National Semiconductor Corporation

Steve Lo

Christian Volf Olgaard

Dennis M Rose

Wai Lau

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.

Voltage sample and hold circuit for low leakage charge pump

An apparatus for reducing the power consumed by, and the noise generated at the output of a charge pump which is part of a phase locked loop circuit. A reduction in the power consumed by the charge pump is achieved by synchronizing the powering up of the charge pump bias circuit to the state of the counters which control the timing of the signal which triggers a charge pump cycle. In this way current is supplied to the charge pump circuit only when needed for the operation of the pump. A reduction in the noise present at the output of the charge pump caused by the reloading of the counters while the pump is activated is achieved by loading a portion of the counter bits prior to activating the pump. A circuit detects when the counter is close to a value of zero and produces a signal which loads the most significant bits of the counter. When the counter reaches a value of zero, the remaining bits are loaded. This reduces the number of counter bits which are loaded while the pump circuit is active, thus reducing the switching noise present on the pump output.

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Sunnyvale, CA, United States of America

Steve Lo