Datenblatt-Suchmaschine für elektronische Bauteile |
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LM2745MTC Datenblatt(PDF) 8 Page - National Semiconductor (TI) |
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LM2745MTC Datenblatt(HTML) 8 Page - National Semiconductor (TI) |
8 / 23 page Block Diagram 20137403 Application Information The LM2745/8 is a voltage-mode, high-speed synchronous buck regulator with a PWM control scheme. It is designed for use in set-top boxes, thin clients, DSL/Cable modems, and other applications that require high efficiency buck convert- ers. It has output shutdown (SD), input undervoltage lock-out (UVLO) mode and power good (PWGD) flag (based on overvoltage and undervoltage detection). The overvoltage and undervoltage signals are OR-gated to drive the power good signal and provide a logic signal to the system if the output voltage goes out of regulation. Current limit is achieved by sensing the voltage V DS across the low side MOSFET. The LM2745/8 is also able to start-up with the output pre-biased with a load. The LM2745 also allows for the switching frequency to be synchronized with an external clock source. START UP/SOFT-START When V CC exceeds 2.79V and the shutdown pin (SD) sees a logic high, the soft-start period begins. Then an internal, fixed 10 µA source begins charging the soft-start capacitor. During soft-start the voltage on the soft-start capacitor C SS is connected internally to the non-inverting input of the error amplifier. The soft-start period lasts until the voltage on the soft-start capacitor exceeds the LM2745/8 reference voltage of 0.6V. At this point the reference voltage takes over at the non-inverting error amplifier input. The capacitance of C SS determines the length of the soft-start period, and can be approximated by: Where C SS is in µF and tSS is in ms. During soft start the Power Good flag is forced low and it is released when the FB pin voltage reaches 70% of 0.6V. At this point the chip enters normal operation mode, and the output overvoltage and undervoltage monitoring starts. SETTING THE OUTPUT VOLTAGE The LM2745/8 regulates the output voltage by controlling the duty cycle of the high side and low side MOSFETs (see Typical Application Circuit).The equation governing output voltage is: SETTING THE SWITCHING FREQUENCY During fixed-frequency mode of operation the PWM fre- quency is adjustable between 50 kHz and 1 MHz and is set by an external resistor, R FADJ, between the FREQ/SYNC pin and ground. The resistance needed for a desired frequency www.national.com 8 |
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Ähnliche Beschreibung - LM2745MTC |
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