Performance-enhancing COTS current-sensing LED resistors

Designed to improve LED brightness and efficiency in portable battery-powered applications, Vishay Precision Group (VPG) ultra high-precision Bulk Metal Z-Foil current-sensing resistors are now available through Farnell element14. The LED1625 and LED221 provide low TCR of 0.05ppm/C nominal (0C to +60C) and 0.2ppm/C nominal (-55C to +125C,+25C ref.), load-life stability of ±0.005%, and tight tolerances to ±0.01%. In LED-backlit LCD displays for portable devices, screen brightness is an important function. Brightness is controlled via an LED panel mounted on the rear of the LCD, which boosts the input voltage — usually coming from a Li-ion battery - connected to the panel. A boost converter converts the input voltage to dozens of volts for the LED panel's operation. To avoid brightness flickering and to control the stability of the LED output voltage, stable and precise current resistors with low thermal EMF and low absolute TCR are required, says the company. "Battery life is critical in portable battery-powered applications, so for an LED driver to be useful, it must be efficient," said Hero Faierstain, senior manager, application engineering, Vishay. "Power efficiency is largely determined by the precision of the 4-terminal current sensing resistor and brightness control. The amount of input power required to generate the desired LED brightness can be determined by dividing the power in the LEDs by the input power. Defining the efficiency in this way means that the power dissipated in the current sensing resistor contributes to the power lost in the power supply. Therefore, it is very important to use high-precision current sense resistors, such as the LED1625 and LED221." "Beginning in 2015, Farnell element14 will be able to ship the LED Series resistors with no minimum quantities," said Jacky Acoca, sales and marketing director, Vishay. "Through collaboration with VPG, Farnell element14 is helping to set a new standard by shipping the exact number of resistors needed of the precise resistance value and tolerances required." The LED1625 offers a 1W power rating and a resistance range from 0.3ohm to 10ohm, while the LED221 features power ratings to 8W and a resistance range from 0.5ohm to 500ohm. Any conceivable value within the devices' resistance ranges, to six digits, is available with no additional cost or lead time effect. The resistors offer PCR (or ?R due to self-heating) of 5ppm at rated power. Featuring a non-inductive (<0.08µH), non-capacitive design, the LED1625 and LED221 offer a rise time of 1ns with effectively no ringing, voltage coefficient of <0.1ppm/V, current noise of 0.01µVRMS/V of applied voltage (<-40dB), and thermal EMF of <0.05µV/C. The resistors withstand ESD spikes to at least 25kV and are available with lead (Pb)-free and tin/lead alloy terminal finishes. The LED221 is also available with a solderable heatsink tab. Samples, as well as prototype and production quantities, of the LED1625 and LED221 are available from Farnell element14 now, says the company.