Linear voltage regulators have been valuable system components since the early days. One reason is the relatively low noise characteristic vis-à-vis the switching type of regulator. Others are a low parts count and overall simplicity compared to discrete solutions. But, because of their power losses, these linear regulators have also been known for being relatively inefficient.

More recently however, linear IC regulators have been developed with more liberal
(i.e., lower) limits on minimum input-output voltage. This voltage, known more commonly as dropout voltage, has led to what is termed the Low DropOut regulator,
or more popularly, the LDO. Dropout voltage (VMIN) is defined simply as that
minimum input-output differential where the regulator undergoes a 2% reduction in
output voltage. For example, if a nominal 5.0V LDO output drops to 4.9V (-2%) under conditions of an input-output differential of 0.5V, by this definition the LDO’s
VMIN is 0.5V.

The lower the voltage allowable across a regulator while still maintaining a regulated output, the less power the regulator dissipates as a result. A low regulator dropout voltage is the key to this, as it takes this lower dropout to maintain regulation as the input voltage lowers. In performance terms, the bottom line for LDOs is simply that more useful power is delivered to the load and less heat is generated in the regulator. LDOs are key elements of power systems that must provide stable voltages from batteries, such as portable computers, cellular phones, etc. This is simply because they maintain their regulated output down to lower points on the battery’s discharge curve. Or, within classic mainspowered raw DC supplies, LDOs allow lower transformer secondary voltages, reducing system susceptibility to shutdown under brownout conditions as well as allowing cooler operation.

If we call the total power PD, this then becomes:

PD = VIN - VOUT IL + VIN,Iground.

Obviously, the magnitude of the load current and the regulator dropout voltage both greatly influence the power dissipated. However, it is also easy to see that for a given IL, as the dropout voltage is lowered, the first term of PD is reduced. With an intermediate dropout voltage rating of 1V, a 1A load current will produce 1W of heat in this regulator, which may require a heat sink for continuous operation. It is this first term of the regulator power which usually predominates, at least for loaded regulator conditions.

The second term, being proportional to Iground (typically only 1-2 mA, sometimes even less) usually only becomes significant when the regulator is unloaded, and the regulator’s quiescent or standby power then produces a constant drain on the source VIN.

However, it should be noted that in some types of regulators (notably those which have very low pass devices such as lateral PNP transistors) the Iground current under load can actually run quite high. This effect is worst at the onset of regulation, or when the pass device is in saturation, and can be noted by a sudden
Iground current "spike", where the current jumps upward abruptly from a lower low level. All LDO regulators using bipolar transistor pass devices which can be saturated (such as PNPs) can show this effect. It is much less severe in PNP regulators using vertical PNPs (since these have a higher intrinsic ) and doesn’t

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