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LM2670T-3.3 데이터시트(PDF) 9 Page - Texas Instruments |
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LM2670T-3.3 데이터시트(HTML) 9 Page - Texas Instruments |
9 / 36 page LM2670 www.ti.com SNVS036J – APRIL 2000 – REVISED APRIL 2013 APPLICATION HINTS The LM2670 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 3A, and highly efficient operation. The LM2670 is part of the SIMPLE SWITCHER family of power converters. A complete design uses a minimum number 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 supply can be designed quickly. The software is provided free of charge and can be downloaded from Texas Instrument's Internet site located at www.ti.com. SWITCH OUTPUT This is the output of a power MOSFET switch connected directly 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 oscillator. 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 voltage. The voltage on pin 1 switches between Vin (switch ON) and below ground by the voltage drop of the external Schottky diode (switch OFF). 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 LM2670. For ensured performance the input voltage must be in the range of 8V to 40V. For best performance of the power supply the input pin should always be bypassed with an input capacitor located close to pin 2. C BOOST A capacitor must be connected from pin 3 to the switch output, pin 1. This capacitor boosts the gate drive to the internal MOSFET above Vin to fully turn it ON. This minimizes conduction losses in the power switch to maintain high efficiency. The recommended value for C Boost is 0.01 μF. GROUND This is the ground reference connection for all components in the power supply. In fast-switching, high-current applications such as those implemented with the LM2670, it is recommended that a broad ground plane be used to minimize signal coupling throughout the circuit SYNC This input allows control of the switching clock frequency. If left open-circuited the regulator will be switched at the internal oscillator frequency, between 225KHz and 280KHz. An external clock can be used to force the switching frequency and thereby control the output ripple frequency of the regulator. This capability provides for consistent filtering of the output ripple from system to system as well as precise frequency spectrum positioning of the ripple frequency which is often desired in communications and radio applications. This external frequency must be greater than the LM2670 internal oscillator frequency, which could be as high as 280KHz, to prevent an erroneous reset of the internal ramp oscillator and PWM control of the power switch. The ramp oscillator is reset on the positive going edge of the sync input signal. It is recommended that the external TTL or CMOS compatible clock (between 0V and a level greater than 3V) be ac coupled to the sync input through a 100pf capacitor and a 1K Ω resistor to ground at pin 5 as shown in Figure 19. When the SYNC function is used, current limit frequency foldback is not active. Therefore, the device my not be fully protected against extreme output short circuit conditions. See ADDITIONAL APPLICATION INFORMATION. Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2670 |
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