1. Linear Regulator Fundamentals
2.3 PNP
Author's Original Notes:

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. PNP-LDO Linear Regulator
Low drop-out voltage (VSAT of PNP)
Higher ground pin current (load dependent)
Requires output capacitor
The classic PNP LDO was developed by National in response to the requirements of the automotive industry which needed regulators which were able to withstand reverse-battery conditions without external protection diodes. An additional benefit was the lower dropout voltage which became the dominant advantage in modern applications as supply voltages have fallen.
The low-dropout (LDO) regulator operates exactly the same as the NPN, with the exception that the NPN Darlington pass transistor has been replaced by a single PNP transistor (see above). The big advantage is that the PNP pass transistor can maintain output regulation with very little voltage drop across it:
VDROP = VSAT
Full-load dropout voltages < 500 mV are typical.

4. Simple Model of Losses in the PNP LDO Regulator
Driving the PNP LDO Pass Element
Author's Original Notes:

5. 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.

6. Summary The NPN-Quasi-LDO has the following Characteristics:
Requires that the input voltage be only 100mV to 700mV higher than the output voltage
Has a higher Ground pin current than NPN type
Requires careful selection of the output capacitor value and ESR ratings
The PNP pass transistor in the LDO uses the collector as the output pin. This high impedance output forces the need for a specific capacitor value (uF) with a specific Effective Series Resistance (ESR), to maintain stability across the output load current range.
Tantalum capacitors are usually recommended as they are most likely type to have the needed capacitance value and ESR value within a wide temperature range. Aluminum capacitors are usually not recommended, but can be used in a limited temperature environment.

7. Thank you!
Author's Original Notes: