What is dropout voltage?

**Lucifre** · 19th November 2006, 05:10

What is drop out voltage in voltage regulators? on every datasheet they give a graph of drop out voltage versus temperature... is it the minimum difference between input and output voltages?

**IanP** · 19th November 2006, 05:23

The dropout voltage is defined as the minimum voltage drop required across the
regulator to maintain output voltage regulation ..

Regards,
IanP

**wptfks** · 20th November 2006, 13:24

i have saw a definition: the dropout voltage is defined as the minimum voltage when the output voltage has dropped 2% while sustain fixed output current

**FSio** · 20th November 2006, 13:50

Hi Lucifre,
The DropOut Voltage in Linear Regulator Voltage depends on its characteristics, for example, in Regulators LMxxx it is 2V aprox. in Reg with Low DropOut Voltage it is <1V aprox.

If you need obtain a Power Supply with linear regulators, you should to do:

VOreq + Vdor + 2 Vd = Vmin wihout ripple
where:
VOreq is Voltage output required
Vdor is DropOut Voltage on IC regulator
Vd is dropouto across diode

rgds

**liuyonggen_1** · 21st November 2006, 02:50

Originally Posted by IanP

The dropout voltage is defined as the minimum voltage drop required across the
regulator to maintain output voltage regulation ..

Regards,
IanP

could you introduce some useful books about LDO? thanks a lot!

Added after 1 minutes:

Originally Posted by IanP

The dropout voltage is defined as the minimum voltage drop required across the
regulator to maintain output voltage regulation ..

Regards,
IanP

could you introduce some useful books to me ? thanks a lot!

**huojinsi** · 21st November 2006, 08:44

Dropout voltage is the input-to-ouput differential voltage at which the circuit ceases to regulate against further reductions in input voltage;this point occurs when the input voltage approaches the output voltage. The below figure shows a typical LDO regulator circuit. In the dropout region, the PMOS pass element(assume output transistor is PMOS) is simply a resistor, and dropout is expressed in terms of its on-resitance(Ron).
Vdropout=Io×Ron

**xxf_86** · 21st November 2006, 09:17

Drop out voltage is the minimum input/output differentialvoltage where the circuit just ceases to regulate.

**hung_wai_ming@hotmail.com** · 24th March 2007, 13:23

Hi

huojinsi is correct.
For simplicity, there's nothing need to remember.
Just remind yourself, when the INPUT voltage close to OUTPUT voltage such that the PASS element changes from saturation to linear region, that is, the VDS(sat), this is AROUND the drop-out voltage. What does it mean?
The name already explains its meaning.
The input/output voltage difference when output ceases to regulate itself and then it DROPS OUT of the regulation.

**shirin_br** · 10th May 2007, 12:55

lTo: iuyonggen_1
The most useful book is Rinconmora book. You can download it from this link.

http://www.edaboard.com/viewtopic.php?p=802274#802274

Success.

**waxtomato** · 11th May 2007, 05:43

from the name, it is so easy to tell that is the diff of supply voltage and the output voltage. Things seem more difficult when VDD shrinks to 1.5v (+/- 10%) but you may need output 1.2v which big current output requirement

**krishnareddy** · 18th December 2007, 17:46

the minimum difference required between input and output of ldo when it is regulating is dropout voltage

What is dropout voltage?

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