Low Dropout voltage Regulator | LDO Regulator basics

This page covers types of linear regulators viz. standard and low dropout voltage regulator. It mentions LDO regulator block diagram. The term LDO stands for low dropout regulator. Various terminologies related to LDO regulator are also mentioned. It includes dropout voltage, quiescent current, standby current, efficiency etc.

What is Low Dropout Regulator:
The DC voltage regulator which regulates output voltage even if supply voltage input is very near to the desired output.

Linear regulators are of two types viz. standard linear regulator and low dropout regulator (LDO). The difference between them lies in use of pass element and amount of drop-out voltage. The output voltage in regulator equals input voltage minus voltage drop across the pass element.

LDO pass elements

LDO pass elements

The figure-1 depicts LDO regulator pass elements. For standard regulator pass element consists of either darlington NPN or PNP stage. The standard regulator has higher voltage drop usually about 2 V due to two Base to Emitter voltage drops in darlington configuration.

Most LDO regulators use either N-channel or P-channel FET as pass element. They have very low dropout voltages which are less than 100mV. The typical application of this LDO regulator is to generate 3.3V from 3.6V Li-Ion battery. This requires very low drop-out voltage. As shown in the figure, the drop-out voltage depends on minimum voltage drop across FET device. This voltage drop is function of RDS(ON) of the FET device.

LDO voltage regulator block diagram

LDO regulator block diagram

The figure-2 depicts LDO regulator block diagram. As shown the input is applied to the pass element. This pass element operates in linear region to drop input voltage down to the level needed as output voltage. The resulting desired output voltage is sensed by error amplifier. This is compared with reference voltage. The error amplifier is used to drive the gate of the pass element to appropriate operating point to ensure correct output voltage. The error amplifier modulates pass element to maintain constant output voltage when operating current or input voltage changes. LDO regulator acts as a simple resistor in steady state operating conditions.

LDO Regulator Terms | dropout voltage, quiescent current, standby current, efficiency

Following are the useful terminologies/terms used in LDO regulator.
dropout voltage: It is the minimum voltage needed across the regulator to maintain regulated voltage output or regulation. For example, if LDO regulator has 3.3V regulated output and if it has 1V as dropout voltage then input voltage should be atleast 4.3V to maintain regulation. It is the input to output differential voltage at which circuit ceases to regulate against any further input voltage reductions. This point occurs when input voltage reaches near to the output voltage. It is expressed in terms of ON-resistance as follows. Here Io is output current.
Vdropout = Io * RON
quiescent current: It is the difference between input current and output current. It is expressed as follows.
Iq = Ii - Io
Low quiescent current is required to maximize the current efficiency of LDO regulator. It consists of bias current as well as gate drive current of series pass element.
standby current: It is the input current drawn by the LDO regulator when output voltage is being disabled by shutdown signal.
efficiency: The LDO regulator efficiency is expressed as follows.
Efficiency = (Io*Vo)/((Io+Iq)*Vi)}*100
The dropout voltage and quiescent current should be minimized in order to achieve higher efficiency. Moreover voltage difference between input and output also should be minimized.


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