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CDCM7005 PRECP settling time

Kazuhiko Hirano
Genius 15118 points
Other Parts Discussed in Thread: CDCM7005
Hello,
 
A customer would like to use PRECP bit at CDCM7005 to shorten lock time.
How much time does it take CP_OUT to get VCC_CP/2.
In other word, how much time users have to wait to make PRECP bit "0" after making PRECP bit "1"?
 
Best regards,
 
K.Hirano
  • 0
    TI__Expert 4150 points

    Hi Hirano-san,

    I think the time to charge to Vcc/2 would be roughly:

    (C1+C2+C3) * Vcc / (2*Icp)

     

    You could simulate this by driving a current source into the loop filter in a Spice simulation.

     

    Best regards, Fritz

  • 0 in reply to Fritz
    TI__Genius 15118 points
    Fritz,
     
    Thank you for your response.
     
    However, I would like to confirm one thing.
    I thought Charge Pump circuit is disconnected from CP_OUT pin and other circuit charge the pin when PRECP bit = 1 to shorten PLL lock time.
    But, even if PRECP bit = 1, the Charge Pump circuit charges CP_OUT pin with Icp?
     
    Best regards,
     
    K.Hirano
  • 0 in reply to Kazuhiko Hirano
    TI__Expert 4150 points

    Hirano-san,

    you probably have a valid comment here. Let me check with the product expert and get back to you. BR. Fritz

  • 0 in reply to Fritz
    TI__Mastermind 20900 points

    Hirano-san,

    you are right, if PRECP is set to '1' the CP circuit is switched off. A 1kOhm resistor to VCC_CP and a 1kOhm to GND will force the voltage at pin CP_OUT to VCC_CP/2. This results in a constant current which charges the caps of the loopfilter. After the loopfilter caps are charge you can turn off the PRECP bit to '0' to be able to achieve a lock condition.

    Best regards,

    Julian