Circuits and methods for controlling current in a light emitting diode array

Abstract

In one embodiment, a circuit comprises a current source to produce current to a light emitting diode array. An analog dimming circuit generates a continuous control signal to the current source to control the current in the light emitting diode array according to a range of control signal values when the control signal is above a threshold. Below the threshold, a digital modulation circuit generates an additional modulated digital signal to the current source to control the current in the light emitting diode array according to a range of modulation values when the continuous control signal is below the threshold. The continuous control signal produces current from the current source into the light emitting diode array above a first value. The combination of the continuous control signal and the modulated digital signal produces current in the light emitting diode array below the first value.

What is claimed is: 
     
       1. A circuit comprising:
 a current source to produce current to a light emitting diode array; 
 an analog dimming circuit to generate a continuous control signal to the current source to control the current in the light emitting diode array according to a range of control signal values when an input signal is above a threshold and to generate the continuous control signal according to a control signal value when the input signal is below the threshold; and 
 a digital modulation circuit to generate a modulated digital signal to the current source to control the current in the light emitting diode array according to a range of modulation values when the continuous control signal is below the threshold, wherein the digital modulation circuit comprises a normalization circuit to generate a normalized signal in response to the input signal and a first modulator coupled with the normalization circuit to generate the modulated digital signal in response to the normalized signal; 
 wherein the continuous control signal produces current from the current source into the light emitting diode array above a first value, and the modulated digital signal in combination with the continuous control signal produces current from the current source into the light emitting diode array below the first value. 
 
     
     
       2. The circuit of  claim 1  wherein the analog dimming circuit comprises:
 a second modulator to generate the continuous control signal in response to the input signal. 
 
     
     
       3. The circuit of  claim 1  wherein the normalization circuit comprises:
 a summing block to add a bit to the input signal to generate an incremented signal; and 
 a multiplying block coupled to the summing block to generate a normalized signal in response to the incremented signal. 
 
     
     
       4. The circuit of  claim 1  wherein the normalization circuit comprises:
 a shift left block to shift left the input signal to generate a normalized signal. 
 
     
     
       5. The circuit of  claim 4  wherein the normalization circuit further comprises:
 a summing block coupled to the shift left block to add a bit to the input signal. 
 
     
     
       6. A circuit comprising:
 a current source to produce current to a light emitting diode array; 
 a switching circuit to receive a first reference voltage and produce a second reference voltage that sets a current in the current source, wherein the second reference voltage is based on a modulation value of a first modulation signal to the switching circuit; 
 a multiplexer to receive a threshold and a dimming control signal; 
 a comparison circuit to receive the threshold and the dimming control signal, the comparison circuit causing the multiplexer to output the dimming control signal when the dimming control signal is greater than the threshold; 
 a first modulator coupled to an output of the comparison circuit to receive one of the threshold and the dimming control signal and produce the first modulated signal, the first modulated signal having a modulation value corresponding to the greater of the threshold and the dimming control signal from the multiplexer; and 
 a second modulator to receive a signal corresponding to the dimming control signal, and in accordance therewith, produce a second modulation signal having a range of modulation values corresponding to dimming control signal values below the threshold; 
 wherein the switching circuit generates a continuous range of reference voltages to adjust the current in the light emitting diode array when the dimming control signal is above the threshold, and clamps the value at or below the threshold, and 
 wherein the second modulation signal turns the current source on and off across a range of modulations values when the dimming control signal is below the threshold. 
 
     
     
       7. The circuit of  claim 6  further comprising a shift circuit configured between the dimmer control signal and the second modulator to normalize the dimmer control signal, wherein the dimmer control signal is equal to powers of two (2). 
     
     
       8. The circuit of  claim 6  further comprising:
 a third modulator coupled between the dimmer control signal and the multiplexer to generate a first modulated signal in response to the dimmer control signal, 
 wherein the second modulator generates the continuous control signal in response to the first modulated signal. 
 
     
     
       9. The circuit of  claim 6  further comprising:
 a third modulator and an accumulate and dump circuit coupled between the dimmer control signal and the multiplexer, 
 wherein the third modulator generates a first modulated signal in response to the dimmer control signal, 
 wherein the accumulate and dump circuit is coupled to the third modulator to generate an accumulated modulated signal in response to the first modulated signal; 
 a second modulator coupled to the accumulate and dump circuit to generate the continuous control signal in response to the accumulated modulated signal. 
 
     
     
       10. The circuit of  claim 6  further comprising a normalization circuit coupled between the dimming control signal and the second modulator to provide the signal corresponding to the dimming control signal to the second modulator. 
     
     
       11. The circuit of  claim 10  wherein the normalization circuit comprises:
 a summing block to add a bit to the dimming control signal to generate an incremented signal; and 
 a multiplying block coupled to the summing block to generate the signal corresponding to the dimming control signal in response to the incremented signal. 
 
     
     
       12. The circuit of  claim 10  wherein the normalization circuit comprises:
 a shift left block to shift left the dimming control signal to the signal corresponding to the dimming control signal in response to the dimming control signal. 
 
     
     
       13. A method comprising:
 generating a continuous control signal to control current in a light emitting diode array according to a range of control signal values when an input signal is above a threshold and to generate the continuous control signal according to a control signal value when the input signal is below the threshold; and 
 additionally generating a modulated digital signal to control the current in the light emitting diode array according to a range of modulation values when the continuous control signal is below the threshold, wherein generating the modulated digital signal further comprises generating a normalized signal in response to the input signal and generating the modulated digital signal in response to the normalized signal, 
 wherein the continuous control signal controls current in the light emitting diode array above a first value, and the combination of the continuous control signal and the modulated digital signal produces current in the light emitting diode array below the first value. 
 
     
     
       14. The method of  claim 13  wherein generating the modulated digital signal includes:
 adding a bit to the input signal to generate an incremented signal; and 
 multiplying the incremented signal by a reciprocal of the threshold to normalize the incremented signal. 
 
     
     
       15. The method of  claim 14  wherein generating the modulated digital signal includes:
 pulse width modulating the incremented signal to generate modulated digital signal. 
 
     
     
       16. The method of  claim 13  wherein generating the modulated digital signal includes:
 shifting the input signal to normalize the continuous control signal, wherein the continuous control signal is equal to powers of two. 
 
     
     
       17. The method of  claim 16  wherein generating the modulated digital signal includes:
 pulse width modulating the incremented signal to generate modulated digital signal.