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LM25010SD Scheda tecnica(PDF) 10 Page - National Semiconductor (TI) |
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LM25010SD Scheda tecnica(HTML) 10 Page - National Semiconductor (TI) |
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10 / 17 page  Regulation Comparator The feedback voltage at the FB pin is compared to the voltage at the SS pin (2.5V, ±2%). In normal operation an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch conducts for the on-time programmed by R ON, causing the FB voltage to rise above 2.5V. After the on-time period the buck switch remains off until the FB voltage falls below 2.5V. Input bias current at the FB pin is less than 5 nA over temperature. Over-Voltage Comparator The feedback voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the on-time is immediately terminated. This condition can occur if the input voltage, or the output load, changes suddenly. The buck switch remains off until the voltage at FB falls below 2.5V. ON-Time Control The on-time of the internal buck switch is determined by the R ON resistor and the input voltage (VIN), and is calculated as follows: (5) The R ON resistor can be determined from the desired on- time by re-arranging Equation 5 to the following: (6) To set a specific continuous conduction mode switching frequency (Fs), the R ON resistor is determined from the following: (7) In high frequency applications the minimum value for t ON 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 V OUT 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 3. 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 V CC bypass switch is turned off. The V CC regulator is not disabled in the shutdown mode. Releasing the RON/SD pin allows normal operation to re- sume. 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. Current Limit Current limit detection occurs during the off-time by monitor- ing the recirculating current through the internal current sense resistor (R SENSE). 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 4 illustrates the inductor current waveform during normal operation and during current limit. The output current I O is the average of the inductor ripple current waveform. The Low Load Current waveform illustrates continuous con- duction mode operation with peak and valley inductor cur- rents below the current limit threshold. When the load cur- rent is increased (High Load Current), the ripple waveform maintains the same amplitude and frequency since the cur- rent 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 4 is above the current limit thresh- old. 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 4). Both V OUT and the switching frequency are reduced as the circuit operates in a constant current 20172718 FIGURE 3. Shutdown Implementation www.national.com 10 |
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