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LM2695SD Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM2695SD Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Start-Up Regulator, V CC The start-up regulator is integral to the LM2695. The input pin (VIN) can be connected directly to line voltage up to 30V, with transient capability to 33V. The V CC output regulates at 7.0V, and is current limited at 9.7 mA. Upon power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC pin reaches the under- voltage lockout threshold of 5.7V, the buck switch is enabled and the Softstart pin is released to allow the Softstart capaci- tor (C6) to charge up. The minimum input voltage is determined by the regulator’s dropout voltage, the V CC UVLO falling threshold ( )5.5V), and the frequency. When V CC falls below the falling thresh- old the V CC UVLO activates to shut off the output. If VCC is externally loaded, the minimum input voltage increases since the output impedance at V CC is )140 Ω. To reduce power dissipation in the start-up regulator, an auxiliary voltage can be diode connected to the V CC pin. Setting the auxiliary voltage to between 8V and 14V shuts off the internal regulator, reducing internal power dissipation. The sum of the auxiliary voltage and the input voltage (V CC +V IN) cannot exceed 47V. Internally, a diode connects VCC to VIN. See Figure 5. 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 LM2695 is determined by the R ON resis- tor and the input voltage (V IN), and is calculated from: (4) See Figure 2. 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 (FS), the R ON resistor is determined from the following: (5) In high frequency applicatons the minimum value for t ON is limited by the maximum duty cycle required for regulation and the minimum off-time of (250 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) 20170411 FIGURE 5. Self Biased Configuration www.national.com 9 |
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