Drop Out Voltage is how many volts (millivolts) the regulator holds
across its input and output.

Example:

Input voltage is battery 9Vdc and discharging, going down.
Regulator Dropout Voltage: 1Vdc
Regulator Output Voltage: +5Vdc
Minimum battery voltage to be regulated to +5Vdc: 6Vdc.

Consider from 0.5 to 1.0 more for safety reasons, so the minimum battery
Voltage to regulate a steady and confortable +5Vdc at output would be
7Vdc.

If using a NiCad 9V battery, in real it is 7.2V (some of them), so right
after the first use, the battery would not be in "safe voltage" to feed
a 1 Vdc dropout regulator, because its Voltage would be lower than
7Vdc...

In any case, it also means power lost, since this dropout times current
means waste of power on the regulator.
In a regular regulator that uses to reduce voltage by voltage control,
it doesn't matter so much, since it is already wasting power to deliver
a lower voltage, but if it is being used in a inteligent regulator with
switching power supply or some other scheme, hmmm, that power loss is
considerable...

Wagner.


Greg Wiley wrote:
>
> Paul B. Webster VK2BZC <paulb@midcoast.com.au> wrote:
>
> > Stewart Pye wrote:
>
> > > Should I use a 78L05 regulator, or resistor voltage divider, or
> > > resistor/zener diode?
>
> >   You can get low-power and micropower regulators which are then limited
> > by their *maximum* output current and tend to have significant drop-out
> > voltage, but there probably is one suitable for this job (others may
> > offer examples).
>
> I was following the discussion just fine until this point.  Forgive my
> ignorance but what is a "drop-out voltage" and why is a high one bad?
>
> Thanks,
>   -greg