Lighting Solution with PowerWise® LED Drivers (LM3410)
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This document provides an overview of the LM3410 LED driver from National Semiconductor. It discusses LED driver design considerations such as using constant current and current matching. It then describes inductive boost regulators and the features of the LM3410 driver, including high efficiency and various protection mechanisms. Finally, it gives examples of using the LM3410 driver in a 3-LED string application and a high brightness illumination application, listing the recommended components for each.
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Lighting Solution with PowerWise® LED Drivers (LM3410)
- 8. Typical Boost Application Circuit I LED V IN = 2.7V to 5.5V I L
- 12. Recommended Components for Example 1 Part ID Part Value Manufacturer Part Number C1, Input Cap 10μF, 6.3V, X5R TDK C2012X5R0J106M C2, Output Cap 2.2μF, 25V, X5R TDK C2012X5R1E225M D1, Catch Diode 0.4V f Schottky 500mA, 30V R Diodes Inc MBR0530 L1 10μH 1.2A Coilcraft DO1608C-103 R1 1.00Ω, 1% Vishay CRCW08051R00F R2 100kΩ, 1% Vishay CRCW08051003F R3 1.50Ω, 1% Vishay CRCW08051R50F HB LEDs 350mA, V f ≊ 3 .6V CREE XREWHT-L1-0000-0901
- 13. Design Example 2 High Brightness Illumination Application
- 14. Recommended Components for Example 2 Part ID Part Value Manufacturer Part Number C1, Input Cap 10μF, 6.3V, X5R TDK C2012X5R0J106K C2, Output Cap 10μF, 6.3V, X5R TDK C2012X5R0J106K C3, Cap 2.2μF, 25V, X5R TDK C2012X5R1E225M D1, Catch Diode 0.4V f , Schottky 1A, 20V R Diodes Inc DFLS120L L1 & L2 4.7μH 3A Coilcraft MSS6132-472 R1 665 mΩ, 1% Vishay CRCW0805R665F R2 100kΩ, 1% Vishay CRCW08051003F HB LEDs 350mA, Vf ≊ 3 .6V CREE XREWHT-L1-0000-0901
Editor's Notes
- Welcome to the training module on National Semiconductor LM3410 LED driver. This training module provides an overview of the LM3410 LED driver, and introduce the high brightness LED driving application and design examples.
- The amount of current through an LED determines the light it emits. The LED characteristics determine the forward voltage needed to achieve the needed level of current. Due to the variation in LED voltage versus current characteristics, controlling only the voltage across the LED leads to variability in light output. Therefore, most LED drivers use current regulation. Other challenges with driving LEDs depend on the LED configuration. Placing all the LEDs in a series string ensures the exact same current flows through each LED. The driving circuitry for the series configuration is simplified due to guaranteed current matching, but the number of LEDs that can be driven off an output rail is limited. With an array of LEDs, every parallel LED string must be driven with the same current to obtain even brightness and color. This configuration allows many more LEDs to be driven off an output rail but also increases the driving circuitry complexity.
- Since all of the backlighting technologies require voltages higher than that provided by a typical portable power supply such as a Lithium Ion battery, boost regulators are more commonly found in portable, battery input applications. Here is the topology of an inductive boost regulator. When the switch is closed, the inductor current increases; the inductor gains and stores energy. When the switch is open, the inductor energy is released to the output load; the inductor current decreases. As the current decreases, the inductor voltage reverses. The input voltage added by the reversed magnetic voltage leads to the output voltage higher than the input voltage. When the switch is closed again, the diode reverse biases, the load is supported by the energy stored in the output capacitor. The inductive boost regulators preferred when large boost is desired and efficiency is more important than size.
- The PowerWise brand LED drivers have exceptional performance-to power ratios, which means high performance with low power consumption. Since LEDs are proportional to the current through them and they have a nonlinear voltage to current relationship, maintaining constant current is essential to maintain a given light output and color purity. PowerWise LED drivers provide greater than 90% efficiency with good performance. Along with efficiency, these LED drivers also have power-saving features such as sophisticated dimming control, low power shutdown and standby, and open/short circuit protection. Using National’s PowerWise LED drivers, coupled with the latest LED industry trends, results in energy-efficient solutions that reduce power consumption.
- The LM3410 constant current LED driver is a monolithic, high frequency, PWM DC/DC converter in 5-pin SOT23, 6-pin LLP, and 8-pin eMSOP packages. It can drive 2.8A typical peak currents with an internal 170mΩ NMOS switch. Switching frequency is internally set to either 525 kHz or 1.60 MHz, allowing the use of extremely small surface mount inductors and chip capacitors. Even though the operating frequency is high, efficiencies up to 88% are easy to achieve. External shutdown is included, featuring an ultra-low standby current of 80nA. The LM3410 utilizes current-mode control and internal compensation to provide high-performance over a wide range of operating conditions. Additional features include dimming, cycle-by-cycle current limit, and thermal shutdown.
- The LM3410 supplies a regulated LED current by switching the internal NMOS control switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS control switch. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins.
- Here is the typical boost application circuit with series LED setting. The device operates very similar to a voltage regulated boost converter except that it regulates the output current through LEDs. The current magnitude is set with a series resistor. This series resistor multiplied by the LED current creates the feedback voltage (190 mV) which the converter regulates to. This high frequency allows the LM3410 to operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of board space. During the switch on-time, the SW pin voltage (V SW ) decreases to approximately GND, and the inductor current (I L ) increases with a linear slope. I L is measured by the current sense amplifier, which generates an output proportional to the switch current. During the switch off-time, inductor current discharges through diode D1, which forces the SW pin to swing to the output voltage plus the forward voltage (VD) of the diode. The regulator loop adjusts the duty cycle (D) to maintain a regulated LED current.
- The LED current is set using the equation shown here, where R1 is connected between the FB pin and GND. When using the LM3410 in the typical boost application configuration shown in last page, with LEDs stacked in series between the VOUT and FB pin, the maximum number of LEDs is limited by the maximum LED forward voltage (VF MAX ). The inductor value determines the input ripple current. Lower inductor values decrease the physical size of the inductor, but increase the input ripple current. An increase in the inductor value will decrease the input ripple current. An input capacitor is necessary to ensure that V IN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent Series Inductance). The LM3410 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. The output capacitor is selected based upon the desired output ripple and transient response. The diode (D1) conducts during the switch off time. A Schottky diode is recommended for its fast switching times and low forward voltage drop.
- The LM3410 can easily be converted into a SEPIC converter. A SEPIC converter has the ability to regulate an output voltage that is either larger or smaller in magnitude than the input voltage. This topology is a perfect fit for Lithium Ion battery applications where the input voltage for a single cell Li-Ion battery will vary between 2.7V & 4.5V and the output voltage is somewhere in between. Most of the analysis of the LM3410 Boost Converter is applicable to the LM3410 SEPIC Converter.
- Here is the design example for driving 3 high brightness LEDs using LM3410.
- This table lists the recommended components for example 1 shown in last page.
- Here is the design example for high brightness illumination application using LM3410.
- This table lists the recommended components for example 2 shown in last page.
- Thank you for taking the time to view this presentation on National Semiconductor LM3410 LED drivers. If you would like to learn more or go on to purchase some of these devices, you can either click on the part list link, or simple call our sales hotline. For more technical information you can either visit the National Semiconductor site – link shown – or if you would prefer to speak to someone live, please call our hotline number shown, or even use our ‘live chat’ online facility.