LDO regulator means low dropout
regulator. An LDO voltage regulator is just a DC linear voltage
regulator which can be operated with a very small input-output voltage
differential. This input output voltage differential is called dropout
voltage. In simple words dropout voltage is the voltage dropped by the
regulator circuitry alone for its working. For example, an LM2941 LDO
voltage regulator has a dropout voltage of only around 0.5V, which means
that in order to get 5 volts at the output you need to input only 5.5
volts where an ordinary 7805 linear voltage regulator has a dropout
voltage of around 2V. This means that, in order to get 5V at the output
of 7805 you need to input at least 7V.
LDO regulator working.
Schematic of a LDO regulator
As I said above the working of a LDO
voltage regulator is similar to the working of an ordinary linear
voltage regulator and the only difference is in the schematic topology
of their internal circuitry. Ordinary linear regulators (7805, LM117,
LM317 etc) uses a common collector scheme while the LDO regulators use
an open collector (termed as open drain if a MOSFET is used as the
series pass element) scheme.
Common collector scheme.
Common collector scheme is one of the
three basic transistor configurations. Here base is the input terminal,
emitter is the output, and supply voltage is applied to the collector
terminal. The series pass transistor inside an ordinary linear voltage
regulator is wired in this configuration. Fig A shows the common
collector scheme (also termed as emitter follower scheme).
Common collector and open collector scheme
Open collector scheme.
Many integrated circuits like LDO
voltage regulators this scheme for wiring the series pass element (BJT
or MOSFET). The input signal (which comes from the previous stage inside
the IC) is coupled to the base of the BJT (or gate of the MOSFET),
collector is left open and it is connected to a pin of the IC while the
emitter is connected to the IC ground. This configuration makes it easy
to saturate the series pass element and hence the dropout voltage is
minimized.Fig B shows the open collector / drain scheme.
Practical LDO regulator circuit.
LDO regulator using LM2941
LDO regulator using LM2941
The circuit shown above is of a
practical LDO regulator circuit using IC LM2941 from National
semiconductors. LM2941 is an integrated LDO voltage regulator IC whose
output can be adjusted. The IC has many good features like thermal
shutdown, transient protection, short circuit protection etc. The
reverse polarity protection feature makes this IC very applicable in
automotive applications. The output voltage can be adjusted from 5V to
20V and the dropout voltage is 0.5V at 1A output current.
In the circuit the voltage divider network comprising of R1 and R2 sets the output voltage. Capacitor C2 is the input filter which is very essential if the regulator circuitry is situated away from the rectifier+filter module. Capacitor C1 is the output filter while S1 is the ON/OFF switch. Resistor R3 is used to ensure the necessary 300mA pull up current which is necessary for proper shutdown when the switch S1 is made open.
In the circuit the voltage divider network comprising of R1 and R2 sets the output voltage. Capacitor C2 is the input filter which is very essential if the regulator circuitry is situated away from the rectifier+filter module. Capacitor C1 is the output filter while S1 is the ON/OFF switch. Resistor R3 is used to ensure the necessary 300mA pull up current which is necessary for proper shutdown when the switch S1 is made open.
Notes.
- The circuit must be assembled on a good quality PCB.
- A heat sink is required for application above few hundred milli amps.
- Vout = 1.275 ((R1 + R2)/R1)
- During calculation select R1=1K and solve for R2.
- Selecting R1 = 1K will make the input bias current error of the adj pin negligible.
- Reducing the value of C1 below 22uF will induce instabilities and also this capacitor must be placed as close as possible to the IC on the PCB.