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LM2651MTC-1.8 데이터시트(PDF) 8 Page - National Semiconductor (TI) |
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LM2651MTC-1.8 데이터시트(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Block Diagram Operation The LM2651 operates in a constant frequency (300 kHz), current-mode PWM for moderate to heavy loads; and it automatically switches to hysteretic mode for light loads. In hysteretic mode, the switching frequency is reduced to keep the efficiency high. MAIN OPERATION When the load current is higher than the sleep mode thresh- old, the part is always operating in PWM mode. At the beginning of each switching cycle, the high-side switch is turned on, the current from the high-side switch is sensed and compared with the output of the error amplifier (COMP pin). When the sensed current reaches the COMP pin volt- age level, the high-side switch is turned off; after 40 ns (deadtime), the low-side switch is turned on. At the end of the switching cycle, the low-side switch is turned off; and the same cycle repeats. The current of the top switch is sensed by a patented internal circuitry. This unique technique gets rid of the external sense resistor, saves cost and size, and improves noise immunity of the sensed current. A feedforward from the input voltage is added to reduce the variation of the current limit over the input voltage range. When the load current decreases below the sleep mode threshold, the output voltage will rise slightly, this rise is sensed by the hysteretic mode comparator which makes the part go into the hysteretic mode with both the high and low side switches off. The output voltage starts to drop until it hits the low threshold of the hysteretic comparator, and the part immediately goes back to the PWM operation. The output voltage keeps increasing until it reaches the top hysteretic threshold, then both the high and low side switches turn off again, and the same cycle repeats. PROTECTIONS The cycle-by-cycle current limit circuitry turns off the high- side MOSFET whenever the current in MOSFET reaches 2A. Design Procedure This section presents guidelines for selecting external com- ponents. 10092503 FIGURE 1. LM2651 Block Diagram www.national.com 8 |
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