Pre-regulator for an ldo

Abstract

An electronic device includes a voltage regulator circuit having a power NFET coupled between an upper supply voltage and a pre-regulator output node and a current source coupled in series with a diode element between the upper supply voltage and a lower supply voltage. A gate of the power NFET is coupled to a first node between the current source and a diode element. A bypass circuit includes a power PFET coupled between the upper supply voltage and the pre-regulator output node. A comparison circuit is coupled to turn the bypass circuit off when the upper supply voltage is greater than a regulation threshold voltage.

What is claimed is: 
     
         1 . A voltage regulator circuit comprising:
 (a) a voltage input, a regulated output, and a ground plane;   (b) a first power transistor having a first drain, a first gate, and a first source, the first drain and the first source being coupled between the voltage input and the regulated output;   (c) a second power transistor having a second drain, a second gate, and a second source, the second source and the second drain being coupled between the voltage input and the regulated output;   (d) a diode element coupled between the first gate and the ground plane; and   (e) a comparison circuit having an input coupled to the voltage input, having a reference voltage input, and having a first output coupled to the second gate.   
     
     
         2 . The voltage regulator circuit of  claim 1 :
 in which the comparison circuit has a second output; and   including a pullup circuit having an input coupled to the voltage input, having an input coupled to the second output, and having an output coupled to the output of the comparison circuit and to the second gate.   
     
     
         3 . The voltage regulator circuit of  claim 1  including a current source and a first capacitor coupled in series between the voltage input and the ground plane and including a first node between the current source and the capacitor that is coupled to the diode element and the first gate. 
     
     
         4 . The voltage regulator circuit of  claim 1  including a second capacitor coupled between the regulated output and the ground plane. 
     
     
         5 . The voltage regulator circuit of  claim 1  in which the first power transistor is an N-type field effect transistor. 
     
     
         6 . The voltage regulator circuit of  claim 1  in which the second power transistor is a P-type field effect transistor. 
     
     
         7 . The voltage regulator circuit of  claim 1  in which the diode element is a zener diode. 
     
     
         8 . The voltage regulator circuit of  claim 1  in which the diode element includes stacked diode-connected N-type field effect transistors. 
     
     
         9 . An integrated circuit comprising:
 (a) a voltage input, a ground plane, and a regulated output;   (b) a pre-regulator circuit including:
 (i) an input coupled to the voltage input, a pre-regulated output, and a coupling to the ground plane; 
 (ii) a first power transistor having a first drain, a first gate, and a first source, the first drain and the first source being coupled between the voltage input and the pre-regulated output; 
 (iii) a second power transistor having a second drain, a second gate, and a second source, the second source and the second drain being coupled between the voltage input and the pre-regulated output; 
 (iv) a diode element coupled between the first gate and the ground plane; and 
 (v) a comparison circuit having an input coupled to the voltage input, having a reference voltage input, and having a first output coupled to the second gate; and 
   (c) a low dropout regulator circuit having an input coupled to the pre-regulated output and having an output coupled to the regulated output.   
     
     
         10 . The integrated circuit of  claim 9  including a data core, analog blocks, and an internal low dropout regulator, the internal low dropout regulator having an input coupled to the pre-regulator output and having regulated power outputs coupled to the data core and the analog blocks. 
     
     
         11 . The integrated circuit of  claim 10  in which the data core and the analog blocks include a carbon monoxide detection circuit, a photo-detection circuit, and an ion detection circuit. 
     
     
         12 . The integrated circuit of  claim 9  in which the voltage input is adapted to be coupled to an output of an AC/DC converter. 
     
     
         13 . The integrated circuit of  claim 9  in which the integrated circuit is a smoke detector integrated circuit. 
     
     
         14 . A process of operating a voltage regulator comprising:
 (a) receiving, at a pre-regulator input, a supply voltage having a range above, equal to, and less than a regulation threshold voltage;   (b) regulating a supply voltage that is above the regulation threshold voltage through a regulator circuit to a regulated output;   (c) passing a supply voltage that is equal to or less than the regulation threshold voltage through a bypass circuit to the regulated output.   
     
     
         15 . The process of  claim 14  in which the regulating includes coupling a clamp diode voltage to a gate of a regulator transistor in the regulator circuit. 
     
     
         16 . The process of  claim 14  in which the passing includes passing the supply voltage through a low resistance bypass transistor to the regulated output. 
     
     
         17 . The process of  claim 16  in which the passing includes controlling a gate of the bypass transistor in response to comparing the supply voltage to an internal reference voltage. 
     
     
         18 . The process of  claim 14  including:
 (a) turning on a regulator transistor in the regulator circuit and turning off a bypass transistor in the bypass circuit when the supply voltage is above the regulation threshold voltage; and 
 (b) turning off the regulator transistor in the regulator circuit and turning on the bypass transistor in the bypass circuit when the supply voltage is equal to or less than the regulation threshold voltage. 
 
     
     
         19 . The process of  claim 14  in which the passing includes passing a supply voltage that is equal to or less than the regulation threshold voltage through a bypass circuit that is separate from the regulator circuit.