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What is a 3-terminal low dropout regulator?

Latest Updated:12/01/2006


What is a 3-terminal low dropout regulator?


In a general 3-terminal regulator (uPC78L00 Series, etc.), an NPN-type transistor is Darlington-connected in the output stage. The I/O voltage differential required for operation therefore cannot be lower than the voltage between the base and emitter of the Darlington-connected output stage transistor (0.7 V × 2 = 1.4 V).
In a 3-terminal low dropout regulator, a PNP transistor is used as the output stage transistor, so the device can be used with a small I/O voltage differential.

The power dissipation (Pd) in a 3-terminal regulator is expressed as follows.
Pd= (VIN-VO) ·IO+VIN·(Ibias+ΔIbias1)
(VIN-VO) : I/O voltage differential
IO: Output current
Ibias: Quiescent current
ΔIbias1: Quiescent current change
The smaller the I/O voltage differential (VIN-VO), the smaller the power dissipation (heat generation).

Caution A large-capacity capacitor (at least 47 uF) is required at the output of a 3-terminal low dropout regulator to prevent abnormal oscillation.
Consequently, a comparatively large circuit operating current flows when the input is low, such as at startup. This peak current appears when the output voltage rises, so we do not recommend using this kind of regulator in applications in which a resistor is inserted at GND to shift up the output voltage.

Suitable Products
Power Management Linear