Selecting Linear
Regulator for the Application:
- Maximum Load Current
- Type of Input Voltage Source (Battery or AC)
- Efficiency
- Output Voltage Precision (Tolerance)
- Quiescent (Idling) Current
- Special Features (Shutdown Pin, Enable, Power Good)
MAXIMUM LOAD CURRENT
The load current
specification for an IC regulator will be defined as either a single value or a
value that is dependent on input-output voltage differential. The regulator
selected must be able to provide sufficient current to the load under
worst-case operating conditions, if system performance is to be reliable.
INPUT VOLTAGE SOURCE
The available input
voltage will strongly influence which type of regulator is best suited for an
application.
Battery Input LDO
regulators are usually
the best choice because they
utilize the available input voltage more fully.
If a DC supply is generated from a rectified
AC source, the dropout voltage of the
regulator is not
as critical because
additional regulator input
voltage is easily obtained by increasing the secondary
voltage of the AC transformer (by adding turns to the secondary winding).
In these applications,
a standard regulator is
usually the most economical choice and
can also provide
more load current.
However, in some
cases the additional features and better output voltage
precision of some of the new LDO regulators would still make them the best
choice.
EFFICIENCY
If the Input voltage
and required output voltage having difference of 5V or more will create power
drop on the transistor based on load current. Efficiency will go low. If the
required output voltage is having difference of 5V or more as compared to Input
supply leads to lower efficiency.
OUTPUT VOLTAGE
PRECISION
Typical linear
regulators usually have an output voltage specification that guarantees the
regulated output will be within 5% of nominal.
This level of accuracy is adequate for most applications. Also, many of the new regulators have
separate output specifications that cover room temperature/full operating
temperature range, and full-load/light-load conditions.
QUIESCENT CURRENT
The quiescent
current that a part draws
from the source
when idling (either
shut down or not
delivering significant amounts
of load current)
can be of
critical importance in battery-powered applications.
Many of the new LDO
regulators are optimized for low quiescent current (like 75 to 150 µA),
and provide significant
improvement over typical
regulators which draw several milliamps.
SPECIAL FEATURES
Shutdown/Enable: A low-power shutdown pin allows a regulator
to be switched off by a logic gate or
microcontroller. This feature
also allows wiring
a regulator for "Snap-ON/Snap-OFF" operation
Load-dump Protection: Regulators used in automotive applications
need built-in protection against overvoltage transients (load-dump). In these cases the regulator usually shuts
down its output during the overvoltage transient, then recovers after it has
passed.
Reverse Input Voltage Protection: This prevents damage to the regulator when
the input voltage
is reversed, essential
in applications where
the user can accidentally reverse the polarity of the
batteries.
Error Flag/Power Good: This flag is used to alert monitoring or
control circuitry that the output has dropped about 5% below its nominal
value. It is intended as a "warning
flag" that can alert a controller that supply voltage may be low enough to
cause erratic operation of the CPU or associated logic circuits.
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