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LM34910 데이터시트(PDF) 9 Page - Texas Instruments |
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LM34910 데이터시트(HTML) 9 Page - Texas Instruments |
9 / 17 page 1.25A Load Current Increases IPK IOCL IO 'I Normal Operation Current Limited LM34910 www.ti.com SNVS297A – OCTOBER 2004 – REVISED FEBRUARY 2005 Figure 7 illustrates the inductor current waveform. During normal operation the load current is Io, the average of the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches 1.25A. During the Current Limited portion of Figure 7, the current ramps down to 1.25A during each off-time, initiating the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an amount equal to: ΔI = (VIN - VOUT) x tON / L1 (7) During this time the LM34910 is in a constant current mode, with an average load current (IOCL) equal to 1.25A + ΔI/2. Figure 7. Inductor Current - Current Limit Operation The current limit threshold can be increased by connecting an external resistor between SGND and ISEN. The external resistor will typically be less than 1 Ω. The peak current out of SW and ISEN must not exceed 3.5A. The average current out of SW must be less than 3A, and the average current out of ISEN must be less than 2A. Therefore IPK in Figure 7 must not exceed 3.5A, and IOCL must not exceed 2A. N - Channel Buck Switch and Driver The LM34910 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak current allowed through the buck switch is 3.5A, and the maximum allowed average current is 3A. 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 charges from VCC through the internal diode. The minimum off-time of 280 ns 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 11.5 µA current source charges up the external capacitor at the SS pin to 2.5V. 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, if a thermal shutdown occurs, or if the RON/SD pin is grounded. Thermal Shutdown The LM34910 should be operated so the junction temperature does not exceed 125°C. If the junction temperature increases, an internal Thermal Shutdown circuit, which activates (typically) at 175°C, takes the controller to a low power reset state by disabling the buck switch and the on-timer, and grounding the Softstart pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces below 155°C (typical hysteresis = 20°C), the Softstart pin is released and normal operation resumes. Copyright © 2004–2005, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM34910 |
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