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LM2679S-5.0 датащи(PDF) 9 Page - National Semiconductor (TI) |
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LM2679S-5.0 датащи(HTML) 9 Page - National Semiconductor (TI) |
9 / 24 page Application Hints The LM2679 provides all of the active functions required for a step-down (buck) switching regulator. The internal power switch is a DMOS power MOSFET to provide power supply designs with high current capability, up to 5A, and highly ef- ficient operation. The LM2679 is part of the SIMPLE SWITCHER family of power converters. A complete design uses a minimum num- ber of external components, which have been pre-determined from a variety of manufacturers. Using either this data sheet or a design software program called LM267X Made Simple (version 2.0) a complete switching power sup- ply can be designed quickly. The software is provided free of charge and can be downloaded from National Semiconduc- tor’s Internet site located at http://www.national.com. PIN 1 - Switch Output This is the output of a power MOSFET switch connected di- rectly to the input voltage. The switch provides energy to an inductor, an output capacitor and the load circuitry under control of an internal pulse-width-modulator (PWM). The PWM controller is internally clocked by a fixed 260KHz oscil- lator. In a standard step-down application the duty cycle (Time ON/Time OFF) of the power switch is proportional to the ratio of the power supply output voltage to the input volt- age. The voltage on pin 1 switches between Vin (switch ON) and below ground by the voltage drop of the external Schot- tky diode (switch OFF). PIN 2 - Input The input voltage for the power supply is connected to pin 2. In addition to providing energy to the load the input voltage also provides bias for the internal circuitry of the LM2679. For guaranteed performance the input voltage must be in the range of 8V to 40V. For best performance of the power sup- ply the input pin should always be bypassed with an input ca- pacitor located close to pin 2. PIN3-C Boost A capacitor must be connected from pin 3 to the switch out- put, pin 1. This capacitor boosts the gate drive to the internal MOSFET above Vin to fully turn it ON. This minimizes con- duction losses in the power switch to maintain high effi- ciency. The recommended value for C Boost is 0.01µF. PIN 4 - Ground This is the ground reference connection for all components in the power supply. In fast-switching, high-current applica- tions such as those implemented with the LM2679, it is rec- ommended that a broad ground plane be used to minimize signal coupling throughout the circuit PIN 5 - Current Adjust A key feature of the LM2679 is the ability to tailor the peak switch current limit to a level required by a particular applica- tion. This alleviates the need to use external components that must be physically sized to accommodate current levels (under shorted output conditions for example) that may be much higher than the normal circuit operating current re- quirements. A resistor connected from pin 5 to ground establishes a cur- rent (I (pin 5) = 1.2V / RADJ) that sets the peak current through the power switch. The maximum switch current is fixed at a level of 37,125 / R ADJ. PIN 6 - Feedback This is the input to a two-stage high gain amplifier, which drives the PWM controller. It is necessary to connect pin 6 to the actual output of the power supply to set the dc output voltage. For the fixed output devices (3.3V, 5V and 12V out- puts), a direct wire connection to the output is all that is re- quired as internal gain setting resistors are provided inside the LM2679. For the adjustable output version two external resistors are required to set the dc output voltage. For stable operation of the power supply it is important to prevent cou- pling of any inductor flux to the feedback input. PIN 7 - Softstart A capacitor connected from pin 7 to ground allows for a slow turn-on of the switching regulator. The capacitor sets a time delay to gradually increase the duty cycle of the internal power switch. This can significantly reduce the amount of surge current required from the input supply during an abrupt application of the input voltage. If softstart is not required this pin should be left open circuited. DESIGN CONSIDERATIONS DS100847-23 FIGURE 1. Basic circuit for fixed output voltage applications. www.national.com 9 |
Аналогичный номер детали - LM2679S-5.0 |
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Аналогичное описание - LM2679S-5.0 |
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