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How to test the PLL charge pump current?

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Newbie level 4
Oct 13, 2004
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I want to test the PLL charge pump current,The PLL loop filter out of the can I test the CP current?

Well, it all depends on how your chip is designed. Assuming it is an up/down combination output terminal, what I do is try to find the "average" voltage of the charge pump output circuit. If you are running off a 5 volt supply, it will be somewhere around 2.5 volts. I would them put a resistor (maybe 1 K) from the charge pump output to a well bypassed supply at that "average" voltage, and use a scope probe to measure the voltage drop across the resistor. Voltage across the resistor is proportional to charge pump output current.


there is another way to check this. connect the intended loop filter capacitor at the output of the charge pump. first check for charge pumping in action by keeping the intial voltage at the output as the minimum possible value (usually 0v) and turning on the pmos switch. use the formula i/c=dv/dt to check for how much time the intended voltage change takes place. then check for charge removal action by keeping the initial voltage at the output as the maximum possible value and turn on the nmos switch. use the above equation to verify the working. i used this method for verifying the charge pump action.


To monitor your CP current , connect your CP output to a ideal voltage source; set the value at the average VTUNE of the VCO and monitor the current in this source. you can then use a STEADY STATE ( PSS in spectre) anaysis, which will allow also to run noise analysis.

---------- Post added at 09:37 ---------- Previous post was at 09:36 ----------

the ideal voltage source emulates the loop filter for the charge pump

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