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Low Drop Out Voltage Regulator having a Low Noise Bypassed Band Gap Reference with Fast Enable Transient Response

IP.com Disclosure Number: IPCOM000007547D
Publication Date: 2002-Apr-04
Document File: 4 page(s) / 52K

Publishing Venue

The IP.com Prior Art Database

Abstract

Low Drop Out (LDO) voltage regulators having a low noise characteristic are becoming increasingly desirable in the consumer electronics market. Because the main source of noise in prior art voltage regulators is from the band gap circuit that is used, a large value capacitor Cbypass is often placed between the band gap and error amplifier to reduce noise. These bypass capcitors introduce other problems and are costly. Reducing the size of the bypass capacitor and the deleterious effects upon the circuit are one objetcive of the invention. This LDO, especially in cellular telephones, provides a faster enable transient response than prior art yet retains or improves noise rejection and suppression. An example of a new low noise band gap reference with pre-charge circuit, having a means to boost bias current of a reference error amplifier upon receiving an enable signal, is shown. There is not any appreciable overshoot or undershoot, because auxiliary charge current becomes the part of the regulation loop.

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Low Drop Out Voltage Regulator having a Low Noise Bypassed Band Gap Reference with Fast Enable Transient Response

Low Drop Out (LDO) voltage regulators having a  low noise characteristic are becoming increasingly desirable in the consumer electronics market.  Because the main source of noise in prior art voltage regulators is from the band gap circuit that is used, a large value capacitor Cbypass is often placed between the band gap and error amplifier  to reduce noise (figure 1).   This capacitor,  together with  a dynamic impedance rd creates a filter, filters the noise at frequencies described by:

      However, many applications of LDO’s, especially in cellular telephones,  require a faster enable transient response than prior art yet retain or improve noise rejection or suppression.  In prior art low power designs, a  relatively large Cbypass (more than 10nF) is often charged by a relatively small current, and thus the time required to charge is excessively long (another undesirible feature of prior art solutions).  The problem of long turn-on time has been solved  in prior art by using a Cbypass pre-charge circuit (figures 2a and 2b).  However, these prior art examples have a common weakness,  either band gap output voltage overshoot or undershoot.  Overshoot and undershoot results in the further undesrable  extension of enable response time in either case. 

      An example of a new low noise band gap reference with pre-charge circuit,  having a means to  boost bias current of a reference error amplifier upon receiving an enable signal, is shown in figure 3. The band gap reference shunt regulator is  based on the difference emitter area of Q6 and Q7 (8:1). Value of the output reference is determined by the dVbe Q6 and Q7, Vbe of Q18 and resis­tors R3, R4 and R5.  Voltage drops  on the resistor R1 and R2 a...