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LTC6948-2 unlock

 i use register as follow,but it does not lock , the ref is 40M,pfd is 20M  output frequency is 985M,integer mode , how can slove this problem

LTC6948_write(0x01,0x04); 
LTC6948_write(0x02,0x0c); 
LTC6948_write(0x03,0Xbd); 
LTC6948_write(0x04,0X28); 
LTC6948_write(0x05,0X00); 
LTC6948_write(0x06,0X10); 
LTC6948_write(0x07,0Xc5); 
LTC6948_write(0x08,0X00); 
LTC6948_write(0x09,0X00); 
LTC6948_write(0x0a,0X00); 
LTC6948_write(0x0b,0Xbc); 
LTC6948_write(0x0c,0X4f); 
LTC6948_write(0x0d,0Xe4); 



add register data
[edited by: FreedomSabre at 12:47 PM (GMT 0) on 26 Dec 2019]
  • PDF

    Hello,

    CPRST tri-states the charge pump and CPMID forces a mid-supply voltage on the charge pump. Both CPRST and CPMID should be set to 0 to allow the charge pump to operate properly.

    Also, if you are using the LTC6948-2 demo board, its loop filter was designed for 50M pfd frequency.  If you haven't redesigned your loop filter for a  20M pfd, please see the attached pdf.  This will show how to design a loop filter in the FracNWizard.

    Chris