Constant Current Supply Options for LED Applications
By Ali Salih, Design Engineering Director for the Standard Products Group • ON Semiconductor
LEDs have emerged as the technology of choice for many lighting applications due to the energy saving, high brightness, wide color range, design flexibility and longevity they offer. However, to help designers implement effective and reliable LED lighting designs, cost-effective current regulators are required.
The voltage source in LED lighting systems is different depending on the type of application. For architectural and buildings applications, we can normally expect the supply voltage to be AC mains. Outside lighting meanwhile may be supplied by AC mains, unregulated supplies such as 12 V lead acid batteries or perhaps solar power. For automotive the power source is typically a 14 V battery.
Although possible, driving LEDs from a voltage source without some form of power conversion is not a good idea as normal fluctuations in the voltage can result in dramatic differences in LED current and brightness. Factors such as the very steep voltage/current curve and a wide variation in forward voltage from lot-to-lot of LEDs necessitate the use of an isolated or non-isolated power conversion stage.
The primary function of an LED driver is to limit the current regardless of input condition and forward voltage variation across a range of operating conditions. The driver itself, as well as the overall system solution, must also meet the application requirements in terms of efficiency, current tolerance, form factor, size, cost and safety. The chosen approach must also be easy to implement and robust enough to meet the environmental extremes of the specific application.
Regulation by series resistor represents a low cost solution, but with this simplistic approach the LED current and brightness increases with fluctuations in the supply voltage. This fact renders the approach unsuitable for the vast majority of designs that require even light performance. An additional and serious concern is that a fluctuating LED current has a significant impact on the life expectancy and reliability of the LED or LED string. The use of resistors also necessitates the need for the costly and time consuming practice of LED binning.
Many existing designs use conventional Field Effect Transistors (FETs) and Bipolar Junction Transistors (BJTs) but these exhibit large current variations over device, voltage and temperature. This results in different levels of brightness from module to module and fluctuations during use depending on the junction temperature of the transistor and the instability of the supply voltage.
High-end switching regulators that use advanced integrated circuits and power conversion topologies offer high efficiency and are effective in controlling LED current. However they are also costly, have EMI issues associated with non-linear regulation, and can occupy an unacceptable amount of board space within the design.
Constant current regulators (CCRs) presented in this article emerge as an option that can provide a simple, cost-effective, but still high performance LED driver solution. Their design is based on semiconductor FET technology, but includes features allowing them to have high device-to-device current uniformity over a wide voltage range while significantly reducing the current temperature coefficient. The efficient and cost-effective design of this type of semiconductor regulator is enabled by low wafer cost, small die size and a small footprint two-terminal package. CCR devices design can achieve high yield with high die-to-die current accuracy without the need for CCR sorting. They also negate the need for LED binning.
The internal design and construction of CCRs for a given package is carefully tailored to enable steady state current uniformity over a wide voltage headroom range over 20 V. Furthermore, slight negative current coefficient, which offers a built-in circuit protection, can intentionally be designed into the device. CCRs can typically withstand 50 V to over 100 V and regulation voltage range is only limited by the thermal mass of the application circuit board. The CCRs themselves are housed in industry-standard, two-terminal packages that are quick and easy to place in designs with restricted PCB real estate.
Traditional FET and BJT current sources may vary by some ±50 percent and advanced LED drivers are typically specified with ±15 percent or 10 percent variations. Constant current regulators exceed these uniformity levels using a proprietary and simple design. CCRs are enhanced to achieve better than ±5 percent uniformity without sorting, which is unprecedented for discrete LED drivers. The internal design of the chip is capable of tighter distribution but the specified 5 percent variation is not all due to the chip itself, but may be partially attributed to the external connections to the package.
Current levels used in the latest automotive, back lighting, signage and architectural lighting applications are typically in the range 10 mA to 30 mA. This is well within the scope of available CCRs that offer current ratings extended up to around 350 mA with intermediate levels of 60 mA and 160 mA. In addition to extremely tight current distribution, CCR technology has enabled the reduction of temperature coefficients by approximately 60 percent compared to conventional transistor-based regulators. This uniformity allows the customer lighting application to display very uniform brightness over wide array panels that use many LEDs. In some special applications where extreme uniformity or very specific brightness of light is a requirement, LEDs may still need to be sorted. Where this is the case, CCR LED drivers lend themselves to current adjustment to further optimize the output brightness. The design of CCRs enables the current to be accurately fixed over a wide input voltage range spanning around 3 V to over 40 V. A low ‘on’ voltage of less than 2 V can be advantageous in many designs – a good example being low battery indicators in automotive applications.
