
IDT5V49EE904
EEPROM PROGRAMMABLE CLOCK GENERATOR
CLOCK SYNTHESIZER
IDT EEPROM PROGRAMMABLE CLOCK GENERATOR
11
IDT5V49EE904
REV P 092412
PLL Loop Bandwidth:
Charge pump gain (K
φ) = Ip / 2π
VCO gain (KVCO) = 900 MHz/V * 2
π
M = Total multiplier value (See the Reference Divider,
Feedback Divider and Output Divider section for more
detail)
ωc = (Rz * Kφ * KVCO * Cz)/(M * (Cz + Cp))
Fc =
ωc / 2π
Note, the phase/frequency detector frequency (FPFD) is
typically seven times the PLL closed-loop bandwidth (Fc)
but too high of a ratio will reduce the phase margin thus
compromising loop stability.
To determine if the loop is stable, the phase margin (
φm)
needs to be calculated as follows.
Phase Margin:
ωz = 1 / (Rz * Cz)
ωp = (Cz + Cp)/(Rz * Cz * Cp)
φm = (360 / 2π) * [tan-1(ωc/ ωz) - tan-1(ωc/ ωp)]
To ensure stability in the loop, the phase margin is
recommended to be > 60° but too high will result in the lock
time being excessively long. Certain loop filter parameters
would need to be compromised to not only meet a required
loop bandwidth but to also maintain loop stability.