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Question

What is the meaning of Dropout Voltage in the specifications of an LDO regulator? In what way can LDO regulators with low dropout voltage be used?

Answer

LDO Linear RegulatorsHow to use

Dropout voltage refers to what degree the input voltage must be higher than the output voltage in order to supply an output current defined as the IOUT condition. The graph below is an example of the dropout voltage of the RP109.

RP109 Dropout Voltage (Condition: IOUT = 150 mA)

Set Output Voltage Dropout Voltage Typ. Dropout Voltage Max.
VOUT = 0.8V 0.70 V 1.00 V
VOUT = 0.9V 0.62 V 0.91 V
1.0V ≤ VOUT < 1.2V 0.56 V 0.82 V
1.2V ≤ VOUT < 1.4V 0.47 V 0.67 V
1.4V ≤ VOUT < 1.8V 0.39 V 0.54 V
1.8V ≤ VOUT < 2.1V 0.33 V 0.48 V
2.1V ≤ VOUT < 2.5V 0.28 V 0.40 V
2.5V ≤ VOUT < 3.0V 0.25 V 0.35 V
3.0V ≤ VOUT ≤ 3.6V 0.23 V 0.32 V

In the case of a 3.0 V output product, for instance, the required dropout voltage is typically 0.23 V @IOUT = 150 mA. The boxed '0.32 V' is the maximum value including thermal conditions into account.

In short, this LDO regulator can output a stable 150 mA with more than 3.32 V input (3.0 V output + 0.32 V dropout). If the input voltage is lower than the sum of the output and dropout voltages, the output voltage may decrease.

Dropout voltage varies according to output voltage and output current. There are various situations where the low dropout voltage is preferred. Three typical occasions are : (1) where the input voltage to the LDO regulator is low, and the difference between the input and output voltages is small; (2) where the LDO regulator should maintain the output voltage to support long time operation with discharging input devices such as batteries and large capacitors; (3) where the switching noise generated by a DC/DC converter may affect the performance of the device, an LDO is used in between DC/DC converter output and the device.

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