Low Power-High-Bandwidth LDO Voltage Regulator with No External Capacitor
A low-dropout (LDO) voltage regulator for low-power applications is designed without an external capacitor for compensation. The regulator has two stages, the first a folded cascode amplifier and the second a large pass transistor acting as a common-source amplifier. To better explore the tradeoff between bandwidth and power supply rejection, transistor dimensions are modified to support three different bias current levels for the same topology. Tradeoffs involving phase margin and load capacitance are also explored. In simulation, the regulator provided an output of 1.3 V from an unregulated 1.8 V supply, using a 0.75 V reference. By exploiting the tradeoffs between PSRR, bandwidth, and power consumption, a PSRR between 40-60 dB is achieved with a bandwidth between 10 kHz-350 kHz while burning no more than 150 mA of current. The output voltage is stable for load currents between 18-174 mA.
As demand rises for electronic devices to be smaller, faster, and more efficient, increasing importance is placed on well designed voltage regulators for power supplies. When space is limited, as in the case of portable devices, circuitry for multiple functions requires multiple voltage levels on the same chip. Voltage regulators are needed to protect the rest of the circuitry from fluctuations in the power supply, which can occur due to crosstalk or digital switching. Large variations in the power supply are extremely detrimental; voltages that are too high can damage sensitive semiconductor devices, while voltages that are too low may disrupt biasing or even prevent the circuitry from working at all. This chapter begins by describing the desirable properties of voltage regulators, and gives an overview of the state of the art. To conclude, the goals of this work are laid out in detail.
1.1 Design considerations for voltage regulators
Many factors must be considered when designing a voltage regulator. Minimizing power
consumption is always desirable, particularly with portable consumer electronics. Less power
consumption allows the device’s battery to last longer, meaning the user needs to charge the
battery less often. A related factor is efficiency, which is determined by the dropout voltage. 8
Dropout voltage is defined as the difference between the unregulated supply voltage and
regulated output voltage . Lowering the dropout voltage can lower the required voltage of the
unregulated power supply, and this in turn lowers the power consumption of the regulator. A less
power-hungry device may also use a smaller battery, leading to better portability.
Decreasing the area of a voltage regulator also improves portability. A device that uses
several voltage regulators benefits greatly from a voltage regulator design that takes up minimal
area, because the structure that houses the circuitry can be made smaller and hence easier to
carry around. Smaller area also means that more devices will fit onto one wafer, decreasing the
cost of manufacturing.
Bandwidth is another important specification in voltage regulator design. The higher the
bandwidth of a regulator, the more quickly it can react to changes in input and power supply and
keep the output voltage constant. High bandwidth also improves the power supply rejection ratio
(PSRR) of the regulator, which is a measure of how well the regulator attenuates noise on the
power supply. The better the power supply rejection, the less the output voltage changes in
response to fluctuations in the supply. The PSRR can be characterized by the magnitude of
attenuation as well as the range of frequencies over which the attenuation occurs. Usually PSRR
is greatest at low frequencies and rolls off as the frequency increases.
Yet another factor to consider in voltage regulator design is stability. Since the purpose of
a voltage regulator is to provide a steady voltage to other components, a regulator prone to
oscillation is not desirable. Since stability va ries with load conditions (such as output current and
load capacitance), which may be incompletely specified or unknown, a regulator design that has
good phase margin for a wide range of output loads is best. Associated with stability is load