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Design >> Mixed-Signal Design >> charge pump current and loop filter resistance in CPPLL
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Message started by heaven_wb on Aug 19^th, 2009, 12:40pm

Title: charge pump current and loop filter resistance in CPPLL
Post by heaven_wb on Aug 19^th, 2009, 12:40pm

I am designing a integer N, second order CPPLL. The loop bandwidth is 100 KHz. the charge pump current is 300 uA, the resistance in loop filter is 12 K. Then the strange problem occurs. the voltage dropping on the resistance would be 3.6V( 300 uA * 12 K ) when charging the loop filter capacitance, but the power supply is just 1.8 V. Who can tell me how to understand this problem? I also see this situation in some paper. I don't know if its CPPLL can work. Thank you very much.