Wai Ho Lau

62626108, Jul 17, 2001

Aug 25, 1999

9/383162

Steve Lo - Sunnyvale CA
Christian Olgaard - Sunnyvale CA
Wai Lau - Sunnyvale CA

National Semiconductor Corporation - Santa Clara CA

G11C 2702

327157

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.