Calibration of up and down charge-pump currents using a sample-and-hold circuit during idle times

Abstract

A charge pump for a phase-locked loop (PLL) has accurate matching of charge and discharge currents applied to the PLL's loop filter. A variable current-sink transistor has its gate-to-source voltage adjusted to match a source current from a fixed current source. An intermediate node in-between series transistors between the current source and sink is sampled by a sampling transistor that connects the intermediate node to a sampling capacitor. The sampling capacitor's voltage is the gate-to-source voltage of the variable current-sink transistor. The variable current-sink transistor has its gate and drain coupled together through the sampling transistor during calibration periods when the charge pump is otherwise idle. When the source current exactly matches the sink current, the gate-to-source voltage stored on the sampling capacitor reaches steady state. Up and down currents are balanced in driver transistors that match the series transistors.

Description

technical field [0001] The present invention is related to phase-locked loop (PLL) charge pump technology, and in particular to sample-and-hold charge pumps. Background technique [0002] Digital systems often rely on precise clocks and data transfers synchronized with the timing of operations. Crystal oscillators are often used to generate a base frequency clock, which can then be divided or multiplied to generate one or more frequencies depending on the desired frequency. An external clock can be used as an internal clock after frequency division or multiplication. Usually the clock is generated from the oscillator output by using a phase-locked loop (hereinafter referred to as PLL). A PLL is a unit circuit widely used in today's digital systems. [0003] figure 1 Shown is a typical PLL. Phase detector 10 receives a reference clock from an external oscillator or clock source. The phase and frequency of the reference clock are compared with the feedback clock generate...

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