Transcript Power Fundamentals: Linear Regulator Fundamentals

Linear Regulator Fundamentals
2.2 NPN
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
VIN
VOUT
PASS
TRANSISTOR
R1
CIN
ERROR
AMP
COUT
R
LOAD
VREF
R2
2
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
3
Losses in the NPN "Quasi-LDO"
Simple Model of Losses in the NPN "Quasi-LDO"
4
Driving the NPN "Quasi-LDO" Pass
Element
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
6
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
7
Thank you!
8