The first step to understanding POWER SUPPLIES ICs! | ③ Improving the efficiency of series regulators! How LDOs (Low Drop Outs) work | NEXTY Electronics, an electronic components and semiconductor trading company

Dropout Voltage

The minimum voltage difference between the input voltage (Vin) and output voltage (Vout) of a series regulator is called the dropout voltage.
Dropout voltage = Vin - Vout

The dropout voltage varies depending on the output voltage, output current (Iout), and ambient temperature.

The higher the output voltage, the lower the dropout voltage.
The larger the output current, the larger the dropout voltage.
The higher the ambient temperature, the higher the dropout voltage.

Figure 1 Dropout voltage
Figure 2 Dropout voltage vs. Vout

What is an LDO (Low Drop Out)?

A series regulator with a low dropout voltage is called an LDO (Low Dropout).
The loss in a series regulator is (Vin - Vout) × Iout + Vin × current consumption, with (Vin - Vout) × Iout accounting for the majority of the loss.
Therefore, by using an LDO to reduce the input/output voltage difference, losses can be reduced and efficiency can be increased.

Circuit configuration

As shown in Figure 3, when the control element (output driver) is an Nch MOS FETS, the source is Vout.
To operate MOS FETS, a Vgs corresponding to Ids is required, as shown in Figure 4. Therefore, Vout cannot exceed the gate voltage - Vgs.
Also, since the gate voltage can only be raised up to Vin, Vout will be Vin-Vgs or less.
Therefore, the dropout voltage (Vin-Vout) must be greater than or equal to Vgs.

Vout ≦ Gate voltage (Vin) - Vgs
Vin - Vout (dropout voltage) ≥ Vgs

Figure 3 Nch output driver
Figure 4 Ids vs Vgs

As shown in Figure 5, when the control element (output driver) is a Pch MOS FETS, the source is Vin, so Vout does not affect Vgs.
In addition, the gate voltage can be lowered to GND, and the dropout voltage can be lowered to the ON resistance × Iout corresponding to Vgs (Vin), allowing it to operate as an LDO.

Vgs＝Vin - Gate voltage (0V) =Vin
Vin - Vout (dropout voltage) = Vin - Ron x Iout

When Vout is set low, the input voltage is also low and Vgs is also low, which increases the ON resistance and the dropout voltage.

*There are also POWER SUPPLIES use a dual-power supply, where the output driver is Nch but the gate drive circuit is a separate POWER SUPPLIES supply, and LDOs that use a boost circuit to raise the gate voltage above Vin, thereby reducing the dropout voltage.

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The first step to understanding POWER SUPPLIES ICs! | ③ Improving the efficiency of series regulators! How LDOs (Low Drop Outs) work

Dropout Voltage

What is an LDO (Low Drop Out)?

Circuit configuration

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