1. Linear Regulator Fundamentals
2.2 NPN
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2. Linear-Regulator Operation
Voltage feedback samples the output R1 and R2 may be internal or external
Feedback controls pass transistor’s current to the load V IN
REF C
OUT R
LOAD
ERROR
AMP
PASS
TRANSISTOR R1 R2
Every linear regulator uses a configuration similar to the one shown in this figure. The primary difference between all of the linear-regulator topologies is the particular pass-transistor configuration utilized. They affect the following key specifications:
DROPOUT VOLTAGE: Dropout voltage, defined as the minimum input-to-output difference of voltage required to keep a linear regulator’s output in regulation, is dependent both on load current and temperature. It is important to note that the electrical performance of the regulator (PSRR, load and line regulation) degrade as the input voltage approaches the point of dropout. A linear regulator will not perform well operated in or very near dropout.
GROUND PIN CURRENT: Ground Pin current (also sometimes called quiescent or no load current) is the current used by the device which does not flow to the load. It is measured as the current flowing out of the ground pin. The term “quiescent current” normally means the ground pin current when the regulator is not driving the load (or is in standby mode).
POWER SUPPLY RIPPLE REJECTION (PSRR): This specifies how much noise on the input gets filtered out by the control loop of the regulator before reaching the output. In general, wider gain bandwidth means better PSRR. A high value of PSRR at a frequency around 1 kHz is useful in some cell phone applications due to the 800 Hz transmit burst.
BROADBAND NOISE: Total noise energy over a specific frequency range is used to specify broadband noise. Lower is always better, and low noise is required for things like PLLs and sensitive analog circuitry. Low Iq regulators can have higher noise because their reference is noisier and contributes the main noise component. Some LDOs attain lower noise performance by connecting the internal reference node to a package pin, allowing a bypass capacitor to be added to reduce reference noise.
STABILITY REQUIREMENTS: The various pass transistors affect the loop’s AC characteristics and must be compensated differently. The LDO regulators generally have more restrictive requirements to assure stability in comparison to NPN regulators.

3. NPN "Quasi-LDO" Voltage Regulator
The NPN-Quasi-LDO has the following Characteristics:
Requires the input voltage to be at least 0.9V to 1.5V higher than the output voltage
Has higher Ground pin current than the NPN-Darlington, but less than the PNP-LDO Voltage Regulators
Requires an output capacitor, but, generally, has no specific ESR requirement like the PNP-LDO
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4. Losses in the NPN "Quasi-LDO"
Simple Model of Losses in the NPN "Quasi-LDO"
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5. Driving the NPN "Quasi-LDO" Pass Element
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6. Drive Current vs. Low/High Load Current
The drive current for a load current requirement of 5mA
Increasing the load current requirement to 500mA
Left is 5mA load current while right side is 500mA load current.

7. Summary The NPN-Quasi-LDO has the following Characteristics:
Requires the input voltage to be at least 0.9V to 1.5V higher than the output voltage
Has higher Ground pin current than the NPN-Darlington, but less than the PNP-LDO Voltage Regulators
Requires an output capacitor, but, generally, has no specific ESR requirement like the PNP-LDO
The 'Quasi-LDO' NPN linear regulators are considered to be stable with some nominal capacitance.
Some output capacitance is required for stability, Usually 10uF is adequate for stability No specific Effective Series Resistance limitation.

8. Thank you!
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