Hello everyone, I am trying to simulate current st...
# analog-designa
Amr Samy
05/06/2023, 6:53 PMHello everyone, I am trying to simulate current starved oscillator which gets its supply voltage from charge pump that get its clock signals from ring oscillator. ring oscillator & charge pump together only are working very well. The current starved oscillator works as well when i use a voltage source as supply voltage. but when i use the output voltage of charge pump as the supply voltage for current starved oscillator. it behaves like this. anyone have an idea what could be the problem ?
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Stefan Schippers
05/06/2023, 7:52 PMConnect the current starved oscillator to a voltage supply of voltage
VxIxVx/IxStefan Schippers
05/06/2023, 8:01 PMIf this is the case you can try to reduce the charge pump output resistance by increasing the ringosc frequency or increasing the charge pump capacitors or both...
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Luis Henrique Rodovalho
05/06/2023, 11:28 PMYou're using ideal current sources connected to your charge pump output. That is why it outputs negative voltages. You should never do that. As it seems, your charge pump can't provide enough power.
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Luis Henrique Rodovalho
05/06/2023, 11:30 PMYou could use ideal current sources to a NMOS current mirror and then mirror it to your PMOS biasiing devices. This way you won't connect a ideal current source directly to your charge pump.
Luis Henrique Rodovalho
05/06/2023, 11:32 PMOne thing you should do is simulate your current starved oscillator with an ideal supply voltage. See which current the supply voltage supplies. If you have its voltage and current, you can calculate a resistance. Use this resistance as a load for your charge pump and see if it still works properly.
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Amr Samy
05/07/2023, 2:03 PM@Luis Henrique Rodovalho Thanks a lot that was exactly the problem. but i still don't understand why the ideal current source degraded the voltage like this ?!
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Luis Henrique Rodovalho
05/07/2023, 4:01 PMIdeal current sources will make the transistors output a current, no matter if it makes sense or not. That is why you should use non ideal current sources, with a parallel series resistor.
In this special case, by mirroring your biasing current, the NMOS tries to output a current to the PMOS which is connected to your charge pump output supply. It can't, of course, so the simulation converges to a point where the NMOS is in the linear region and the current mirror doesn't work as intended. Before, the ideal current source must have forced the circuit to operate outside the model ranges. Maybe all the current from the charge pump went to this single current source.
Anyway, you need a solution. You must increase the transistor sizes from the charge pump and the charge pump driver. Sooner or later you will find the right dimensions.
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