1. Field of the Invention
The present invention relates generally to the field of band-gap voltage references, and more particularly to a low noise, high PSRR (power supply rejection ratio) band-gap reference having a fast turn-on time.
2. Description of the Related Art
The basic operation of band-gap circuits is well known in the art. For example, a Brokaw band-gap core is disclosed in U.S. Pat. No. 3,887,863, the disclosure of which is herein incorporated by reference. A band-gap core generally comprises a pair of transistors, which generate a voltage proportional to absolute temperature (PTAT). A network of resistors is connected to the transistors to multiply the PTAT voltage and add the voltage to the base-emitter voltage of one of the transistors, such that the total voltage is constant over temperature. The band-gap core circuit thus provides a temperature compensated reference voltage output.
Many applications, such as a low drop-out (LDO) voltage regulator, need to have low noise characteristics and also have a high PSRR (power supply rejection ratio). The main source of both noise and PSRR in a voltage regulator is from the band-gap circuit. As shown in FIG. 1, a large value capacitor 14 (.gtoreq.10 nF) has been used in prior art circuits to filter the noise from the band-gap 10 from the rest of the circuit 12. Many applications, though, further require a fast "turn-on" time (i.e. the time it takes for the circuit to stabilize the output voltage after being turned on). For example, in cellular telephones, many manufacturers wish to periodically turn off the voltage regulator in order to conserve battery power. If the turn-on time is too large, however, it is not practical to shut off the regulator since this would interrupt the use of the cellular telephone. Since inserting a large capacitor to filter the noise significantly increases the "turn-on" time for the voltage regulator, the solution of FIG. 1 is not preferred for the cellular telephone market and other similar applications.
Thus, it would be desirable to have a low noise, high PSRR band-gap circuit that has a fast turn-on time and that can be incorporated into a voltage regulator or other circuits utilizing a band-gap reference.