Low-dropout regulator
The disadvantage is that linear DC regulators must dissipate heat in order to operate.
The article was written by Robert Dobkin, an IC designer then working for National Semiconductor.
One input of the differential amplifier monitors the fraction of the output determined by the resistor ratio of R1 and R2.
For high voltages under very low In-Out difference there will be significant power loss in the control circuit.
It is important to keep thermal considerations in mind when using a low drop-out linear regulator.
Having high current and/or a wide differential between input and output voltage could lead to large power dissipation.
Depending on the package, excessive power dissipation could damage the LDO or cause it to go into thermal shutdown.
The series pass element, topologies, and ambient temperature are the primary contributors to quiescent current.
[6] Many applications do not require an LDO to be in full operation all of the time (i.e. supplying current to the load).
In this idle state the LDO still draws a small amount of quiescent current in order to keep the internal circuitry ready in case a load is presented.
This is especially useful when a system is using switchers, which introduce a ripple in the output voltage occurring at the switching frequency.
Left alone, this ripple has the potential to adversely affect the performance of oscillators,[7] data converters,[8] and RF systems[9] being powered by the switcher.
Two specifications that should be considered when using an LDO as a filter are power supply rejection ratio (PSRR) and output noise.
An LDO is characterized by its drop-out voltage, quiescent current, load regulation, line regulation, maximum current (which is decided by the size of the pass transistor), speed (how fast it can respond as the load varies), voltage variations in the output because of sudden transients in the load current, output capacitor and its equivalent series resistance.
PSRR refers to the LDO's ability to reject ripple it sees at its input.
[11] As part of its regulation, the error amplifier and bandgap attenuate any spikes in the input voltage that deviate from the internal reference to which it is compared.
[12] In an ideal LDO, the output voltage would be solely composed of the DC frequency.
However, the error amplifier is limited in its ability to gain small spikes at high frequencies.
As an example, an LDO that has a PSRR of 55 dB at 1 MHz attenuates a 1 mV input ripple at this frequency to just 1.78 μV at the output.
Most LDOs have relatively high PSRR at lower frequencies (10 Hz – 1 kHz).
However, a Performance LDO is distinguished in having high PSRR over a broad frequency spectrum (10 Hz – 5 MHz).
Having high PSRR over a wide band allows the LDO to reject high-frequency noise like that arising from a switcher.
[9] Each of these phenomena contribute noise to the output voltage, mostly concentrated over the lower end of the frequency spectrum.
In order to properly filter AC frequencies, an LDO must both reject ripple at the input while introducing minimal noise at the output.
Efforts to attenuate ripple from the input voltage could be in vain if a noisy LDO just adds that noise back again at the output.
The worst case of the output voltage variations occurs as the load current transitions from zero to its maximum rated value or vice versa.
Increasing DC open-loop current gain improves the line regulation.
[6] The transient response is the maximum allowable output voltage variation for a load current step change.
) that is usually added to the output capacitor to improve the load transient response, and the maximum load-current (
IVRs combine a switching voltage regulator with all necessary control circuitry into a single device which results in a 10× size reduction and 10–50% energy savings.
