|Sunday, 27 August 2006|
Voltage regulators play a critical role in the proper operation of a large number of modern electronic circuits. A voltage regulator is used for supplying an output voltage with a regulated voltage level from a DC voltage source by appropriately controlling a duty cycle of a power switch transistor. A voltage regulator ensures that the voltage requirement of a load is maintained. Voltage regulators function to maintain voltage within a range of output values. Voltage regulators are often used to provide a relatively constant voltage source to other electronic circuits. It would be virtually impossible to operate the numerous electronic devices such as, for example, PCs or cell phones in the absence of integrated circuit low dropout voltage regulators. Power regulation for a microelectronic device such as a processor must include a steady voltage and an ability to respond to dynamic current demands of the processor. Voltage regulator circuits can serve many purposes in integrated circuit devices. Voltage regulators are typically designed to react to changes in load condition to ensure that enough power is provided to the circuit so that the voltage of the supplied power signal remains at or very near the desired level. Voltage regulator circuits can be as a regulated internal power supply voltage for certain sections of an integrated circuit device. Voltage regulators can supply a power supply voltage to memory cell arrays within memory devices, such as dynamic random access memories (DRAMs) and static RAMs (SRAMs), as but two of the many possible applications. Voltage regulators prevent surges or spikes in line voltage during start-up when an electrical device is switched on. The surges or spikes of voltage typically cause damage or failure of electrical or electronic circuits within the device unless a voltage regulator is included to control the spikes and surges. Voltage regulators provide a constant voltage over a variety of load impedances. As the impedance of the load increases, the voltage regulator requires less output current to keep the load at a constant voltage. As the impedance of the load decreases, more current is required to maintain a same constant voltage. Depending on requirements of practical applications, the regulated output voltage may be higher or lower than the original DC voltage source. Therefore, voltage regulators are important components of electrical circuits, particularly in regard to integrated circuits that are widely used in many electrical devices.
A voltage regulator may convert power that is received from a power supply at first voltage and current levels to second voltage and current levels. Voltage regulation is accomplished by a voltage regulator control circuit that monitors the output voltage and uses feedback to keep the output voltage constant. Voltage regulation is achieved by sensing the output voltage via a feedback control loop. Voltage regulators generally use a current sensing circuit to detect and control the voltage and current levels of power supplied thereby. Increases in the accuracy of current sensing may allow the use of voltage regulator elements that are rated for lower power use, and therefore may increase voltage regulator reliability and decrease voltage regulator cost. he voltage regulator is equipped with a control unit which monitors the input and output voltages of the voltage regulator. A voltage regulator feedback control loop is used to adjust the exciter field current of the generator in response to the error signal. By coupling the load voltage to the voltage regulator as a feedback loop, the load condition can be monitored. The feedback enables the voltage regulator to quickly react to a voltage transient. The voltage regulator circuit feeds back the regulated output voltage to a comparator through a resistor circuit. The resistor circuit varies its resistance such that a voltage regulator circuit provides various required voltage levels for operating a semiconductor memory device. For implementing current limiting, the regulator circuit includes an arrangement to sense the current conducted by the output transistor and limit that current to a predetermined safe maximum value when overload occurs. A voltage regulator circuit includes a voltage divider having a plurality of resistors and a plurality of switches. Only one switch is selected at any given time to control the resistance value of the voltage divider. A voltage reference provides a reference voltage that is compared against the output of the voltage regulator. Circuitry within the voltage regulator adjusts the output of the voltage regulator to have a desirable relationship to the voltage reference. The sensing circuit output signal is subtracted from a reference signal to develop an error signal proportional to the difference between the actual generator output and a desired generator output. Power supply rejection ratio (PSRR) is a measure of the effectiveness of a voltage regulator, which measures the amount of noise present on the power supply to the voltage regulator which is transmitted to the regulated voltage.
A variety of voltage regulators are commonly used today to provide tight static and dynamic tolerance supply voltages to digital electronic systems. The voltage regulator may have different forms, such as a device form on large apparatus or a chip form on a circuit system. Among the various types of voltage regulators are replica biased voltage regulators. Generally, in a replica biased voltage regulator a voltage established in one portion of a circuit is replicated typically by larger sized devices to present a output voltage. Integrated circuit voltage regulators are common components which typically have an input terminal for receiving an input voltage, a common terminal, and an output terminal which supplies current to a load. Central processing unit (CPU) voltage regulators receive one voltage and supply a second voltage to a processor, also known as a microprocessor. High voltage regulators use a transformer or possibly another similar device to increase the supply voltage to the desired high voltage for driving the load. Applications of these high voltage regulators include charging a photoflash capacitor, such as those commonly used in cameras. A step voltage regulator is an autotransformer used to maintain a relatively constant voltage level within a power distribution system. Step voltage regulators include an input voltage which may fluctuate from the desired operating voltage, depending upon the existing load conditions. Point-of-load (POL) regulators, which are also referred to as voltage regulators or DC/DC converters, are commonly used in conjunction with electronic circuits. A known POL regulator type is a synchronous buck regulator. Synchronous buck voltage regulators are very efficient and typically provide an acceptable performance/cost trade-off. A PFM switching control mode and a pulse width modulated (PWM) switching control mode are the most frequently used methods for controlling the duty cycle of the power switch transistor in the voltage regulator. The PFM voltage regulator turns on the power switch transistor each time when the output voltage decreases to become equal to a target voltage, thereby regulating the output voltage. The PWM voltage regulator controls the on and off states of the power switch transistor by a rectangular wave having a predetermined duty cycle to achieve the effect of regulating the output voltage. The pulse width modulation (PWM) power converter has been widely applied to various electronic products. This is because the voltage/current requirements of electronic circuits typically differ from the voltage that is readily available or the current that can practically be delivered. A buck regulator normally employs an inductor to convert the pulse width modulated (PWM) square waves generated by a control circuit of a synchronous buck converter to an average value.
