient time.

Paul