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LM2576D2T-3.3 Fiches technique(PDF) 9 Page - ON Semiconductor |
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LM2576D2T-3.3 Fiches technique(HTML) 9 Page - ON Semiconductor |
9 / 26 page LM2576 http://onsemi.com 9 PIN FUNCTION DESCRIPTION Pin Symbol Description (Refer to Figure 1) 1 Vin This pin is the positive input supply for the LM2576 step−down switching regulator. In order to minimize voltage transients and to supply the switching currents needed by the regulator, a suitable input bypass capacitor must be present (Cin in Figure 1). 2 Output This is the emitter of the internal switch. The saturation voltage Vsat of this output switch is typically 1.5 V. It should be kept in mind that the PCB area connected to this pin should be kept to a minimum in order to minimize coupling to sensitive circuitry. 3 GND Circuit ground pin. See the information about the printed circuit board layout. 4 Feedback This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the internal resistor divider network R2, R1 and applied to the non−inverting input of the internal error amplifier. In the Adjustable version of the LM2576 switching regulator this pin is the direct input of the error amplifier and the resistor network R2, R1 is connected externally to allow programming of the output voltage. 5 ON/OFF It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total input supply current to approximately 80 mA. The threshold voltage is typically 1.4 V. Applying a voltage above this value (up to +Vin) shuts the regulator off. If the voltage applied to this pin is lower than 1.4 V or if this pin is left open, the regulator will be in the “on” condition. DESIGN PROCEDURE Buck Converter Basics The LM2576 is a “Buck” or Step−Down Converter which is the most elementary forward−mode converter. Its basic schematic can be seen in Figure 16. The operation of this regulator topology has two distinct time periods. The first one occurs when the series switch is on, the input voltage is connected to the input of the inductor. The output of the inductor is the output voltage, and the rectifier (or catch diode) is reverse biased. During this period, since there is a constant voltage source connected across the inductor, the inductor current begins to linearly ramp upwards, as described by the following equation: I L(on) + V in –Vout ton L During this “on” period, energy is stored within the core material in the form of magnetic flux. If the inductor is properly designed, there is sufficient energy stored to carry the requirements of the load during the “off” period. Figure 16. Basic Buck Converter D Vin RLoad L Cout Power Switch The next period is the “off” period of the power switch. When the power switch turns off, the voltage across the inductor reverses its polarity and is clamped at one diode voltage drop below ground by the catch diode. The current now flows through the catch diode thus maintaining the load current loop. This removes the stored energy from the inductor. The inductor current during this time is: I L(off) + Vout –VD toff L This period ends when the power switch is once again turned on. Regulation of the converter is accomplished by varying the duty cycle of the power switch. It is possible to describe the duty cycle as follows: d + ton T , where T is the period of switching. For the buck converter with ideal components, the duty cycle can also be described as: d + Vout V in Figure 17 shows the buck converter, idealized waveforms of the catch diode voltage and the inductor current. Power Switch Figure 17. Buck Converter Idealized Waveforms Power Switch Off Power Switch Off Power Switch On Power Switch On Von(SW) VD(FWD) Time Time ILoad(AV) Imin Ipk Diode Diode Power Switch |
Numéro de pièce similaire - LM2576D2T-3.3 |
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Description similaire - LM2576D2T-3.3 |
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