LDOs for Portables and Wearables

As electronic devices get increasingly smaller in form factor, they are also becoming more portable and relying more on battery power. These devices include security systems, fitness trackers, and Internet of Things or IoT devices. The design of such tiny devices demands high-efficiency power regulators that can make use of every milliwatt of power from each charge for extending the working life of the device. The efficiency of traditional linear regulators and switch-mode power regulators falls woefully short of the requirements. Moreover, transient voltages and noise in switch-mode power regulators are detrimental to their performance.

The most recent addition to switching and linear regulators is the LDO or the low-dropout voltage regulator. It lowers thermal dissipation while improving efficiency by operating with a very low voltage drop across the regulator. Low-to-medium power applications are well-served by various types of LDOs, as they are available in minuscule packages of 3 x 3 x 0.6 mm. In addition, there are LDOs with fixed or adjustable output voltages, including some versions with on-off control of the output.

A voltage regulator must maintain a constant output voltage even when the source or load voltages change. Traditional voltage regulator devices operate in one of two ways—linear or switched mode. While LDO regulators are linear regulators, they operate with a very low voltage difference between their output and input terminals. As with other linear voltage regulators, LDOs also function with feedback control.

This feedback control of the LDO functions via a resistive voltage divider that scales the output voltage. The scaled voltage enters an error amplifier that compares it to a reference voltage. The resulting output of the error amplifier drives the series pass element to maintain the output terminal with the desired voltage. The dropout voltage of the LDO is the difference between the input and output voltages, and this appears across the series pass element.

The series pass element of an LDO functions like a resistor whose value varies with the applied voltage from the error amplifier. LDO manufacturers use various devices for the series pass element. It can be a PMOS device, NMOS device, or a PNP bipolar transistor. While it is possible to drive into saturation the PMOS and PNP devices, the dropout voltage for PMOS-type FET devices depends on the drain-to-source on resistance. Although each of these devices has its own advantages and disadvantages, using PMOS devices for the series pass element has the lowest implementation cost. For instance, positive LDO regulators from Diodes Incorporated offer LDOs with PMOS pass devices featuring dropout voltages of about 300 mV, when their output voltage is 3.3 V and the load current is 1 A.

The output of the LDO must have an output capacitor. The inherent ESR or effective series resistance of the capacitor affects the stability of the circuit. That means the capacitor used must have an ESR of 10 ohms or lower for guaranteeing stability covering the entire operating temperature range. Typically, these capacitors are of the type multilayer ceramic, solid-state E-CAPs, or tantalum, with values upwards of 2.2 µF.