Selasa, 12 Juli 2011

Low Drop-out

Important considerations when selecting a voltage regulator include: 1) the desired output voltage level and its regulation capability; 2) the output current capacity; 3) the applicable input voltages; 4) conversion efficiency (Pout/Pin); 5) the transient response time; 6) ease of use; and if applicable, 7) the ability to step-down or step-up output voltages. In switch-mode regulators, the switching frequency is also a consideration.

There are several types of voltage regulators, which may be classified in terms of how they operate or what type of regulation they offer. The most common regulator IC is the standard linear regulator. A typical linear voltage regulator operates by forcing a fixed voltage at the output through a voltage-controlled current source. It has a feedback mechanism that continuously adjusts the current source output based on the level of the output voltage. A drop in voltage would excite the current source into delivering more current to the load to maintain the output voltage. Thus, the capacity of this current source is generally the limiting factor for the maximum load current that the linear regulator can deliver while maintaining the required output level. The amount of time needed for the output to adjust to a change in the input or load is the transient response time of the regulator.

The feedback loop used by linear regulators need some form of compensation for stability. In most linear regulator IC's, the required feedback loop compensation is already built into the circuit, thereby requiring no external components for this purpose. However, some regulator IC's, like the low-dropout ones, do require that a capacitor be connected between the output and ground to ensure stability. The main disadvantage of linear regulators is their low efficiency, since they are constantly conducting.

The switching voltage regulator is another type of regulator IC. It differs from the linear regulator in the sense that it employs pulse width modulation (PWM) to regulate its output. The output is controlled by current that is switched at a fixed frequency ranging from a few Hz to a few kHz but with varying duty cycle. The duty cycle of the pulses increase if the output of the regulator needs to supply more load current to maintain the output voltage and decreases if the output needs to be reduced. Switching regulators are more efficient than linear regulators because they only supply power when necessary. Complexity, output ripples, and limited current capacity are the disadvantages of switching regulators.

There is also a group of regulator IC's known as Low Drop-out (LDO) regulators. The drop-out voltage is the minimum voltage across the regulator that's required to maintain the output voltage at the correct level. The lower the drop-out voltage, the less power is dissipated internally within the regulator, the higher is the regulation efficiency. In LDO regulators, the drop-out voltage is typically just about 0.6 V. Even at maximum current, the drop-out voltage increases to just about 0.7-0.8 V.

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