LM3404_2

COT Drivers Control LED Ripple Current COT Drivers Control LED Ripple Current The constant on-time (COT) control method used by the LM3402 and LM3404 constant-current buck regulators provides a balance between control over switching frequency and fast transient response. Normally this "quasi-hysteretic" control senses the input voltage and adjusts the on-time tON of the power MOSFET as needed to keep fSW constant. Investigating a little more deeply reveals that tON is in fact proportional to the current flowing into the RON pin. The addition of a single, general purpose PNP transistor forces tON to be proportional to (VIN - VO) and provides two benefits that are particularly useful to LED drivers: improved tolerance of the average LED current, IF, and constant LED ripple current, ΔiF. National Semiconductor Application Note 1853 Chris Richardson September 23, 2008 LEDs have a relationship between their luminous flux and forward current, IF, that is linear up to a point. Beyond that point, increasing IF causes more heat than light. High ripple current forces the LED to spend half of the time at a high peak current, putting it in the lower lm/W region of the flux curve. This reduces the light output when compared to a purely DC drive current even though the average forward current remains the same. Close inspection of LED datasheets also reveals that the absolute maximum ratings for peak current are close to or often equal to the ratings for average current. High current density in the LED junction lowers lumen maintenance, providing yet another incentive for keeping the ripple current under control. Benefits of Constant Ripple The luminous flux and dominant wavelength (or color temperature for white LEDs) of LED light are controlled by average current. The constant-ripple LED driver in Figure 1 is much better at controlling average LED current over changes in both input voltage and changes in output voltage because it fixes the valley of the inductor current and also fixes the current ripple. Controlling LED ripple current implies control over peak LED current, which in turn affects the luminous flux of an LED. All Circuit Performance The circuit of Figure 1 uses the PNP-based constant ripple concept to take an input voltage of 24VDC ±10% and drive 1A through as many LEDs in series as the maximum output voltage will allow. For a circuit with 'n' LEDs of forward voltage VF in series, the output voltage is: VO = 0.2 + n x VF 30064501 FIGURE 1. Constant Ripple LED Driver Using the LM3404 Buck Regulator The maximum voltage that can be achieved is then: VO-MAX = VIN-MIN x (1 - fSW x 300 ns) In the above equation, the 300 ns term reflects the minimum off -time of the LM3402 and LM3404 buck regulators. in output voltage is effectively a change in the number of series-connected LEDs that the circuit drives. One circuit with both average current and ripple current controlled independently of VO can now power anything from a single infrared LED (VF-TYP of ~1.8V) to as many as five white LEDs in series, yielding a VO of ~18V. Such a circuit would be ideal for an LED-driving power-supply module. Many of the existing, commercial AC-input 'brick' modules for driving LEDs are specified to provide a constant current of 'x' mA at a voltage up to 'y' volts. Depending on the need for galvanic www.national.com Making a "Universal" Current Source Figure 2 and Figure 3 show the dependence of ripple current and switching frequency against output voltage. This change © 2008 National Semiconductor Corporation 300645 AN-1853 AN-1853 isolation and/or power factor correction, the LM3402 or LM3404 buck regulator could be paired with an existing ACDC regulator to provide the 24V, resulting in a high-quality universal current source. desired peak-to-peak inductor ripple current, ΔiL. The required inductance is then: 2. Select the closest standard inductor value to L and call it LSTD. RON can then be calculated with the following expression: 3. 4. Use the closest 1% resistor value for RON. Design for the remaining components (input capacitor, Schottky diode, etc.) remains the same, and is outlined in the LM3402 and LM3404 datasheets. Switching Frequency Changes When using the LM3402 and LM3404 buck regulators in the constant-ripple configuration, the switching frequency will change with VIN and VO. Careful attention to PCB layout and proper filtering must be employed will all switching converters, and particular care is needed for systems where fSW changes. The following steps can be used to predict the switching frequency: 1. Calculate the on-time at the minimum and maximum values of VIN and VO using the actual 1% resistor value of RON and the following equation: 30064502 FIGURE 2. Ripple Current vs. Output Voltage 2. The switching frequency can then be determined using tON and the following expression: Conclusion 30064503 FIGURE 3. Switching Frequency vs. Output Voltage Design Procedure Designing for constant ripple in a COT converter requires a change in the selection of the on-time setting resistor RON: 1. Start with the typical input voltage, VIN-TYP, and an output voltage that is at the center between the minimum and maximum expected value, VO-CTR. Use the maximum permissible switching frequency, fSW-MAX, and the A pure DC LED drive current would be ideal for LEDs, but in practice the majority of LED lighting is powered from the AC mains and includes at least one switching regulator between the wall and the LEDs. Even battery or solar-powered systems are likely to employ a switching regulator in the interest of power efficiency. Therefore, some amount of ripple current will be present in almost every LED driver design. Allowing higher ripple current reduces the size and cost of the drive circuit, but comes at the expense of light output and reliability. Armed with the ability to control both LED ripple current and switching frequency, the LED lighting designer can make his/ her own trade-offs between solution size, cost, and quality based on the needs of the application. www.national.com 2 AN-1853 BOM ID U1 Q1 L1 D1 CF CB CIN RSNS RON Part Number LM3404 CMPT3906 VLF10040T-330M2R1 CMSH2-40M VJ0603Y104KXXAT VJ0603Y103KXXAT C4532X7R1H685M ERJ8RQFR20V CRCW06035762F Type LED Driver PNP Inductor Schottky Diode Capacitor Capacitor Capacitor Resistor Resistor Size SO-8 SOT23-6 10 x 10 x 4.0 mm SMA 0603 0603 1812 1206 0603 Parameters 42V, 1.2A 40 VCE, 10 mA 40V, 2A 100 nF, 10% 10 nF, 10% 6.8 µF, 50V 0.2Ω, 1% 57.6 kΩ, 1% Qty 1 1 1 1 1 1 1 1 Vendor NSC Central Semi TDK Central Semi Vishay Vishay TDK Panasonic Vishay 33 µH, 2.1A, 80 Ω 1 3 www.national.com COT Drivers Control LED Ripple Current Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock Conditioners Data Converters Displays Ethernet Interface LVDS Power Management Switching Regulators LDOs LED Lighting PowerWise Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/displays www.national.com/ethernet www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH Analog University App Notes Distributors Green Compliance Packaging Design Support www.national.com/webench www.national.com/AU www.national.com/appnotes www.national.com/contacts www.national.com/quality/green www.national.com/packaging www.national.com/quality www.national.com/refdesigns www.national.com/feedback Quality and Reliability Reference Designs Feedback THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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