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LM5010SDX Datasheet(PDF) 9 Page - Texas Instruments |
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LM5010SDX Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 26 page  tON = 1.18 x 10 -10 x (RON + 1.4k) VIN - 1.4V + 67 ns FB SW L1 C2 R1 R2 R3 BST VCC D2 C3 C4 D1 LM5010 VOUT2 VOUT1 S GND I SEN LM5010 www.ti.com SNVS307E – SEPTEMBER 2004 – REVISED FEBRUARY 2005 Start-up Regulator (VCC) The startup regulator is integral to the LM5010. The input pin (VIN) can be connected directly to line voltages up to 75V. The VCC output is regulated at 7.0V, ±6%, and is current limited to 10 mA. Upon power up the regulator sources current into the external capacitor at VCC (C3). With a 0.1 µF capacitor at VCC, approximately 58 µs are required for the VCC voltage to reach the under-voltage lockout threshold (UVLO) of 5.8V (t1 in Figure 8), at which time the buck switch is enabled, and the soft start pin is released to allow the soft start capacitor (C6) to charge up. VOUT then increases to its regulated value as the soft start voltage increases (t2 in Figure 8). The minimum input operating voltage is determined by the regulator’s dropout voltage, the VCC UVLO falling threshold ( ≊5.65V), and the frequency. When VCC falls below the falling threshold the VCC UVLO activates to shut off the buck switch and ground the soft start pin. If VCC is externally loaded, the minimum input voltage increases since the output impedance at VCC is ≊140Ω at low VIN. SeeFigure 2 and Figure 3. In applications involving a high value for VIN where power dissipation in the startup regulator is a concern, an auxiliary voltage can be diode connected to the VCC pin (Figure 10). Setting the auxiliary voltage to between 7.5V and 14V shuts off the internal regulator, reducing internal power dissipation. The current required into the VCC pin is shown in Figure 4. Internally a diode connects VCC to VIN. Figure 10. Self Biased Configuration 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 determined by the RON resistor and the input voltage (VIN), calculated from the following: (5) Copyright © 2004–2005, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM5010 |
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