Low-Cost, Small-Size Multi-String LED Driver Circuit Architecture

Description:

Overview

The University of Texas at Dallas is seeking companies interested in commercializing a novel LED driver circuit architecture which promises to provide a low-cost, small-footprint Light Emitting Diode (LED) driver solution for a large variety of applications, including backlighting for portable devices, electronic displays, street lighting, traffic lights, and automotive lighting.

LED lighting systems, comprised of LED drivers and LED display modules, have become very attractive due to numerous advantages such as low power consumption, high optical efficiency, low cost, long lifetime, and environmentally friendly operation. An LED driver is an integrated circuit (IC) which converts an incoming Direct Current (DC) supply voltage into the DC current and voltage levels required by the LED module. Typically, LED modules are composed of multiple LED “strings” connected in parallel, with each string composed of several individual LED elements connected in series. The number of LEDs per string and the total number of strings are dependent upon the desired level of illumination.

Conventional LED lighting systems require sophisticated LED driver circuitry in order to ensure that the current delivered to an LED string is constant, irrespective of variations in the LED fabrication process, operating temperature, device aging, and usage. Additionally, because the LEDs in a string are connected in series, LED drivers have to regulate their output voltage at several tens of volts, which is much higher than the maximum voltage capacity of modern transistors produced using the standard, low-cost CMOS fabrication process. Conventional LED drivers address this high voltage stress issue in two different ways:

CMOS Process: Low Cost / Large Microchip Size

Using the low-cost CMOS fabrication process, each power switch in this type of LED driver is composed of numerous low-power transistors, thereby dividing the voltage load equally across many CMOS transistors. This results in a physically large LED driver chip that is inexpensive to fabricate.

DMOS Process: High Cost / Small Microchip Size

Using the high-cost DMOS fabrication process, each power switch in this type of LED driver is composed of a single (or a few) high-power transistors. This results in a physically small LED driver chip that is very expensive to fabricate.

Further, for applications such as display backlighting which require multiple LED strings to be driven, conventional LED drivers offer two design options:

One LED driver per string: Moderate Efficiency / High Cost / Large Packaging

Most commonly, a dedicated LED driver is used for each separate string. This allows for simplified implementation, particularly when driving RGB LEDs, but requires numerous driver components, resulting in bulky packaging, high total component cost, and only moderate efficiency for the overall system.

One LED driver for all strings: Low Efficiency / Low Cost / Small Packaging

Multi-string LED driver are capable of driving multiple strings using one driver. As mentioned previously, fabrication, material, and aging variations affect the power requirements of each string, with some requiring a higher voltage than others. However, because conventional multi-string LED driver architecture is limited to providing the same voltage to all power outputs, the output voltage of a multi-string LED driver is defined by the string which requires the highest voltage. For strings which require a lower voltage, the excess power is sent to ground (wasted), thus resulting in multi-string LED drivers being very low in power efficiency.

Technology

To overcome the tradeoffs of conventional LED drivers note above, researchers in the University of Texas at Dallas Integrated Systems Design Lab have developed a novel LED driver power stage architecture that is compatible with the low-cost CMOS fabrication process, but which is capable of tolerating the high voltage stresses required to drive conventional LED strings using only a minimum number of transistors. This is achieved using a unique circuit design which ensures that the voltage stress across the power switches is reduced to half of the maximum voltage level, thereby providing an LED driver solution that is both low-cost and small in physical size.

This circuit architecture is also scalable – it can be extended to provide multiple voltages to simultaneously regulate several LED strings by the addition of simple repeating circuit units. Because each LED string is driven by an independently regulated supply voltage, the result is a very power-efficient multi-string LED driver circuit that is low-cost and small in size.

Features & Benefits

Low Cost	· Requires industry-standard, low-cost CMOS fabrication process – estimated 80% cost savings over DMOS process. · Multi-string LED Driver configuration requires fewer parts (inductors, etc.).
Small Size	· Compared to conventional CMOS-fabricated LED drivers, the present LED driver requires far fewer power switches, and is therefore smaller in size. · Multi-string LED Driver configuration requires fewer parts (inductors, etc.).
Power Efficient	· Multi-string LED Driver configuration maximizes power efficiency for each LED string.
High Performance	· Circuit architecture provides lower inductor current ripple, lower output voltage ripple, and faster transient response.