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LM2841 数据表(PDF) 8 Page - Texas Instruments |
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LM2841 数据表(HTML) 8 Page - Texas Instruments |
8 / 19 page L = (VIN - VOUT)VOUT VIN x IRIPPLE x fSW LM2841, LM2842 SNVS540H – MARCH 2009 – REVISED APRIL 2013 www.ti.com INPUT CAPACITOR A low ESR ceramic capacitor (CIN) is needed between the VIN pin and GND pin. This capacitor prevents large voltage transients from appearing at the input. Use a 2.2 µF-10 µF value with X5R or X7R dielectric. Depending on construction, a ceramic capacitor’s value can decrease up to 50% of its nominal value when rated voltage is applied. Consult with the capacitor manufacturer's data sheet for information on capacitor derating over voltage and temperature. INDUCTOR SELECTION The most critical parameters for the inductor are the inductance, peak current, and the DC resistance. The inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages. (1) A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, and current stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the ripple current increases with the input voltage, the maximum input voltage is always used to determine the inductance. The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is available with a bigger winding area. A good tradeoff between the efficiency and the core size is letting the inductor copper loss equal 2% of the output power. See AN-1197 SNVA038 for more information on selecting inductors. A good starting point for most applications is a 10 µH to 22 µH with 1.1A or greater current rating for the LM2842 or a 0.7A or greater current rating for the LM2840/41. Using such a rating will enable the LM2840/1/2 to current limit without saturating the inductor. This is preferable to the LM2840/1/2 going into thermal shutdown mode and the possibility of damaging the inductor if the output is shorted to ground or other longterm overload. OUTPUT CAPACITOR The selection of COUT is driven by the maximum allowable output voltage ripple. The output ripple in the constant frequency, PWM mode is approximated by: VRIPPLE = IRIPPLE(ESR+(1/(8fSWCOUT))) The ESR term usually plays the dominant role in determining the voltage ripple. Low ESR ceramic capacitors are recommended. Capacitors in the range of 22 µF-100 µF are a good starting point with an ESR of 0.1 Ω or less. BOOTSTRAP CAPACITOR A 0.15 µF ceramic capacitor or larger is recommended for the bootstrap capacitor (CBOOT). For applications where the input voltage is less than twice the output voltage a larger capacitor is recommended, generally 0.15 µF to 1 µF to ensure plenty of gate drive for the internal switches and a consistently low RDSON. SOFT-START COMPONENTS The LM2840/1/2 has circuitry that is used in conjunction with the SHDN pin to limit the inrush current on start-up of the DC/DC switching regulator. The SHDN pin in conjunction with a RC filter is used to tailor the soft-start for a specific application. When a voltage applied to the SHDN pin is between 0V and up to 2.3V it will cause the cycle by cycle current limit in the power stage to be modulated for minimum current limit at 0V up to the rated current limit at 2.3V. Thus controlling the output rise time and inrush current at startup. The resistor value should be selected so the current sourced into the SHDN pin will be greater then the leakage current of the SHDN pin (1.5 µA ) when the voltage at SHDN is equal or greater then 2.3V. SHUTDOWN OPERATION The SHDN pin of the LM2840/1/2 is designed so that it may be controlled using 2.3V or higher logic signals. If the shutdown function is not to be used the SHDN pin may be tied to VIN. The maximum voltage to the SHDN pin should not exceed 42V. If the use of a higher voltage is desired due to system or other constraints it may be used, however a 100 k Ω or larger resistor is recommended between the applied voltage and the SHDN pin to protect the device. 8 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Product Folder Links: LM2841 LM2842 |
类似零件编号 - LM2841_13 |
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类似说明 - LM2841_13 |
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