Single Inductor Serial-Parallel LED Driver

Abstract

An LED driver circuit is disclosed that can drive a plurality of LED strings that are arranged in parallel, each LED string having a plurality of component LEDs that are series-connected. The LED strings can be the same type of LEDs in each string, or have different types of LEDs from one string to another. The LED driver includes a voltage control loop that dynamically regulates the output voltage across the parallel arrangement of LED strings. The output voltage is dynamically adjusted to accommodate the LED string with the largest operational voltage drop. This enables LED displays to constructed using different types of LEDs strings, but still supply the LED strings in a power efficient manner. Further, each LED string also includes its own individual current regulation loop so that the current, and therefore brightness, of each LED string can be individually adjusted.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation of U.S. patent application Ser. No. 11 / 790,221, filed on Apr. 24, 2007, now allowed, titled “Single Inductor Serial-Parallel LED Driver”, which claims the benefit of U.S. Provisional Application No. 60 / 841,543, filed on Sep. 1, 2006, which is incorporated by reference herein in its entirety.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention is directed to a driver circuit for a Light Emitting Diode (LED) display system that can be used, for example, in cellular phones, PDAs, and other similar visual display units. More particularly, this invention is directed to providing driver circuitry to drive a multi-string configuration of different types of LEDs used in display systems.[0004]2. Background Art[0005]Modern day display systems increasingly rely upon LEDs to render a high quality visual display. The applications can include displays for cell phones, personal digital ...

AI Technical Summary

Benefits of technology

"The present invention is an LED driver circuit that can drive multiple LED strings that are arranged in parallel. The circuit can control the voltage output across the LED strings and the current in each string. This allows for efficient powering of LED strings with different voltage drops. The current regulation loops ensure that each string has enough headroom for proper current regulation. The circuit also allows for tight and precise current matching across all strings, as well as individual programming and enablement / disability of each string. The design freedom also increases tolerance to LED mismatch due to manufacturing processes. The technical effects of the invention include improved efficiency, precision, and flexibility in driving LED strings."

Problems solved by technology

Since multiple LEDs are in use in display systems, such a variation in the optical output of the LEDs leads to a degradation in the overall quality of the display images and may lead to failure in operation due to reduction in the life of an LED.

Boosting from such low input voltages to 30 Volts is not very efficient and not feasible since the duty cycle required for such a process approaches 100%.

Such a current mismatch again leads to a degradation in the output of the display.

In other words, the LED driver 200 does not have any method to regulate the current in the individual LED strings, and thus falls short of attaining maximum optical output efficiency of the display system.

Additionally, the LED driver 200 is not power efficient.

However, if one of the LED strings only requires 3 volts / per LED for a total of 12 Volts, then the extra 4 Volts is dissipated across one of the resistors 216a-n, which is an efficiency loss of 25% (4 Volts / 16 Volts*100).

Images