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LM3401 데이터시트(PDF) 7 Page - Texas Instruments |
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LM3401 데이터시트(HTML) 7 Page - Texas Instruments |
7 / 27 page VCS 10V/Div VSNS 100 mV/Div ILED 100 mA/Div 400 ns/DIV VIN SNS HG DIM ILIM GND + - CS on 20 PA ILIM VDD regulator + - + - -0.13V UVLO HYS 5.5 PA X 0.2 VREF 200 mV LM3401 www.ti.com SNVS516C – AUGUST 2007 – REVISED MAY 2013 BLOCK DIAGRAM Figure 14. Block Diagram Operational Description HYSTERETIC CONTROL The LM3401 is a step-down DC-DC controller designed to provide a constant current source for driving a high power LED string. The LM3401 uses comparator-based voltage mode hysteretic control for a simple and stable design. Hysteretic control does not utilize an internal oscillator, but relies on output conditions to directly control switching. The LM3401 controls LED current within the adjustable hysteresis window by monitoring peak and valley voltage at the SNS pin. A dual sided hysteresis window is used to optimize accuracy. Regulated LED current flows to ground through a sense resistor at the SNS pin. The voltage generated at the SNS pin is compared to the 200 mV internal reference. When the SNS voltage falls below the reference voltage minus hysteresis, the output of the hysteretic comparator goes low. This results in the driver output, HG, pulling the gate of the PFET low and turning on the PFET. With the PFET on, LED current ramps up through the PFET and the inductor. As the LED current increases, the SNS voltage reaches its upper threshold (reference voltage plus hysteresis). This forces the output of the comparator and HG to go high, which turns the PFET off. When the PFET turns off, current flows through the catch diode, and LED and inductor current ramp down. When the SNS voltage falls to its lower threshold, the cycle repeats. The resulting LED current, SNS, and switch node waveforms are shown in Figure 15. Figure 15. Hysteretic Switching Waveforms Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 7 Product Folder Links: LM3401 |
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