LDL212 undervoltage mode

ryanharm · ‎2023-12-04

Hi, can you tell me what happens to the LDL212 when the input voltage drops below the output voltage+dropout? Does it track the input voltage (minus the dropout voltage)? Or does it drop out?

Also, in the datasheet it mentions undervoltage lockout in the features but doesn't explain anywhere how it works. Do you have more info on this feature?

Peter BENSCH · ‎2023-12-11

First of all, it should be noted that the LDL212 has a maximum dropout voltage of 0.7V.

Although it is not explicitly mentioned in the datasheet, an LDO typically does not turn off when the input voltage falls below Vout+Vdrop. As long as the input voltage is above the limit of UVLO and the LDO enabled, the opamp contained in the LDO will attempt to regulate the output voltage. The conductivity of the PFET controlled for this purpose is increased as the input voltage falls until the dropout voltage is reached. From this point the output voltage will follow the input voltage by the dropot voltage until it has fallen below Vout-Vf, then the scenario mentioned in the first answer applies.

tl;dr: yes, it will track the input, as long as Vin > Vout-Vf.

Good luck!
/Peter

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Peter BENSCH · ‎2023-12-05

Welcome @ryanharm, to the community!

The LDL212 is an LDO with a p-channel series MOSFET that has a parasitic antiparallel diode path from drain to source, i.e. from output to input. As soon as the input voltage has a value that is below the output voltage by the forward voltage of a p-n junction, this diode will conduct - which is an unwanted situation.

Undervoltage lockout:

The data sheet specifies a switch-on threshold of typ. 2.3V and a hysteresis of 200mV. This means that the LDL212 is switched on at typically 2.3V when the input voltage rises and switched off below typically 2.3V-0.2V = 2.1V.

Does it answer your questions?

Regards
/Peter

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ryanharm · ‎2023-12-10

Hi Peter

Thanks for your reply. Actually, I didn't mean there would be an output voltage higher than the input voltage. Obviously this would not occur.

The question was what happens to the output when the input voltage falls below the dropout voltage. For example, lets say 12V regulator, 1V dropout. So what happens when the input voltage is less than 13V? Does the output track the input?

Regards

Ryan

Peter BENSCH · ‎2023-12-11

First of all, it should be noted that the LDL212 has a maximum dropout voltage of 0.7V.

Although it is not explicitly mentioned in the datasheet, an LDO typically does not turn off when the input voltage falls below Vout+Vdrop. As long as the input voltage is above the limit of UVLO and the LDO enabled, the opamp contained in the LDO will attempt to regulate the output voltage. The conductivity of the PFET controlled for this purpose is increased as the input voltage falls until the dropout voltage is reached. From this point the output voltage will follow the input voltage by the dropot voltage until it has fallen below Vout-Vf, then the scenario mentioned in the first answer applies.

tl;dr: yes, it will track the input, as long as Vin > Vout-Vf.

Good luck!
/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

ryanharm · ‎2023-12-12

Perfect, thanks Peter.