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LM34914 数据表(PDF) 11 Page - Texas Instruments |
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LM34914 数据表(HTML) 11 Page - Texas Instruments |
11 / 24 page 'I IOUT2 IOUT3 Normal Operation Load Current Decreases Current Limited Load Current Increases Normal Operation Current Limit Threshold IOUT1 TON 2.5V TON 2 LM34914 www.ti.com SNVS453B – MAY 2006 – REVISED MARCH 2013 Figure 10. Inductor Current - Normal and Current Limit Operation N - Channel Buck Switch and Driver The LM34914 integrates an N-Channel buck switch and associated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022 µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During each off-time, the SW pin is at approximately -1V, and C4 is recharged for the next on-time from VCC through the internal diode. The minimum off-time ensures a minimum time each cycle to recharge the bootstrap capacitor. Softstart The softstart feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold, an internal 12.5 µA current source charges up the external capacitor at the SS pin to 2.5V (t2 in Figure 8). The ramping voltage at SS (and the non-inverting input of the regulation comparator) ramps up the output voltage in a controlled manner. An internal switch grounds the SS pin if VCC is below the under-voltage lockout threshold, or if the RON/SD pin is grounded. Thermal Shutdown The LM34914 should be operated so the junction temperature does not exceed 125°C. If the junction temperature increases above that, an internal Thermal Shutdown circuit activates (typically) at 175°C, taking the controller to a low power reset state 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 155°C (typical hysteresis = 20°C), normal operation resumes. Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM34914 |
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