Led drive system for controlling an off-chip power supply

Abstract

A LED drive system for controlling an off-chip power supply enables the power supply function to be located ‘off-chip’—i.e., on an IC which is separate from the IC containing the other LED drive system components. The off-chip supply provides a common line voltage for LED strings connected in series with respective current sink circuits at respective junctions, in response to a signal applied to a control input. The system includes a ‘minimum’ circuit which outputs the least of the voltages at the junctions, and an I/O pin which receives a signal that varies with the output of the minimum circuit. When the external supply's control input is coupled to the system's I/O pin, the present system controls the output of the external power supply as needed to provide a desired common line voltage.

1 . An LED drive system which resides on a first integrated circuit (IC) and is arranged to control an off-chip power supply that supplies a common line voltage for two or more LED strings that are connected in series with respective current sink circuits at respective junctions, each of said current sink circuits arranged to cause a desired current to be conducted by the LED string to which it is connected, said LED drive system comprising:
 a minimum circuit which receives the voltages at each of said junctions at respective inputs and which outputs a minimum voltage which is proportional to the least of the received voltages; and   an I/O pin on said first IC which receives a signal that varies with the output of said minimum circuit, wherein said I/O pin is a feedback I/O pin;   said LED drive system arranged such that, when said I/O pin is coupled to the control input of an external power supply which produces an output voltage that varies with a signal applied to said control input, said LED drive system controls said power supply's output voltage, the output voltage of said external power supply providing said common line voltage; and   said LED drive system further arranged such that, when said external power supply which produces an output voltage that varies with a signal applied to a control input is a DC-DC converter having a transformer with a primary side and a secondary side, with said secondary side coupled to an output node, and an optocoupler having an input which is coupled to said feedback I/O pin and with the voltage at said output node being said common line voltage, said LED drive system varies the signal at said feedback I/O pin as needed to produce a common line voltage which makes said minimum voltage equal to said desired junction voltage.   
     
     
         2 . The LED drive system of  claim 1 , further comprising an error amplifier which receives said minimum voltage and a reference desired junction voltage at respective inputs and which outputs a voltage that varies with the difference between said inputs, said signal which varies with the output of said minimum circuit provided to said feedback I/O pin being the output of said error amplifier. 
     
     
         3 . The LED drive system of  claim 2 , further comprising a variable current source connected to said feedback I/O pin and arranged to conduct a current via said feedback I/O pin that varies with the output of said error amplifier. 
     
     
         4 . The LED drive system of  claim 3 , further comprising a compensation node, said variable current source arranged to conduct a current that varies with a signal applied to a control input, the output of said error amplifier and said control input connected to said compensation node. 
     
     
         5 . The LED drive system of  claim 4 , further comprising one or compensation components connected to said compensation node. 
     
     
         6 . The LED drive system of  claim 1 , further comprising a DC-DC converter having a transformer with a primary side and a secondary side, with said secondary side connected to an output node, and an optocoupler having an input which is coupled to said feedback I/O pin and with the voltage at said output node being said common line voltage, said DC-DC converter being said external power supply. 
     
     
         7 . The LED drive system of  claim 6 , wherein said DC-DC converter is a boost converter, a buck converter, or a low dropout voltage regulator. 
     
     
         8 . An LED drive system which resides on a first integrated circuit (IC) and is arranged to control an off-chip power supply that supplies a common line voltage for two or more LED strings that are connected in series with respective current sink circuits at respective junctions, each of said current sink circuits arranged to cause a desired current to be conducted by the LED string to which it is connected, comprising:
 a minimum circuit which receives the voltages at each of said junctions at respective inputs and which outputs a minimum voltage which is proportional to the least of the received voltages;   a first I/O pin on said first IC which receives a signal that varies with the output of said minimum circuit, wherein said I/O pin is a feedback I/O pin;   an error amplifier which receives said minimum voltage and a reference desired junction voltage at respective inputs and which outputs a voltage that varies with the difference between said inputs; and   a variable current source connected to said feedback I/O pin and arranged to conduct a current via said feedback I/O pin that varies with the output of said error amplifier;   said LED drive system arranged such that, when said feedback I/O pin is coupled to the control input of an external power supply which produces an output voltage that varies with a signal applied to said control input, said LED drive system controls said power supply's output voltage, the output voltage of said external power supply providing said common line voltage.   
     
     
         9 . The LED drive system of  claim 8 , wherein, said signal which varies with the output of said minimum circuit provided to said feedback I/O pin being the output of said error amplifier. 
     
     
         10 . The LED drive system of  claim 1 , said LED drive system further arranged such that, when said external power supply which produces an output voltage that varies with a signal applied to a control input is a DC-DC converter having an error amplifier which is coupled to said I/O pin and to a reference voltage at respective inputs and which produces an output voltage at an output node that varies with the difference between said ‘minimum’ voltage and said reference voltage, the voltage at said output node being said common line voltage, said LED drive system varies the signal at said I/O pin as needed to produce a common line voltage which makes said ‘minimum’ voltage equal to said desired junction voltage. 
     
     
         11 . The LED drive system of  claim 10 , further comprising a DC-DC converter having an error amplifier which is coupled to said I/O pin and to a reference voltage at respective inputs and which produces an output voltage at an output node that varies with the difference between said ‘minimum’ voltage and said reference voltage, the voltage at said output node being said common line voltage, said DC-DC converter being said external power supply. 
     
     
         12 . The LED drive system of  claim 11 , wherein said DC-DC converter is a boost converter, a buck converter, or a low dropout voltage regulator.