Maintaining the Resistor Divider Ratio During Start-up

Abstract

Circuits and methods to maintain a resistive voltage divider ratio during start-up of an electronic circuit comprising a feed-forward capacitor across a feedback resistor using a dynamic start-up circuit are disclosed as e.g. a LDO or an amplifier. In a preferred embodiment of the disclosure is applied to an LDO. Modification of the resistive voltage divider ratio caused by the feed-forward capacitor during start-up is prevented while the voltage level of a voltage access point of the voltage divider on the feed-forward capacitor is maintained. Embodiments of the disclosure presented comprise using a start-up buffer or a start-up capacitor during the start-up phase.

What is claimed is: 
     
         1 . A method to maintain a resistive voltage divider ratio during start-up of any electronic circuit comprising a feed-forward capacitor across a feedback resistor of a resistive voltage divider using a dynamic start-up circuit, comprising the following steps:
 (1) providing an electronic circuit comprising a feed-forward capacitor across a first feedback resistor of a resistive voltage divider and a start-up circuit;   (2) avoiding modification of resistive voltage divider ratio caused by feed-forward capacitor during start-up phase;   (3) monitoring output voltage and finish start-up-phase when desired output voltage of the electronic circuit is reached; and   (4) processing normal operation after start-up phase is finished wherein feed-forward capacitor is connected across the feedback resistor of the resistive voltage divider after end of start-up phase.   
     
     
         2 . The method of  claim 1  wherein said electronic circuit is a LDO. 
     
     
         3 . The method of  claim 1  wherein said electronic circuit is an amplifier. 
     
     
         4 . The method of  claim 1  wherein a start-up comparator detects if a desired output voltage is reached and hence the start-up phase is finished. 
     
     
         5 . The method of  claim 4  wherein the start-up comparator detects if the start-up phase is finished by comparing a voltage representing the output voltage of the electronic circuit with a reference voltage. 
     
     
         6 . The method of  claim 5  wherein the reference voltage is a variable voltage. 
     
     
         7 . The method of  claim 1  wherein said modification of resistive voltage divider ratio is avoided by disconnecting the feed-forward capacitor from a voltage access point of the resistive voltage divider during start-up phase only, wherein the voltage of the voltage access point of the resistive voltage divider represents the output voltage of the electronic circuit. 
     
     
         8 . The method of  claim 1  wherein a voltage level of the voltage access point of the resistive voltage divider is maintained on the feed-forward capacitor by a start-up buffer during start-up phase, wherein the voltage of the voltage access point of the resistive voltage divider represents the output voltage of the electronic circuit and wherein a bias current of the buffer amplifier is disconnected at the end of the start-up phase, wherein an input of the start-up buffer is connected to the voltage access point of the voltage divider. 
     
     
         9 . The method of  claim 8  wherein a start-up comparator detects if a desired output voltage of the electronic circuit is reached and, in this case, initiates disconnecting the bias current of the start-up buffer and connecting the feed-forward capacitor directly to the voltage access point of the resistive voltage divider. 
     
     
         10 . The method of  claim 1  wherein said modification of resistive voltage divider ratio is prevented by connecting a start-up capacitor across a second resistor of the resistive voltage divider during the start-up phase only, wherein the start-up capacitor has a capacitance of C FF ×R 2 /R 1 , wherein C FF  is a capacitance of the feed-forward capacitor, R 1  is a resistance of the part of the voltage divider between access point and ground and R 2  is a resistance of the part of the voltage diver between output voltage and access point. 
     
     
         11 . The method of  claim 10  wherein a start-up comparator detects if a desired output voltage is reached and, in this case, disconnects the start-up capacitor from the voltage access point of the voltage divider. 
     
     
         12 . The method of  claim 10  wherein the start-up capacitor has a capacitance of C FF ×R 2 /R 1 , wherein C FF  is a capacitance of the feed-forward capacitor, R 1  is a resistance of a part of the voltage divider between a voltage access point of the resistive voltage divider and ground and R 2  is a resistance of the part of the voltage divider between output voltage and access point. 
     
     
         13 . The method of  claim 1  wherein said first feedback resistor is a part of the resistive voltage divider. 
     
     
         14 . A circuit to maintain a resistive voltage divider ratio during start-up of any electronic circuit comprising a feed-forward capacitor across a feedback resistor and a resistive voltage divider using a dynamic start-up circuit, comprising:
 the feed-forward capacitor, wherein a first plate of the feed-forward capacitor is connected to an output port of the circuit and a second plate is connected to a voltage access point of the resistive voltage divider after the start-up phase of the electronic circuit is finished;   said resistive voltage divider, being connected between an output port of the circuit and ground, comprising the feedback resistor having a resistance R 2  connected between the output port of the circuit and the voltage access point in-between the voltage divider having a fraction of the output voltage and further comprising a second resistor having a resistance R 1 ;   a start-up comparator detecting if the start-up phase is finished by comparing a voltage representing the output voltage of the electronic circuit with a reference voltage;   a means to maintain a voltage level of the voltage access point across the feed-forward capacitor during start-up of the circuit, wherein the means to maintain a voltage level of the voltage access point comprises switching means to connect or disconnect components of the circuit at beginning and end of the start-up process, wherein the switching means are activated by the start-up comparator.   
     
     
         15 . The circuit of  claim 14  wherein said circuit is an LDO. 
     
     
         16 . The circuit of  claim 14  wherein said circuit is an amplifier. 
     
     
         17 . The circuit of  claim 14  wherein the end of the start-up phase is reached when the voltage of the voltage access point equals a reference voltage. 
     
     
         18 . The circuit of  claim 17  wherein the reference voltage is a fixed voltage. 
     
     
         19 . The circuit of  claim 17  wherein the reference voltage is a variable voltage. 
     
     
         20 . The circuit of  claim 14  wherein said means to maintain the voltage level further comprises a buffer amplifier wherein the start-up comparator initiates both connecting a second plate of the feed-forward capacitor to the voltage access point and disconnecting a bias current of the buffer amplifier at the end of the start-up phase using said switching means, wherein an input of the start-up buffer is connected to the voltage access point of the voltage divider. 
     
     
         21 . The circuit of  claim 20  wherein said switching means comprises a first switching means connecting the second plate of the feed-forward capacitor to the voltage access point at the end of the start-up phase and a second switching means disconnecting the bias current of the buffer amplifier. 
     
     
         22 . The circuit of  claim 21  wherein said switching means further comprises a third switching means providing an extra bias current to a differential amplifier of the circuit during start-up only. 
     
     
         23 . The circuit of  claim 14  wherein said means to maintain the voltage level comprises a second capacitor connected between the voltage access point and ground. 
     
     
         24 . The circuit of  claim 23  wherein the second capacitor has a capacitance C=C FF ×R 2 /R 1 , wherein C FF  is the capacitance of the feed-forward capacitor and wherein the start-up comparator initiates disconnecting the second capacitor from the voltage access point via a switching means at the end of the start-up phase.