LDO Basics
- awasch@uwaterloo.ca (Deactivated)
- lwbantot@uwaterloo.ca (Deactivated)
What are LDOs?
LDOs aka low-dropout voltage regulators are used to stabilize voltage. They are used to regulate the output voltage when the input voltage is too high or when the input voltage is close to the output voltage. LDOs have a very low voltage voltage drop when outputting a regulated voltage. This allows LDOs to be used in applications where the input voltage is close to the required regulated output voltage.
INTERVIEW QUESTION: What is the efficiency of an LDO/linear regulator?
Efficiency = Output Voltage / Input Voltage
How do LDOs work?
LDOs use a closed feedback loop to bias a pass element to maintain a constant voltage.
There are 2 main components in an LDO: the Op Amp and the MOFSET
Op Amp in an LDO
Op Amps have many applications in electronics, but for an LDO, it is used as a non-inverting/error amplifier which can be used as a comparator.
This op amp takes a reference voltage and compares it to the output of the LDO. The op-amp compares the two values and outputs the error to the gate of a MOFSET. The resistors are used to set the output of the LDO by changing the amount of feedback that the op-amp gets.
MOFSET in an LDO
The MOFSET is a transistor, but in this application, it acts like a voltage controlled resistor or a variable resistor. With a greater voltage input in the gain, there is a greater resistance in the drain of the MOFSET.
The gain voltage changes based off of the output of the Op-Amp, so the output of the MOFSET will also change along with it.
All together
The output of the LDO (Vout) is used as an input in the Op-Amp to always ensure there is a stable voltage. The LDO is always comparing the output with what the output should be so it can adjust accordingly.
Advantages
- Simple.
- Cheap.
- Power supply rejection ratio. Linear regulators respond quickly to changes in input voltage, producing an output voltage that is mostly free of any ripple on the input.
- Respond quickly to changes in load voltage.
- No switching noise. Other voltage conversion circuits, known as DC-DC converters, have high-frequency switching noise. Linear regulators don’t have this characteristic.
- Under-Voltage lockout (protects against under-voltage)
- Can be used to limit current
- Thermal Shut down
Disadvantages
- Simple.
- Cheap.
- Power supply rejection ratio. Linear regulators respond quickly to changes in input voltage, producing an output voltage that is mostly free of any ripple on the input.
- Respond quickly to changes in load voltage.
- No switching noise. Other voltage conversion circuits, known as DC-DC converters, have high-frequency switching noise. Linear regulators don’t have this characteristic.