LED Resistors (Z-Foil) (LED1625 and LED221)LED Resistors (Z-Foil) (LED1625 and LED221) Ultra High PrUltra High Precision LED Bulk Metal Foil Resistors ecision LED Bulk Metal Foil Resistors Rating to 8W, TCR of 0.2 ppm/C and Stability of 0.005% FEATURES AND BENEFITS LED1625 Temperature coefficient of resistance (TCR) (Table 1): 0.05 ppm/C nominal (0C to +60C) 0.2 ppm/C nominal (55C to +125C,+25C ref.) Resistance range: 0.3 to 500 (for higher and lower values, please contact us) LED221 Bulk Metal Foil resistors are not restricted to standard values we can supply specific as required values at For improved LED brightness. no extra cost or delivery (e.g., 50R1234 vs. 50 ) Resistance tolerance: to 0.01% Power coefficient R due to self heating: INTRODUCTION 5 ppm at rated power Load life stability: 0.005% (50 ppm) In the modern LED-backed LCD, screen brightness is a very important function. The brightness is obtained by the Electrostatic discharge (ESD): at least to 25 kV LED panel mounted to the rear of the LCD, which controls Thermal stabilization time: <1 s (within 10 ppm of brightness by boosting the input voltage connected to steady state value) the panel. The input voltage usually comes from an Li-ion Power rating: LED1625 to 1 W LED221 to 8 W (Table 2) battery. A boost converter converts the Li-ion battery Non-inductive, non-capacitive design voltage to dozens of volts for the LED panel operation. To avoid brightness flickering and to control the stability Rise time: 1 ns effectively no ringing of the LED output voltage, stable and precise current Current noise: 0.010 V /V of applied voltage RMS sense resistors with low thermal EMF and low absolute <40 dB temperature coefficient of resistance are used. Voltage coefficient: <0.1 ppm/V BC Non-inductive: <0.08 H R Non hot spot design I I Thermal EMF: <0.05 V/C A D Low harmonic distortion, linear behavior LED221 is available with a solderable heat-sink tab V ~ (Ni/Tin-plated) Zin = Figure 1Power Derating Curves LED1625 LED221 + 70 C 55 C 55 C 100 100 75 75 50 25 50 0 75 50 25 0 +25 +50 +75 +100 +125 +150 +175 Ambient Temperature (C) 25 0 75 50 25 0 +25 +50 +75 +100 +125 +150 Ambient Temperature (C) Note * This datasheet provides information about parts that are RoHS-compliant and/or parts that are non-RoHS-compliant. For example, parts with lead (Pb) terminations are not RoHS compliant. Please see the information/tables in this datasheet for details. Document No.: 63256Document No.: 63256 For any questions, contact www.vishayfoilresistors.com Revision: 08-Aug-2014Revision: 08-Aug-2014 foil vpgsensors.com 1 Percent of Rated Power Percent of Rated PowerNew LED ResistorsLED Resistors (Z-Foil) (LED1625 and LED221) (LED1625 and LED221) (1) Table 1aSpecifications Resistance Nominal TCR and Max. Spread (2) Model Tolerance Power Rating Maximum Range (55C to +125C, +25C Ref.) (3) Number (%) +70C Current () (ppm/C) 0.2% >2.0 to 10 0.5% 1 W on LED1625 1.0% 0.2 2.8 1.8 A (4) FR4 PCB 0.5% 0.3 to 2.0 1.0% Notes (1) Tighter performances are available. Please contact us: foil vpgsensors.com. (2) 0.05% and 0.1% are also available per special request. (3) Power, currentwhichever is lower. (4) See solder pad layout in Figure 5a. Table 1bSpecifications Resistance Tightest (1) Model Nominal TCR and Max. Spread (2) Range Tolerance Number (ppm/C) () (%) 0.5 to <1 0.05% 0.2 2.8 LED221 1 to <10 0.02% 0.2 2.3 10 to 500 0.01% 0.2 1.8 Note (1) MIL-Range (55C to +125C, +25C Ref.) current. Multiple LEDs should be connected in a series THE LED PRECISION RESISTORS configuration to keep an identical current flowing in each Direct control of current is provided by current sensing LED. Driving LEDs in parallel requires a ballast resistor foil resistors. LEDs are current-driven devices whose in each LED string, which leads to lower efficiency and brightness is proportional to their forward current. uneven current matching. Forward current can be controlled in two ways. The first Battery life is critical in portable applications. For an LED method is to use the LED V-I curve to determine what driver to be useful, it must be efficient. Power efficiency is voltage needs to be applied to the LED to generate the largely determined by the precision of the current sensing desired forward current. This is typically accomplished and brightness control. The amount of input power by applying a voltage source and using a ballast resistor required to generate the desired LED brightness can be as shown in Figure 2. However, this method has several easily determined by dividing the power in the LEDs drawbacks. Any change in LED forward voltage creates by the input power. Defining the efficiency in this way a change in LED current. With a nominal forward battery means that the power dissipated in the current sensing voltage of 3.6 V, the LED in Figure 2 has 20 mA of resistor contributes to the power lost in the power supply. current. If this voltage changes to 4.0 V, which is within Therefore, it is very important to use VPG s high-precision the specified voltage tolerance due to temperature or current sense resistors. In the case of a preferable and manufacturing changes, the forward current drops to calculated value of 10 , the best choice is the LED1625 14 mA. This 11% change in forward voltage causes or LED221 10 resistor with 0.1% tolerance. a much larger 30 % change in forward current. Also, depending upon the available input voltage, the voltage Figure 4 shows a typical LED driver application that drives drop and power dissipation across the ballast resistor a number of LEDs with 20 mA of forward current and waste power and reduce battery life. operates from an input voltage range of 2.8 V to 6.0 V. The entire circuit consists of the control IC, two small ceramic The second, preferred method of regulating LED current capacitors, an inductor, a diode, and a 10 current sense is to drive the LED with a constant current source. The resistor. For this application, the LED1625 or LED221 constant current source eliminates changes in current due 10 resistor would be ideal. This small circuit shows to variations in forward voltage, which translates into a the high level of integration that is achieved with todays constant LED brightness. Generating a constant current LED drivers. The primary power supply functions and the source is fairly simple. Rather than regulating the output secondary features such as load disconnect, overvoltage voltage, the input power supply regulates the voltage protection, and PWM dimming have been implemented across a current sense resistor. Figure 3 shows this with a control IC and five small surface-mount passive implementation. The power supply reference voltage and components. the value of the current sense resistor determine the LED www.vishayfoilresistors.com For any questions, contact Document No.: 63256 2 foil vpgsensors.com Revision: 08-Aug-2014