I have a PLL that is operating unstably at some temperatures. I have been able to show that reducing the charge pump current from 128uA to 64uA ensures that the PLL will operate stably at the same temperature point where it would otherwise be unable to lock.
Rather than have a software based solution I would like to achieve the same result (a locking PLL across my temperature range) by changing the loop filter components.
----+----+---- | | Cs --- --- Cp --- --- | | Rs \ | / | \ | | | ----+----+--- Please excuse the diagram, not access to imgur.
Since halving the charge pump current was sufficient to stabilize my loop would doubling the loop filter capacitance Cp (shown above) provide the same result. Currently my component values are Cs = 3300pF, Rs = 6.81kOhm, and Cp = 33pF.
$$ \Delta V_1 = \Delta V_2 = \Delta V, $$ $$ i_1 = 2i_2, $$ If the period of time where the charge pump is conducting (phase difference) is fixed then $$ Q_1=2Q_2, $$ $$ \Delta V = \Delta Q/C $$ As a result the doubling of the value of C with the same charge pump current is effectively the same as halving the charge pump current and leaving C fixed.
I realize that this will change the loop filter bandwidth as well as impact the settling time. Is there a way of getting a rough estimate of how this will impact the settling time? I was hoping to get an answer as a delta from the existing implementation (e.g. the settling time will double.) What other impacts might such a change have to the PLL's behaviour?