Light emitting element driving apparatus, method of driving a light emitting element and computer readable recording medium

Abstract

A light emitting element driving apparatus for driving a light emitting element, includes driving means for a executing PWM driving operation to drive the light emitting element at a predetermined duty cycle and for executing a soft-start driving operation during each ON-period in the PWM driving operation. During the soft-start driving operation, a current value of current supplied to the light emitting element is increased from a first constant current value up to a second constant current value. The driving apparatus includes parameter adjusting means for controlling the driving means such that the second constant current value is supplied to the light emitting element for a period that is longer than a period from a beginning of the ON-period to a time at which the current value of the current supplied to the light emitting element reaches the second constant current value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of a light emitting element driving apparatus according to the invention, and together with the detailed description of the preferred embodiments given below serves to explain the principles of the invention.

FIG. 1 is a diagram showing functional blocks of a light emitting element driving apparatus.

FIG. 2 is a diagram illustrating relation between a rated value and a minimum value of current supplied to an LED.

FIG. 3 is a flowchart for explaining operation of a parameter adjuster.

FIG. 4 is a diagram illustrating adjustment of the second constant current value.

FIG. 5 is a diagram illustrating each time period of a PWM duty cycle with a soft-start driving operation when a period for holding the first constant current value is added to a period for holding the second constant current value.

FIG. 6 is a table illustrating an example of adjustment second constant current value and the turned-on period.

FIG. 7 is an example of a connection of multiple LEDs.

FIG. 8A is a diagram illustrating a PWM driving operation, and FIG. 8B is a diagram illustrating a “Soft-Start” driving operation.

FIG. 9 is an example of a variation of characteristics of current.

FIG. 10 is a diagram illustrating a “Soft-Start” driving operation executed during the turned on period in the PWM driving operation in which the time period during which the second predetermined current is supplied is extremely short with respect to the total turned-on period.

Claims

1. A light emitting element driving apparatus for driving a light emitting element, comprising: driving means for executing a PWM driving operation to drive the light emitting element at a predetermined duty cycle and for executing a soft-start driving operation during each ON-period in the PWM driving operation, wherein during the soft-start driving operation a current value of current supplied to the light emitting element is increased from a first constant current value up to a second constant current value; andparameter adjusting means for controlling the driving means such that the second constant current value is supplied to the light emitting element for a period that is not shorter than a period from a beginning of the ON-period to a time at which the current value of the current supplied to the light emitting element reaches the second constant current value.

2. The light emitting element driving apparatus according to claim 1, wherein a rated value defined in terms of an average current value that is supplied to the light-emitting element by the driving means during each of duty periods in the PWM driving operation is set lower than a lowest limit value of the current supplied to the light emitting element by the driving means during each ON-period in the PWM driving operation.

3. The light emitting element driving apparatus according to claim 2, wherein the parameter adjusting means comprises: second constant current adjusting means for setting the second constant current value to a current value not lower than the lowest limit value; andduty cycle adjusting means for adjusting the ON-period in the PWM driving operation such that an average value of the current supplied to the light-emitting element when the PWM driving operation is executed with the second constant current value adjusted by the second constant current adjusting means if the soft-start driving operation is not executed is substantially equivalent to the rated value.

4. The light emitting element driving apparatus according to claim 3, wherein: the second constant current adjusting means adjusts the second constant current value such that the driving means supplies the second constant current value to the light emitting element at least for a period that is longer than the period from the beginning of the ON-period to the time at which the current value of the current supplied to the light emitting element reaches the second current value during the ON-period set by the duty cycle adjusting means; andthe driving means executes the PWM driving operation in accordance with the ON-period set by the duty cycle adjusting means and executes the soft-start driving operation with the second current value adjusted by the second constant current adjusting means.

5. The light emitting element driving apparatus according to claim 2, wherein the lowest limit value is set to a current value such that the current output from the driving means shows a predetermined variation characteristic under a predetermined condition.

6. The light emitting element driving apparatus according to claim 3, wherein the lowest limit value is set to a current value such that the current output from the driving means shows a predetermined variation characteristic under predetermined condition.

7. The light emitting element driving apparatus according to claim 3, wherein the second constant current adjusting means adjusts the second constant current value based on a variation characteristic of the current output from the driving means.

8. The light emitting element driving apparatus according to claim 4, wherein the second constant current adjusting means adjusts the second constant current value based on a variation characteristic of the current output from the driving means.

9. The light emitting element driving apparatus according to claim 4, wherein the first constant current value is substantially equivalent to the lowest limit value.

10. A method of driving a light emitting element, comprising: executing a PWM driving operation to drive the light emitting element at a predetermined duty cycle, and executing a soft-start driving operation during each ON-period in the PWM driving operation, wherein during the soft-start driving operation a current value of current supplied to the light emitting element is increased from a first constant current value up to a second constant current value; andcontrolling parameters of the duty cycle and the second constant current value such that the second constant current value is supplied to the light emitting element for a period that is longer than a period from a beginning of the ON-period to a time at which the current value of the current supplied to the light emitting element reaches the second current value.

11. A computer readable recording medium storing a computer program, which is executable by a computer, to cause the computer to control a light emitting element by executing functions comprising: executing a PWM driving operation to drive the light emitting element at a predetermined duty cycle;executing a soft-start driving operation during each ON-period in the PWM driving operation, wherein during the soft-start driving operation a current value of current supplied to the light emitting element is increased from a first constant current value up to a second constant current value; andcontrolling parameters of the duty cycle and the second constant current value such that the second constant current value is supplied to the light emitting element for a period that is longer than a period from a beginning of the ON-period to a time at which the current value of the current supplied to the light emitting element the second current value.