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LM5010SD Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM5010SD Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 19 page Start-up Regulator (V CC) (Continued) Regulation Comparator The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V, ±2%). In normal operation (the output voltage is regulated) an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch stays on for the on-time causing the FB voltage to rise above 2.5V. After the on-time period the buck switch stays off until the FB voltage falls below 2.5V. 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, change suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5V. ON-Time Control The on-time of the internal switch (see Figure 5) is deter- mined by the R ON resistor and the input voltage (VIN), cal- culated from the following: (5) The inverse relationship of t ON vs. VIN results in a nearly constant frequency as V IN is varied. If the application re- quires a high frequency the minimum value for t ON, and consequently R ON, is limited by the off-time (265 ns, ±15%) which limits the maximum duty cycle at minimum V IN. The tolerance for Equation 5 is ±25%. Frequencies in excess of 1 MHz are possible with the LM5010. Shutdown The LM5010 can be remotely shut down by taking the R ON/SD pin below 0.65V. See Figure 11. In this mode the softstart pin is internally grounded, the on-timer is disabled, and the input current at V IN is reduced (Figure 7). Releasing the R ON/SD pin allows normal operation to resume. When the switch is open, the nominal voltage at R ON/SD is shown in Figure 6. 20119916 FIGURE 10. Self Biased Configuration 20119918 FIGURE 11. Shutdown Implementation www.national.com 11 |
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