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LM25011AQ Datasheet(PDF) 11 Page - Texas Instruments |
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LM25011AQ Datasheet(HTML) 11 Page - Texas Instruments |
11 / 26 page Current Limit Threshold Normal Operation Load Current Increases Current Limited 'I Inductor Current Decreases Load Current Normal Operation Voltage at the FB Pin 2.51V Voltage at the CS Pin 0V IPK IO1 IO2 IOCL LM25011, LM25011-Q1 www.ti.com SNVS617F – MAY 2009 – REVISED JANUARY 2011 The on-time must be chosen greater than 90 ns for proper operation. Equation 1, Equation 5, and Equation 6 are valid only during normal operation; that is, the circuit is not in current limit. When the LM25011 operates in current limit, the on-time is reduced by approximately 40% (this feature is not present in LM25011A). This feature reduces the peak inductor current which may be excessively high if the load current and the input voltage are simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the output voltage resumes its normal regulated value. The maximum continuous current into the RT pin must be less than 2 mA. For high frequency applications, the maximum switching frequency is limited at the maximum input voltage by the minimum on-time one-shot period (90 ns). At minimum input voltage the maximum switching frequency is limited by the minimum off-time one-shot period, which, if reached, prevents achievement of the proper duty cycle. CURRENT LIMIT Current limit detection occurs during the off-time by monitoring the voltage across the external current sense resistor RS. Referring to the Block Diagram, during the off-time the recirculating current flows through the inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across the sense resistor exceeds the threshold (VILIM) the current limit comparator output switches to delay the start of the next on-time period. The next on-time starts when the recirculating current decreases such that the voltage across RS reduces to the threshold and the voltage at FB is below 2.51V. The operating frequency is typically lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by approximately 40% (only in LM25011) if the voltage at the FB pin is below its threshold when the voltage across RS reduces to its threshold (VOUT is low due to current limiting). Figure 4 illustrates the inductor current waveform during normal operation and in current limit. During the first “Normal Operation” the load current is I01, the average of the inductor current waveform. As the load resistance is reduced, the inductor current increases until the lower peak of the inductor ripple current exceeds the threshold. During the “Current Limited” portion of Figure 4, each on-time is reduced by approximately 40%, resulting in lower ripple amplitude for the inductor’s current. During this time the LM25011 is in a constant current mode with an average load current equal to the current limit threshold plus half the ripple amplitude (IOCL), and the output voltage is below the normal regulated value. Normal operation resumes when the load current is reduced (to IO2), allowing VOUT and the on-time to return to their normal values. Note that in the second period of “Normal Operation”, even though the inductor’s peak current exceeds the current limit threshold during part of each cycle, the circuit is not in current limit since the inductor current falls below the current limit threshold during each off time. The peak current allowed through the buck switch is 3.5A, and the maximum allowed average current is 2.0A. Figure 4. Normal and Current Limit Operation Copyright © 2009–2011, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM25011 LM25011-Q1 |
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