Datenblatt-Suchmaschine für elektronische Bauteile |
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LM25010 Datenblatt(PDF) 9 Page - Texas Instruments |
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LM25010 Datenblatt(HTML) 9 Page - Texas Instruments |
9 / 27 page RON = (tON - 67 ns) x (VIN - 1.4V) 1.18 x 10 -10 - 1.4 k: 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 LM25010 VOUT SGND ISEN LM25010 www.ti.com SNVS419C – DECEMBER 2005 – REVISED DECEMBER 2008 When VIN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 - 3 µs. The capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to the LM25010 pins. In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An auxiliary voltage of between 7.5V and 14V can be diode connected to the VCC pin (D2 in Figure 2) to shut off the VCC regulator, reducing internal power dissipation. The current required into the VCC pin is shown in the Typical Performance Characteristics. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less than VIN. The turn-on sequence is shown in Figure 1. When VCC exceeds the under-voltage lock-out threshold (UVLO) of 5.25V (t1 in Figure 1), the buck switch is enabled, and the SS pin is released to allow the soft-start capacitor (C6) to charge up. The output voltage VOUT is regulated at a reduced level which increases to the desired value as the soft-start voltage increases (t2 in Figure 1). Figure 2. Self Biased Configuration 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 RON, 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 RON resistor and the input voltage (VIN), and is calculated as follows: (5) The RON 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 RON resistor is determined from the following: Copyright © 2005–2008, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM25010 |
Ähnliche Teilenummer - LM25010_14 |
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Ähnliche Beschreibung - LM25010_14 |
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