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LM3406 Scheda tecnica(PDF) 8 Page - Texas Instruments |
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LM3406 Scheda tecnica(HTML) 8 Page - Texas Instruments |
8 / 37 page LM3406/06HV CS VSNS IF RSNS IF + - One-shot VO VF VF LED 1 LED n 0.2V + - COMP tON = 1 x 10 -11 x R ON x VO VIN LM3406, LM3406HV SNVS512C – SEPTEMBER 2008 – REVISED FEBRUARY 2010 www.ti.com APPLICATION INFORMATION THEORY OF OPERATION The LM3406 and LM3406HV are buck regulators with a wide input voltage range, low voltage reference, and a fast output enable/disable function. These features combine to make them ideal for use as a constant current source for LEDs with forward currents as high as 1.5A. The controlled on-time (COT) architecture uses a comparator and a one-shot on-timer that varies inversely with input and output voltage instead of a fixed clock. The LM3406/06HV also employs an integrator circuit that averages the output current. When the converter runs in continuous conduction mode (CCM) the controlled on-time maintains a constant switching frequency over changes in both input and output voltage. These features combine to give the LM3406/06HV an accurate output current, fast transient response, and constant switching frequency over a wide range of conditions. CONTROLLED ON-TIME OVERVIEW Figure 12 shows a simplified version of the feedback system used to control the current through an array of LEDs. A differential voltage signal, VSNS, is created as the LED current flows through the current setting resistor, RSNS. VSNS is fed back by the CS pin, where it is integrated and compared against an error amplifier-generated reference. The error amplifier is a transconductance (Gm) amplifier which adjusts the voltage on COMP to maintain a 200 mV average at the CS pin. The on-comparator turns on the power MOSFET when VSNS falls below the reference created by the Gm amp. The power MOSFET conducts for a controlled on-time, tON, set by an external resistor, RON, the input voltage, VIN and the output voltage, VO. On-time can be estimated by the following simplified equation (for the most accurate version of this expression see the Appendix): (1) At the conclusion of tON the power MOSFET turns off and must remain off for a minimum of 230 ns. Once this tOFF-MIN is complete the CS comparator compares the integrated VSNS and reference again, waiting to begin the next cycle. Figure 12. Comparator and One-Shot 8 Submit Documentation Feedback Copyright © 2008–2010, Texas Instruments Incorporated Product Folder Links: LM3406 LM3406HV |
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