Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1999-12-16
2001-08-21
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S540000, C327S552000
Reexamination Certificate
active
06278320
ABSTRACT:
BACKGROUND OF THE INVENTION
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
(≧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.
SUMMARY OF THE INVENTION
The present invention provides a capacitor charger (or “cap charger”) circuit to quickly charge a filter capacitor when power is applied to a voltage regulator circuit or other similar circuit. The output of a typical band-gap circuit is connected to an input node. A resistor connects the input node to a large filter capacitor, forming an RC filter network. The resistor isolates the filter capacitor from the input node. A comparator circuit has one input connected to the filter capacitor and a circuit output node and a second input connected to the input node. An output of the comparator connects to a gate of a MOS transistor, which is configured to operate as a switch.
At start-up, the comparator is on since the input values are different. The MOS switch is then turned on which charges up the filter capacitor. Once the output node equals or nearly equals the voltage at the input node, the comparator shuts off the transistor switch.
The input terminal of the comparator that is connected to the input node may be set to a predetermined voltage value that is slightly less than the input voltage, in order to compensate for circuit latencies and offsets. The circuit provides a very fast turn-on time, reaching 95% of its steady state value within 200 microseconds. This “turn-on” time may be changed by adjusting the value of a current source used to charge the capacitor. Thus, the present invention may be utilized in power sensitive applications, such as cellular telephones, that wish to shut off the voltage regulator circuit in order to conserve power.
REFERENCES:
patent: 3887863 (1975-06-01), Brokaw
patent: 4823070 (1989-04-01), Nelson
patent: 5892381 (1999-04-01), Koifman et al.
Beyer Weaver & Thomas LLP
Cox Cassandra
National Semiconductor Corporation
Tran Toan
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