Pll with built-in filter-capacitor leakage-tester with current pump and comparator

Abstract

A filter capacitor within a phase-locked loop (PLL) can be tested using a built-in test circuit. The PLL's charge pump is deactivated while a test-current source is activated to supply a test current to the PLL filter capacitor. When the test current is larger than any leakage currents through the capacitor, the capacitor's voltage rises above a reference voltage. A test comparator compares the capacitor's voltage to the reference voltage and signals a good test result when the capacitor's voltage rises above the reference voltage. When leakage current is larger than the test current, the capacitor's voltage cannot rise above the reference voltage and the test comparator signal a leakage failure. The test current source can share a bias voltage with the charge pump and can drive the capacitor to a voltage higher than the charge pump does to increase leakage and stress during testing.