Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display, as well as for general lighting applications.
In a large LCD matrix display, and in large solid state lighting applications, such as street lighting, typically the LEDs are supplied in a plurality of strings of serially connected LEDs, at least in part so that in the event of failure of one string at least some light is still output. The constituent LEDs of each LED string thus share a common current.
In order to supply a white backlight for the matrix display one of two basic techniques are commonly used. In a first technique strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique individual strings of colored LEDs are placed in proximity so that in combination their light is seen as a white light. Often, two strings of green LEDs are utilized to balance each single red and blue LED string.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the matrix display, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused by a diffuser so as to avoid hot spots. In the case of colored LEDs, a mixer is further required, which may be part of the diffuser, to ensure that the light of the colored LEDs is not viewed separately, but rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design and manufacturing is centered on the need to maintain a correct white point in the event that colored LEDs are utilized.
LEDs providing high luminance exhibit a range of forward voltage drops, denoted Vf, and their luminance is primarily a function of current. For example, one manufacturer of LEDs suitable for use with a portable computer, such as a notebook computer, indicates that Vf for a particular high luminance white LED ranges from 2.95 volts to 3.65 volts at 20 mA and an LED junction temperature of 25° C., thus exhibiting a variance in Vf of greater than ±10%. Furthermore, the luminance of the LEDs vary as a function of junction temperature and age, typically exhibiting a reduced luminance as a function of current with increasing temperature and increasing age. In order to provide backlight illumination for a portable computer with an LCD matrix display of at least 25 cm measured diagonally, at least 20, and typically in excess of 40, LEDs are required. In order to provide street lighting, in certain applications over 100 LEDs are required.
In order to provide a balanced overall luminance, it is important to control the current of the various LED strings to be approximately equal. In one embodiment, as described in U.S. patent application Ser. No. 11/676,313 to Korcharz et al, entitled “Voltage Controlled Backlight Driver”, filed Feb. 19, 2007 and published as US 2007/0195025 on Aug. 23, 2007, the entire contents of which is incorporated herein by reference, this is accomplished by a controlled dissipative element placed in series with each of the LED strings, the controlled dissipative element further acting as an enable/disable control for the LED string, which may be advantageously driven by a pulse width modulation (PWM) signal. Additionally, a power source supplying power to a plurality of LED strings is controlled responsive to a function of an electrical characteristic of at least one of the plurality of LED strings receiving power from the power source. The controlled dissipative element is typically embodied in a metal-oxide semiconductor field effect transistor (MOSFET), which may be provided either within the LED driving control integrated circuit, or externally thereof.
In order to ensure that the current flowing through the plurality of LED strings are equal, and to control the power source as described above, it is necessary to sample the voltage at one or more terminals of each of the dissipative elements. In the event that the power source is arranged to produce a voltage in excess of the voltage rating of the LED driving control integrated circuit (IC), care must be taken to ensure that the excess voltage is not experienced by the LED driving control IC. This is particularly acute when external MOSFETs are used. A voltage divider arrangement is known to be used to adjust high voltages for sampling by lower voltage ICs, however in the case of LED strings this is disadvantageous, since the low leakage current passing through the LED strings via the voltage divider produces a weak illumination even when the LED string is to be disabled via the dissipative element, or other switch.