A linear regulator is one type of voltage regulator that includes a linear control element and an electrical feedback element. The linear control element, often a transistor, is coupled in series with the unregulated input voltage. The feedback element is used to maintain a constant output voltage by comparing the output voltage to a stable, known voltage reference. A linear voltage regulator accepts a fluctuating input voltage and provides a substantially constant output voltage at a desirable level. Linear voltage regulators normally incorporate special circuitry for protecting both the load and the regulator under abnormal conditions such as overload. The most common protection mechanism used is current limiting. Linear voltage regulators are used to produce a steady output voltage at a predetermined level from an input voltage which may vary. Linear voltage regulators are widely used in the power supply circuits of electronic designs. Linear regulators are advantageous because they respond very quickly to fluctuations in load current and input voltage. A shunt regulator is a type of linear regulator in which the linear control element is tied from output to ground rather than in series with the load. A shunt regulator includes an alternate current path for regulator current, the regulator supplying constant current during no load conditions as well as during full load. The regulator supplies constant (full load) current with excess current being shunted through the parallel path. Linear regulators often must dissipate a great deal of power. The power dissipated given a constant load current is proportional to the voltage drop between the input and the output. Large power dissipation requires adequate cooling for proper operation of the regulator and surrounding components. In many applications these regulators are required to operate with small input-output voltage differentials. Low dropout (LDO) linear voltage regulators are a class of linear voltage regulators that are specifically designed to operate with small input-output voltage differentials.
A low-dropout (LDO) regulator is a linear voltage regulator characterized by its ability to regulate the output voltage at a low voltage differential across an input terminal and an output terminal of the LDO regulator. The low drop out (LDO) linear voltage regulator is used to reduce power consumption by providing the lowest voltage drop across the linear regulator. A low drop-out regulator utilizes a transistor or FET to generate a regulated output voltage with very low differential between the input voltage and the output voltage. A typical LDO voltage regulator includes a feedback-control loop coupled to a pass element. The feedback-control loop modulates a gate voltage of the pass element to control its impedance. A low drop-out regulator made in an integrated circuit may be used to provide a predetermined voltage with low noise to a set of electronic circuits from a supply voltage provided by a rechargeable battery. Low dropout voltage regulators (LDOs) are used in power supply systems to provide a regulated voltage at a predetermined multiple of a reference voltage. LDO voltage regulators enable power management systems to efficiently supply additional voltage levels that are smaller than the main supply voltage. Low dropout voltage regulators are widely used building blocks in almost any electronic application. LDO regulators are widely used in low-voltage, high current applications, and especially in battery-operated electronic devices because they can provide high-performance linear regulation with significant power savings and reduced external component costs. Low drop-out voltage regulators are widely used in portable electronics equipment such as laptop computers, personal digital assistants, cellular phones, pagers, and digital cameras to provide a constant-voltage power supply for analog/digital circuits.
Another type of voltage regulator is a switching regulator. A switching regulator generates an output voltage by converting an input DC voltage into a high frequency voltage, and filtering the high frequency voltage to generate the output DC voltage. A switching regulator includes a transistor operated as a saturated switch for alternately coupling and de-coupling an input DC voltage source, such as a battery, to a load, such as an integrated circuit. The transistor applies the full unregulated input voltage across an inductor for short intervals. The transistor typically serves as the switch, the state of which is controlled with a fixed-frequency control signal applied to the transistor's base or gate. An output filter, typically including an inductor and a capacitor, is coupled between the input voltage source and the load to filter the output of the switch and thus provide the output DC voltage. A controller measures an electrical characteristic of the circuit. The controller alternately drives one or the other of the transistors to conduct. A switching regulator operates by switching current through an inductor on and off repetitively. The regulator's output voltage is typically maintained by adjusting the duty ratio of the control signal provided to the transistor. The voltage at the output terminal is regulated by adjusting the duty cycle of the complementary signals driving the two transistors. Switching regulators provide a predetermined and substantially constant output voltage from a source voltage that may be poorly-specified or fluctuating. Synchronous switching regulators convert the typically fluctuating input voltage to a regulated output voltage. Synchronous switching regulators include a main switching element and a synchronous switching element that typically are driven by non-overlapping clock pulses to supply current at a regulated voltage to a load. Switching voltage regulators are widely used due to their high efficiency.