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LM2734YMKX датащи(PDF) 7 Page - National Semiconductor (TI) |
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LM2734YMKX датащи(HTML) 7 Page - National Semiconductor (TI) |
7 / 22 page Application Information THEORY OF OPERATION The LM2734 is a constant frequency PWM buck regulator IC that delivers a 1A load current. The regulator has a preset switching frequency of either 550kHz (LM2734Y) or 1.6MHz (LM2734X). These high frequencies allow the LM2734 to operate with small surface mount capacitors and inductors, resulting in DC/DC converters that require a minimum amount of board space. The LM2734 is internally compen- sated, so it is simple to use, and requires few external components. The LM2734 uses current-mode control to regulate the output voltage. The following operating description of the LM2734 will refer to the Simplified Block Diagram (Figure 1) and to the wave- forms in Figure 2. The LM2734 supplies a regulated output voltage by switching the internal NMOS control switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS control switch. During this on-time, the SW pin voltage (V SW) swings up to approximately V IN, and the inductor current (IL) in- creases with a linear slope. I L is measured by the current- sense amplifier, which generates an output proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and compared to the error am- plifier’s output, which is proportional to the difference be- tween the feedback voltage and V REF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through Schottky diode D1, which forces the SW pin to swing below ground by the forward voltage (V D) of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage. BOOST FUNCTION Capacitor C BOOST and diode D2 in Figure 3 are used to generate a voltage V BOOST.VBOOST -VSW is the gate drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time, V BOOST needs to be at least 1.6V greater than V SW. Although the LM2734 will operate with this minimum voltage, it may not have sufficient gate drive to supply large values of output Block Diagram 20102306 FIGURE 1. 20102307 FIGURE 2. LM2734 Waveforms of SW Pin Voltage and Inductor Current www.national.com 7 |
Аналогичный номер детали - LM2734YMKX |
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Аналогичное описание - LM2734YMKX |
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