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LM2694SDX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM2694SDX 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). 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 programmed 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. Input bias current at the FB pin is less than 100 nA over temperature. Over-Voltage Comparator The voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the on-time pulse is immediately terminated. This condition can occur if the input voltage or the output load changes suddenly, or if the induc- tor (L1) saturates. The buck switch remains off until the voltage at FB falls below 2.5V. ON-Time Timer, and Shutdown The on-time for the LM2694 is determined by the R ON resis- tor and the input voltage (V IN), and is calculated from: (4) See Figure 4. The inverse relationship with V IN results in a nearly constant frequency as V IN is varied. To set a specific continuous conduction mode switching frequency (F S), the R ON resistor is determined from the following: (5) 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 265 ns, ±15%. The minimum off-time limits the maximum duty cycle achievable with a low voltage at V IN. The minimum allowed on-time to regulate the desired V OUT at the minimum VIN is determined from the following: (6) The LM2694 can be remotely shut down by taking the RON/SD pin below 0.8V. See Figure 10. In this mode the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the RON/SD pin allows normal operation to resume. The voltage at the RON/SD pin is normally between 1.5V and 3.0V, depending on V IN and the R ON resistor. 20187014 FIGURE 9. Self Biased Configuration 20187019 FIGURE 10. Shutdown Implementation www.national.com 11 |
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