The present invention relates to voltage regulators and, more particularly, to dropper-type regulators providing a soft start function.
Dropper-type regulators are typically used when it is desired to supply an output voltage lower than an input voltage. In some dropper-type regulators, an output transistor is used as a variable resistor, so that an input voltage is lowered to thereby maintain a stable output voltage. Dropper-type regulators may be configured to provide a so-called soft start function that smoothes the rise of the output voltage so that a high current upon power activation (known as an inrush current) may be prevented.
An example of a dropper-type regulator 100 having such a soft start function is illustrated in FIG. 1. An output transistor 1 is connected between an input terminal 102 and an output terminal 104, and a stabilization capacitor 106 is connected to the output terminal 104. The output voltage is divided by resistors 108 and 110, and a feedback voltage derived from a central connection point 112 of the resistors 108, 110 is connected to an inversion input terminal 114 of a differential amplifier 3. The differential amplifier 3 is configured to receive a reference voltage VREF at a non-inversion input terminal 116 and to supply an output voltage corresponding to a difference between the feedback voltage and the reference voltage VREF to a drive circuit 4 to provide voltage feedback, so that the output voltage of the regulator 100 is maintained relatively constant. Moreover, the drive circuit 4 is connected to a duty control circuit 5, which controls a duty ratio of a gate voltage of the output transistor 1 upon activation, so that the output transistor 1 is intermittently turned on/off, thereby providing a soft start function. See, e.g., Japanese Laid-Open Patent Application No. 2004-318339, which is incorporated by reference.
In another example which does not include the duty control circuit, a CR circuit (capacitor-resistor circuit, also known as an RC circuit for resistor-capacitor circuit) is inserted between the differential amplifier 3 and the reference voltage source VREF. The CR circuit reduces the rate of increase of the output voltage when the reference voltage rapidly increases upon power activation, thereby providing the soft start function. See, e.g., Japanese Patent Application Laid-Open No. 2005-327027, which is incorporated by reference.
However, the dropper-type regulator illustrated in FIG. 1 requires additional circuits for duty control which were not originally required in a basic regulator (such as an oscillator, a pulse width modulator, and a frequency sweep circuit) and, thus, the size of the circuit increases. Moreover, since it is typically necessary to change a pulse width modulation rate or a frequency sweep time whenever the capacitance of the output capacitor changes, a control circuit for such control is typically required. Similarly, in devices including a CR circuit, the sizes of the resistor and the capacitor comprising the CR circuit may need to be increased if the capacitance of the output capacitor changes. Thus, when a CR circuit is integrated into an integrated circuit (“IC”), the chip size may increase. As a result, it is difficult to set the circuit parameters with sufficient flexibility. Furthermore, it is typically difficult to design a device in which the output voltage is immediately OFF upon power OFF due to the influence of the CR circuit.