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Need Current Source with User selectable current and voltage compliance

 

Having issues making a current source with limited voltage compliance for an ideal charge pump in a PLL circuit. The methods of clamping the Source max Vc using a diode and voltage source or Zener diode create spikes which disrupt the loop. VCCS or 2 terminal G source would work. 
 
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I uploaded Voltage-Clamped-Current-Source.asc which has four five (three four, really) examples of adding a voltage clamp to an ideal current source.  It is not an algorithmic approach, but it does the job.
 
I don't know why you got spikes with your diode and voltage source method.  Did you use the ideal diode "D", or a real diode model?
 
Andy
 
 
 
 

On 26/12/2024 02:10, Tom via groups.io wrote:
Having issues making a current source with limited voltage compliance for an ideal charge pump in a PLL circuit. The methods of clamping the Source max Vc using a diode and voltage source or Zener diode create spikes which disrupt the loop. VCCS or 2 terminal G source would work. 
Did you try enabling "This is an active load" in the current source properties?

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Regards,
Tony

 

On Thu, Dec 26, 2024 at 08:08 AM, Tony Casey wrote:
Did you try enabling "This is an active load" in the current source properties?
I thought that did something different (shutting off the current source as the voltage approaches zero).  Is there a different explanation?  Can it throttle the current when the terminal voltage exceeds some preset value?
 
Andy
 
 

I didn't use the "ideal" diode so even the minuscule capacitance was causing issues. The LTspice ideal diode is very similar to the XSPICE simple diode code model.
 

LTspice has a few kinds of "ideal" models.  I used both kinds.
 
If you used any "real" diode (one with a part number), there's chances of a few factors that could affect results.
 
The ideal diode "D" follows the Shockley diode law, so it is actually pretty close to an actual silicon diode's behavior, but it lacks capacitance and bulk (ohmic) resistance.  That might be what affected your results.
 
Andy

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