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LM25069 Datasheet(PDF) 10 Page - Texas Instruments |
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LM25069 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 29 page TIMER Pin Load Current Output Voltage (OUT Pin) PGD UVLO Normal Operation GATE Pin Insertion Time POR VSYS VIN ILIMIT 20 PA source 2.5 PA 80 PA t 3 t 2 t 1 In- rush Limiting 5.5 PA 1.72V 2 mA 2 mA pull-down 260 mA pull-down 1.3V LM25069 SNVS607E – FEBRUARY 2011 – REVISED MARCH 2013 www.ti.com If the TIMER pin voltage reaches 1.72V before in-rush current limiting or power limiting ceases (during t2), a fault is declared and Q1 is turned off. See the Fault Timer & Restart section for a complete description of the fault mode. Figure 23. Power Up Sequence (Current Limit only) Gate Control A charge pump provides the voltage at the GATE pin to enhance the N-Channel MOSFET’s gate. During normal operating conditions (t3 in Figure 23) the gate of Q1 is held charged by an internal 20 µA current source. The voltage at the GATE pin (with respect to ground) is limited by an internal 19.5V zener diode. See the graph GATE Pin voltage. Since the gate-to-source voltage applied to Q1 could be as high as 19.5V during various conditions, a zener diode with the appropriate voltage rating must be added between the GATE and OUT pins if the maximum VGS rating of the selected MOSFET is less than 19.5V. The external zener diode must have a forward current rating of at least 260 mA. When the system voltage is initially applied, the GATE pin is held low by a 260 mA pull-down current. This helps prevent an inadvertent turn-on of the MOSFET through its drain-gate capacitance as the applied system voltage increases. During the insertion time (t1 in Figure 23) the GATE pin is held low by a 2 mA pull-down current. This maintains Q1 in the off-state until the end of t1, regardless of the voltage at VIN or UVLO. Following the insertion time, during t2 in Figure 23, the gate voltage of Q1 is modulated to keep the current or power dissipation level from exceeding the programmed levels. While in the current or power limiting mode the TIMER pin capacitor is charging. If the current and power limiting cease before the TIMER pin reaches 1.72V the TIMER pin capacitor then discharges, and the circuit enters normal operation. 10 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated Product Folder Links: LM25069 |
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