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LM25010 데이터시트(PDF) 11 Page - Texas Instruments |
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LM25010 데이터시트(HTML) 11 Page - Texas Instruments |
11 / 27 page Current Limit Threshold Io Normal Operation Current Limited Low Load Current High Load Current 'I IOCL IPK LM25010 www.ti.com SNVS419C – DECEMBER 2005 – REVISED DECEMBER 2008 Figure 4. Inductor Current - Current Limit Operation N - Channel Buck Switch and Driver The LM25010 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak current through the buck switch should not exceed 2A, and the load current should not exceed 1.5A. The gate driver circuit is powered by the external bootstrap capacitor between BST and SW (C4), which is recharged each off-time from VCC through the internal high voltage diode. The minimum off-time, nominally 260 ns, ensures sufficient time during each cycle to recharge the bootstrap capacitor. A 0.022 µF ceramic capacitor is recommended for C4. Soft-Start The soft-start feature allows the regulator to gradually reach a steady state operating point, thereby reducing startup stresses and current surges. At turn-on, while VCC is below the under-voltage threshold (t1 in Figure 1), the SS pin is internally grounded, and VOUT is held at 0V. When VCC exceeds the under-voltage threshold (UVLO) an internal 11.5 µA current source charges the external capacitor (C6) at the SS pin to 2.5V (t2 in Figure 1). The increasing SS voltage at the non-inverting input of the regulation comparator gradually increases the output voltage from zero to the desired value. The soft-start feature keeps the load inductor current from reaching the current limit threshold during start-up, thereby reducing inrush currents. An internal switch grounds the SS pin if VCC is below the under-voltage lock-out threshold, or if the circuit is shutdown using the RON/SD pin. Thermal Shutdown The LM25010 should be operated below the Maximum Operating Junction Temperature rating. If the junction temperature increases during a fault or abnormal operating condition, the internal Thermal Shutdown circuit activates typically at 175°C. The Thermal Shutdown circuit reduces power dissipation by disabling the buck switch and the on-timer. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces below approximately 155°C (20°C typical hysteresis), normal operation resumes. Applications Information EXTERNAL COMPONENTS The procedure for calculating the external components is illustrated with a design example. Referring to the Block Diagram, the circuit is to be configured for the following specifications: • VOUT = 5V • VIN = 6V to 40V • FS = 175 kHz • Minimum load current = 200 mA • Maximum load current = 1.0A • Softstart time = 5 ms. R1 and R2: These resistors set the output voltage, and their ratio is calculated from: Copyright © 2005–2008, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM25010 |
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