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LM25010 bảng dữ liệu(PDF) 10 Page - Texas Instruments |
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LM25010 bảng dữ liệu(HTML) 10 Page - Texas Instruments |
10 / 28 page 'I = (VIN - VOUT) x tON L1 VIN STOP RUN RON Input Voltage LM25010 RON/SD tON(min) = VOUT x 300 ns (VIN(min) ± VOUT) RON = VOUT x (VIN - 1.4V) VIN x FS x 1.18 x 10 -10 - 1.4 k: LM25010 SNVS419D – DECEMBER 2005 – REVISED FEBRUARY 2013 www.ti.com (7) In high frequency applications the minimum value for tON is limited by the maximum duty cycle required for regulation and the minimum off-time of the LM25010 (260 ns, ±15%). The fixed off-time limits the maximum duty cycle achievable with a low voltage at VIN. The minimum allowed on-time to regulate the desired VOUT at the minimum VIN is determined from the following: (8) Shutdown The LM25010 can be remotely shut down by forcing the RON/SD pin below 0.7V with a switch or open drain device. See Figure 9. In the shutdown mode the SS pin is internally grounded, the on-time one-shot is disabled, the input current at VIN is reduced, and the VCC bypass switch is turned off. The VCC regulator is not disabled in the shutdown mode. Releasing the RON/SD pin allows normal operation to resume. The nominal voltage at RON/SD is shown in the Typical Performance Characteristics. When switching the RON/SD pin, the transition time should be faster than one to two cycles of the regulator’s nominal switching frequency. Figure 9. Shutdown Implementation Current Limit Current limit detection occurs during the off-time by monitoring the recirculating current through the internal current sense resistor (RSENSE). The detection threshold is 1.25A, ±0.25A. Referring to the Block Diagram, if the current into SGND during the off-time exceeds the threshold level the current limit comparator delays the start of the next on-time period. The next on-time starts when the current into SGND is below the threshold and the voltage at FB is below 2.5V. Figure 10 illustrates the inductor current waveform during normal operation and during current limit. The output current IO is the average of the inductor ripple current waveform. The Low Load Current waveform illustrates continuous conduction mode operation with peak and valley inductor currents below the current limit threshold. When the load current is increased (High Load Current), the ripple waveform maintains the same amplitude and frequency since the current falls below the current limit threshold at the valley of the ripple waveform. Note the average current in the High Load Current portion of Figure 10 is above the current limit threshold. Since the current reduces below the threshold in the normal off-time each cycle, the start of each on-time is not delayed, and the circuit’s output voltage is regulated at the correct value. When the load current is further increased such that the lower peak would be above the threshold, the off-time is lengthened to allow the current to decrease to the threshold before the next on-time begins (Current Limited portion of Figure 10). Both VOUT and the switching frequency are reduced as the circuit operates in a constant current mode. The load current (IOCL) is equal to the current limit threshold plus half the ripple current (ΔI/2). The ripple amplitude ( ΔI) is calculated from: (9) The current limit threshold can be increased by connecting an external resistor (RCL) between SGND and ISEN. RCL typically is less than 1Ω, and the calculation of its value is explained in the Applications Information section. If the current limit threshold is increased by adding RCL, the maximum continuous load current should not exceed 1.5A, and the peak current out of the SW pin should not exceed 2A. 10 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Product Folder Links: LM25010 |
Số phần tương tự - LM25010_15 |
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Mô tả tương tự - LM25010_15 |
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