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MIC4723YMME Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC4723YMME Datasheet(HTML) 9 Page - Micrel Semiconductor |
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9 / 20 page  Micrel, Inc. MIC4723 June 2008 9 M9999-060308-E Application Information The MIC4723 is a 3A PWM non-synchronous buck regulator. By switching an input voltage supply, and filtering the switched voltage through an Inductor and capacitor, a regulated DC voltage is obtained. Figure 1 shows a simplified example of a non-synchronous buck converter. Figure 1. Example of non-synchronous buck converter For a non-synchronous buck converter, there are two modes of operation; continuous and discontinuous. Continuous or discontinuous refer to the inductor current. If current is continuously flowing through the inductor throughout the switching cycle, it is in continuous operation. If the inductor current drops to zero during the off time, it is in discontinuous operation. Critically continuous is the point where any decrease in output current will cause it to enter discontinuous operation. The critically continuous load current can be calculated as follows; L 2 2.0MHz V V V I IN 2 OUT OUT OUT × × ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ − = Continuous or discontinuous operation determines how we calculate peak inductor current. Continuous Operation Figure 2 illustrates the switch voltage and inductor current during continuous operation. Figure 2. Continuous Operation The output voltage is regulated by pulse width modulating (PWM) the switch voltage to the average required output voltage. The switching can be broken up into two cycles; On and Off. During the on-time, Figure 3 illustrates the high side switch is turned on, current flows from the input supply through the inductor and to the output. The inductor current is: Figure 3. On-Time charged at the rate; ( ) L V V OUT IN − To determine the total on-time, or time at which the inductor charges, the duty cycle needs to be calculated. The duty cycle can be calculated as; IN OUT V V D = and the On time is; 2.0MHz D TON = Therefore, peak to peak ripple current is; () L 2.0MHz V V V V I IN OUT OUT IN pk pk × × − = − Since the average peak to peak current is equal to the load current. The actual peak (or highest current the inductor will see in a steady-state condition) is equal to the output current plus ½ the peak-to-peak current. () L 2.0MHz 2 V V V V I I IN OUT OUT IN OUT pk × × × − + = Figure 4 demonstrates the off-time. During the off-time, the high-side internal P-channel MOSFET turns off. Since the current in the inductor has to discharge, the current flows through the free-wheeling Schottky diode to the output. In this case, the inductor discharge rate is (where VD is the diode forward voltage); |
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