4
Basic Knowledge of
LDO Voltage Regulator
CMOS linear regulators can be categorized as low supply current, large current, high voltage,
high-speed, LDO, and so on. There is no strict definition for these categories, but usually “low
supply current” are the ones with the supply current of a few μA, “large current” are the ones
that can pull 500mA or more, “high voltage” are the ones with the voltage of 15V to 20V or more,
and “high-speed” are the ones with the ripple rejection rate of approximately 60dB@1kHz. “LDO”
does not have an exact definition either. Originally it referred to the low dropout output of PNP
output and P-ch MOSFET output, in comparison to the dropout of NPN emitter follower output
and NPN Darlington output of a bipolar linear regulator. Figure 3 shows the types of output
transistors. These days, the value of less than 2@3.3V in on-resistance conversion is
becoming one standard of definition.
[Figure 3] Output Driver Models
Output Transistor
Control circuit must be higher by 0.6V (base
voltage) than the output pin, in order to flow
base current. The control circuit is operated by
input power source, so dropout voltage of 0.6V
is needed.
A transistor turns on when input voltage is lower than base voltage and/or gate voltage is applied. There is
no limit on input power source voltage in relative to output pin voltage. The dropout voltage is small
because the circuit operates if there is the base voltage or gate voltage, and input power voltage that can
operate control circuit.
1.2V or more dropout voltage is needed as the
circuit consists of 2 emitter follower circuits. The
circuit can output large current because the
base current of load transistor can be amplified
by the predriver.
Out
ut Current
Output Pin
Input Power Source
Base Voltage: 0.6V
NPN Emitter Follower Output
Out
ut Current
Output Pin
Base Voltage: 0.6V
NPN Darlington Output
Base Voltage: 0.6V
Predriver
Input Power Source
Control
Circuit
Control
Circuit
Output Pin
Out
ut Current
PMOS Transistor Output
Gate Voltage: 0.7V
Output Pin
Out
ut Current
PNP Transistor Output
Base Voltage: 0.6V
Input Power Source
Input Power Source
Control
Circuit
Control
Circuit