The patent badge is an abbreviated version of the USPTO patent document. The patent badge does contain a link to the full patent document.
The patent badge is an abbreviated version of the USPTO patent document. The patent badge covers the following: Patent number, Date patent was issued, Date patent was filed, Title of the patent, Applicant, Inventor, Assignee, Attorney firm, Primary examiner, Assistant examiner, CPCs, and Abstract. The patent badge does contain a link to the full patent document (in Adobe Acrobat format, aka pdf). To download or print any patent click here.
Patent No.:
Date of Patent:
Jul. 17, 2001
Filed:
Aug. 25, 1999
National Semiconductor Corporation, Santa Clara, CA (US);
Abstract
A voltage sample and hold circuit for use as part of a low leakage charge pump circuit in a phase lock loop (PLL). During an inactive state of the charge pumping function, a MOSFET switch that normally connects the charge pump output to the loop filter preceding the voltage controlled oscillator (VCO) of the PLL is opened, e.g., for open loop modulation of the VCO. Meanwhile, the sample and hold circuit which has sampled the voltage at the input side of the MOSFET switch now maintains that voltage, thereby forcing a zero-voltage difference across the MOSFET switch. This zero-voltage difference virtually eliminates subthreshold leakage current through the MOSFET switch, thereby significantly reducing loss of charge in the loop filter due to such leakage current. This ensures a significantly more constant DC bias at the input to the VCO and, therefore, a more stable output center, or carrier, frequency from the PLL during open loop modulation.