Abstract:
A power-up control circuit architecture that utilizes zero current under normal operation. The power-up control circuit will sense a common supply voltage, Vcc, and turn an output on and off at a desired threshold voltage, providing a substantially faster on/off switch than that achievable solely by sensing the common supply voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to power-up control circuits, and more particularly to a power-up control circuit having an architecture that senses a rising and falling supply voltage while consuming no current during normal operation. 
     2. Description of the Prior Art 
     Circuits, products and processes that presently employ one or more dedicated power-up control circuits are problematic in that once the circuit(s), product(s) and/or process(es) become fully operational following the initial power-up, current continues to be consumed, and therefore wasted, by the one or more dedicated power-up control circuits. These known power-up circuit solutions and architectures require current flow throughout the power-up circuit(s) at any time the associated circuit, product, or process is in operation; and are limited in that there is presently no power-up circuit solution that requires no current during normal operation while still being able to sense a falling supply voltage. 
     In view of the foregoing, a need exists for a power-up control circuit that requires no current during normal operation while maintaining sense on the supply voltage, regardless of whether the supply voltage is rising or falling. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a power-up control circuit architecture that utilizes zero current under normal operation. The power-up control circuit will sense a common supply voltage, Vcc, and turn an output on and off at a desired threshold voltage, providing a substantially faster on/off switch than that achievable solely by sensing the common supply voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features and advantages of the present invention will be readily appreciated as the invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing figure wherein: 
     FIG. 1 is a schematic diagram illustrating a power-up control circuit according to one embodiment of the present invention. 
     While the above-identified drawing figure sets forth a particular embodiment, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram illustrating a power-up control circuit  10  according to one embodiment of the present invention. Power-up control circuit  10  can be seen connected to a common supply voltage, Vcc, as well as a common ground, GND. Power-up control circuit  10  has two main sections, including a first section  12  that controls the low-to-high common supply voltage transition, and a second section  14  that controls the high-to-low common supply voltage transition. The output signal (OutA)  16  of the first section  12  is combined in a third section  18  with the output signal (OutB)  20  of the second section. The third section  18  comprises a logic AND function for the output signals  16 ,  20  produced by the first section  12  and the second section  14 . 
     Theory of Operation 
     When the common supply voltage, Vcc, is low, output signal (OutA)  16  is low, transistors M 3  and M 4  are on, and transistors Ml and M 2  are off. As the common supply voltage, Vcc, starts to rise, signal OutA  16  remains low due to the pull-up of transistor M 4  at the gates  19 ,  21  of inverter  22 . When a sufficient turn-on voltage is reached at the gate of transistor M 1 , transistor M 1  will turn on and pull-down the gates  19 ,  21  of the inverter  22 , causing the output signal at OutA  16  to be pulled high. The output signal at OutB  20  is already high due to the pulldown resistor R 1  ( 32 ) at the gates  23 ,  25  of its associated inverter  24 , causing the output signal at Out  26  to pull high. After the signal at OutA  16  has transitioned high, it will turn off transistors M 3  and M 4  and turn on transistor M 2 , removing all current flow such that the signal at OutA  16  will be held in a high state. 
     As the common supply voltage, Vcc, stays high, capacitor C 1  ( 28 ) is being charged to Vcc-Vdiode, wherein Vdiode is provided by diode  30 . At this time, transistor M 5  is off. As Vcc starts its transition downward, capacitor  28  remains charged, and transistor M 5  stays off until Vcc goes low enough to turn on transistor M 5 . When transistor M 5  turns on, capacitor  28  discharges into pulldown resistor  32 , and for a sufficiently large pulldown resistance value, bumps up the voltage at the gates  23 ,  25  of inverter  24 . The gates  23 ,  25  to the inverter  24  associated with OutB  20  then go high, causing the signal at OutB to transition low. The action will pull the output signal at Out  26  low. As Vcc continues dropping to zero, the signal at OutA  16  will also drop to zero, turning on transistors M 3  and M 4  and turning off transistor M 2 , placing the power-up control circuit  10  back into its initial condition. 
     In view of the above, it can be seen the present invention presents a significant advancement in the art of power-up control circuits. Further, this invention has been described in considerable detail in order to provide those skilled in the power-up control circuit art with the information needed to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow.