1-9. Differences between a three-terminal voltage regulator and an LDO regulator

Classic three-terminal voltage regulators (also known as standard voltage regulators) use an NPN-type transistor or an N-channel MOSFET as an output transistor.
For these regulators to work, an input-output voltage differential called a dropout voltage (VDO) is necessary as shown below. In the case of NPN-type voltage regulators, the minimum value of VDO must satisfy VIN - VOUT > RIN x IIN + 2 × VBE

Figure 1.9.1 circuitry diagram( VDO = VIN - VOUT = RIN x IIN + 2 x VBE )

In the case of MOSFET-based voltage regulators, the minimum value of VDO must satisfy VIN - VOUT > RIN × IIN + VGS. Suppose RIN = 1 kΩ, IIN = 1 mA, VBE = 0.7 V, and VGS = 1 V. Then, the minimum input voltage required to generate a 5-V output is calculated to be 7.4 V in the case of NPN-type voltage regulators and 7 V in the case of MOSFET-based voltage regulators.

Figure 1.9.2 circuitry diagram( VDO = VIN - VOUT = RIN x IIN + VGS )
Figure 1.9.3 circuitry diagram( VDO = VIN - VOUT = VCE(sat) and  VDO = VIN - VOUT = VDS )

In contrast to these regulators, LDO regulators mainly use a PNP transistor or a P-channel MOSFET as an output transistor. The minimum dropout voltage of such LDO regulators is determined by the collector-emitter voltage (VCE(sat)) and the drain-source voltage (VDS = RDS(ON) × ID). Therefore, LDO regulators can operate with a smaller dropout voltage than three-terminal voltage regulators.

Figure 1.9.4 a new type of LDO regulator that provides an even smaller dropout voltage

There is also a new type of LDO regulator that provides an even smaller dropout voltage, which uses an N-channel MOSFET with a smaller on-resistance than a P-channel MOSFET as an output transistor and has a power supply pin dedicated to the control circuit (including the MOSFET drive circuit). 

Chapter1 Introduction to Low-Dropout (LDO) Regulators

1-1. Types of voltage regulator ICs
1-2. Advantages and disadvantages of linear regulators and switching regulators
1-3. What is an LDO regulator?
1-4. Need of LDO regulators for electronic systems
1-5. What is a linear regulator?
1-6. Operations of linear and switching regulators
1-7. Principle of operation of series regulators
1-8. Circuit configuration of a series regulator

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