Spehro Pefhany wrote...
That's for an ideal regulator. In reality, there's an additional
term in there related to current the regulator uses for its own
purposes- that flows from input to ground, or current that flows
from input to output and must be wasted in a divider network etc.
Right. As Spehro says Jamie, Pd = (Vin-Vout) * I is a best-
case theoretical dissipation for a linear regulator, assuming
that no additional power is required for the circuitry.
To be fully accurate for LM317-style 3-terminal regulators
we must add a Vin * Iadj internal-dissipation term for the
reference (Iadj = 50 to 100uA), and a Vout * 1.22 / R1 term
for the reference resistors. Notice that these terms don't
depend on output current. These terms may be small, but this
doesn't mean the LM317 is performing magic. With a '317 we
have two serious constraints: a typical 2V minimum Vin-Vout
overhead (the dropout voltage), and a minimum load current
(5mA max) required to operate the circuitry. (A safety rule
is to make the second reference-resistor term large enough to
meet the minimum load current spec.)
Comparing the '317 to other types of regulator ICs, they have
different Pd extra terms and different constraints. Common
780x-style regulators have the Vin * Iq quiescent-current
term, where Iq = 8mA max, and is unaffected by load current.
780x types also require a 2V minimum Vin-Vout overhead (the
dropout voltage), but they have no minimum load current.
The 2V dropout-voltage issue for '317 and '7800 regulators
forces a considerable additional loss of power wasted in the
heat sink. We can solve this issue with a low-dropout (LDO)
regulator style. These also have the Vin * Iq quiescent-
current term, plus a Vin * Iout / beta term if they use
a bipolar pass element (beta is the PNP transistor's gain).
Types using p-channel MOSFETs don't have an Iout/beta term.
LDO regulators have a greatly reduced dropout requirement,
some as low as 50 or 100mV. They generally don't have any
minimum load current requirement. Sounds pretty good, but
they do require large output capacitors, without which they
may have inferior load transient response.
Jamie, let's consider the issue of dropout voltage, which is
related to the minimum input - output voltage in a regulator
circuit. Just because you choose an LDO IC with a low 50mV
spec. doesn't mean you can design a power supply with a small
input-output voltage difference, no-way-Bob! Thinking of AC
line power supplies, you have to provide large overheads for
transformer load sag, diode resistance and storage-capacitor
voltage ripple, all of which increase with load current, plus
an allowance for brownout operation at one extreme and excess
line voltage at the other extreme. Considerations like these
can swamp the one or two-volt savings offered by an LDO, but
hey, you take what you can get! Certainly an LDO regulator
can be a real life saver in battery-operated systems.
Thanks,
- Win
whill_at_picovolt-dot-com