A user's guide to compensating low-dropout regulators

The explosive proliferation of battery-powered equipment in the past decade has accelerated the development and usage of Low-Dropout voltage regulators (sometimes referred to as PNP voltage regulators). Compared to the standard NPN regulator, the Low-Dropout Regulator (LDO) can hold its output voltage in regulation with much less voltage across it: the NPN requires about 2 V of "headroom" to regulate, while the LDO will typically work with less than 500 mV of input-to-output voltage differential. The reduced input voltage requirement of an LDO is advantageous in battery-powered systems, since it translates directly into fewer battery cells, less wasted power, smaller size, and lower cost. The LDO's higher level of performance requires the use of external compensation capacitors, something an NPN regulator does not need. Also, the capacitors must be carefully selected for electrical parameters to assure stability. This article will develop an overview of LDO/NPN regulator theory, focusing on loop compensation and the parameters affecting regulator stability.