Post Date : Mar 12, 2023 / Last updated : Apr 06, 2023

Voltage regulator operation maintains a constant output voltage by adjusting the driver transistor/FET resistance in response to the input voltage and output current.

The driver FET resistance control adjusts the driver FET gate voltage (=error amplifier output) using the feedback circuit (negative feedback circuit) that uses the error amplifier.

Specifically, the error amplifier compares the output voltage divided voltage: FB and the voltage reference: VREF and then adjusts the driver FET gate voltage so that the difference is zero to maintain a constant output voltage. This is difficult to envision based on this explanation alone, so let’s use a simple example to look at the error amplifier operation.

Case1: When the output voltage is lower than the set output voltage

When the output voltage is lower than the set output voltage, the FB voltage becomes lower than the error amplifier’s voltage reference VREF, so the error amplifier’s output voltage is lowered.
The error amplifier’s output is input to the Pch driver FET gate to lower the Pch driver FET’s on resistance “Ron.”

When the Pch driver FET’s on resistance “Ron” is lowered, the voltage step-down in the driver FET is decreased to increase the output voltage. This operation raises the output voltage that is lower than the set output voltage.
This operation continues until the output voltage is equivalent to the set output voltage.

VOUT↓ ⇒ FB < VREF
⇒ Error AMP : Low ⇒ Pch Driver “Ron↓”

Therefore,

if VOUT becomes lower than the setting, the Pch driver Ron is lowered to raise the VOUT

to keep the VOUT stable at the set value.

Case2: When the output voltage is higher than the set output voltage

When the output voltage is higher than the set output voltage, the FB voltage becomes higher than the error amplifier’s voltage reference VREF, so the error amplifier’s output voltage is raised.
The error amplifier’s output is input to the Pch driver FET gate to raise the Pch driver FET’s on resistance “Ron.”

When the Pch driver FET’s on resistance “Ron” is raised, the voltage step-down in the driver FET is increased to lower the output voltage. This operation lowers the output voltage that is higher than the set output voltage.
This operation continues until the output voltage is equivalent to the set output voltage.

VOUT↑ ⇒ FB > VREF
⇒ Error AMP : High ⇒ Pch Driver “Ron↑”

Therefore,

if VOUT becomes higher than the setting, the Pch driver Ron is raised to lower the VOUT

to keep the VOUT stable at the set value.

In this way, by monitoring the output voltage and conducting feedback, the linear type voltage regulator controls the output voltage to “keep the set output voltage constant”.