A comparison of CCR performance relative to conventional FETs and bias resistors is illustrated in Figure 1. The 28 mA application shows the resistor is only effective for a singular point (14 V) with low brightness in the low voltage domain below 14 V and high current above 14 V producing highly variable brightness and likely damage to LED reliability. The conventional FET offers some regulation but the variation of current is 6 mA (20 percent) for the package and board used. Meanwhile, the CCR shows steady current regulation with a change of only 2 mA in the 10 V overhead voltage space. The slight negative coefficient is intentional by design in order to provide some overcurrent protection to the LEDs. The coefficient of the regulator can be tailored by design to enable slight current increase to compensate for LED brightness reduction at high temperature.
Figure 2 demonstrates the constant regulation current of a CCR and illustrates turn on at low voltage as well as uniform current over voltage for 60 mA and 100 mA devices. The small increase in current is due to the affect of the heat sinking that is sufficient to overcome the self-heating in the specific device design illustrated.
The temperature coefficient reduction by the CCRs described here is presented in Figure 3 where conventional FETs are compared to two types of CCR. The temperature coefficient (TC) of regular FETs in the same package is about 0.17 mA/°C, while CCR TC is reduced to about 0.07 mA/°C and further reduced to 0.045 mA/°C. This reduction of more than 60 percent of temperature coefficient down to a mere 35 mA /°C is enabled by design elements integrated in the CCR monolithic chip. The small TC designed in these new CCRs is extremely valuable for maintaining current over wide operating temperatures.
Figure 4 illustrates the elegance and system simplification utilizing of CCRs in residential and architectural lighting. Here, LEDs are powered directly from AC mains through a standard rectifier bridge and the current uniformity is sustained by a single CCR. For 110 VAC, 30 to 40 CCRs can be strung in series, for 220 VAC, 60 to 80 LEDs connected in series provide a powerful illumination source. Owing to the circuit simplicity and space saving capabilities of CCRs, very slim and flat lighting panels can be produced. The illumination capacity of one string of 60 mA LEDs driven from 100 VAC is similar to that of a 60 W conventional light bulb but consumes one tenth the power and generates virtually no heat. In addition to energy efficiency and longevity, LED lighting avoids the use of the hazardous materials found in compact fluorescent lighting (CFL) and the bulkiness of transformer powered light tubes. Looking ahead, CCRs will help drive the growth of LED lighting as an effective and viable alternative to traditional lighting approaches.
Ali Salih is the design engineering director for the Standard Products Group of ON Semiconductor, Phoenix, AZ. Dr. Salih has more than 20 years of experience in power semiconductor device design and new products development, having started his career as a device designer and R&D materials scientist at General Instrument, later moving to ON Semiconductor, where his responsibility initially covered all classes of power semiconductor devices. Dr. Salih’s main focuses now are on constant current regulator (CCR) LED drivers, low capacitance ESD protection, integrated power components, IGBT, rectifiers and thyristors. Dr. Salih earned his M.S. and Ph.D. from NC State University in solid state Physics and electronic Materials Science and Eng., respectively. Dr. Salih can be reached at Ali.Salih@onsemi.com.
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Constant Current Supply Options for LED Applications
Regulation by series resistor represents a low cost solution, but with this simplistic approach the LED current and brightness increases with fluctuations in the supply voltage. This fact renders the approach unsuitable for the vast majority of designs that require even light performance. An additional and serious concern is that a fluctuating LED current has a significant impact on the life expectancy and reliability of the LED or LED string. The use of resistors also necessitates the need for the costly and time consuming practice of LED binning.
Constant current regulators (CCRs) presented in this article emerge as an option that can provide a simple, cost-effective, but still high performance LED driver solution. Their design is based on semiconductor FET technology, but includes features allowing them to have high device-to-device current uniformity over a wide voltage range while significantly reducing the current temperature coefficient. The efficient and cost-effective design of this type of semiconductor regulator is enabled by low wafer cost, small die size and a small footprint two-terminal package. CCR devices design can achieve high yield with high die-to-die current accuracy without the need for CCR sorting. They also negate the need for LED binning. 
Current levels used in the latest automotive, back lighting, signage and architectural lighting applications are typically in the range 10 mA to 30 mA. This is well within the scope of available CCRs that offer current ratings extended up to around 350 mA with intermediate levels of 60 mA and 160 mA. In addition to extremely tight current distribution, CCR technology has enabled the reduction of temperature coefficients by approximately 60 percent compared to conventional transistor-based regulators. This uniformity allows the customer lighting application to display very uniform brightness over wide array panels that use many LEDs. In some special applications where extreme uniformity or very specific brightness of light is a requirement, LEDs may still need to be sorted. Where this is the case, CCR LED drivers lend themselves to current adjustment to further optimize the output brightness. The design of CCRs enables the current to be accurately fixed over a wide input voltage range spanning around 3 V to over 40 V. A low ‘on’ voltage of less than 2 V can be advantageous in many designs – a good example being low battery indicators in automotive applications. 
Figure 2 demonstrates the constant regulation current of a CCR and illustrates turn on at low voltage as well as uniform current over voltage for 60 mA and 100 mA devices. The small increase in current is due to the affect of the heat sinking that is sufficient to overcome the self-heating in the specific device design illustrated. 