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LTM8067 Folha de dados(PDF) 14 Page - Linear Technology |
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LTM8067 Folha de dados(HTML) 14 Page - Linear Technology |
14 / 20 page LTM8067 14 8067fc For more information www.linear.com/LTM8067 APPLICATIONS INFORMATION age at the VIN pin of the LTM8067 can ring to more than twice the nominal input voltage, possibly exceeding the LTM8067’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LTM8067 into an energized supply, the input network should be designed to prevent this overshoot. This can be accomplished by installing a small resistor in series to VIN, but the most popular method of controlling input voltage overshoot is adding an electrolytic bulk capacitor to the VIN net. This capacitor’s relatively high equivalent series resistance damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripplefilteringandcanslightlyimprovetheefficiencyofthe circuit, though it can be a large component in the circuit. Thermal Considerations The LTM8067 output current may need to be derated if it is required to operate in a high ambient temperature. The amount of current derating is dependent upon the input voltage, output power and ambient temperature. The temperature rise curves given in the Typical Performance Characteristicssectioncanbeusedasaguide.Thesecurves were generated by the LTM8067 mounted to a 58cm2 4-layer FR4 printed circuit board. Boards of other sizes and layer count can exhibit different thermal behavior, so it is incumbent upon the user to verify proper operation over the intended system’s line, load and environmental operating conditions. Forincreasedaccuracyandfidelitytotheactualapplication, many designers use FEA to predict thermal performance. To that end, the Pin Configuration section of the data sheet typically gives four thermal coefficients: θJA: Thermal resistance from junction to ambient θJCbottom: Thermal resistance from junction to the bot- tom of the product case θJCtop: Thermal resistance from junction to top of the product case θJCboard:Thermalresistancefromjunctiontotheprinted circuit board. While the meaning of each of these coefficients may seem to be intuitive, JEDEC has defined each to avoid Minimum Load Due to the nature of the flyback regulator in general, and the LTM8067 control scheme specifically, the LTM8067 requires a minimum load for proper operation. Other- wise, the output may go out of regulation if the load is too light. The most common way to address this is to place a resistor across the output. The Minimum Load Current vs VOUT Over Full Output Voltage Range graph in the Typical Performance Characteristics section may be used as a guide in selecting the resistor. Note that this graph describes room temperature operation. If the end applicationoperatesatacoldertemperature,theminimum load requirement may be higher and the minimum load condition must be characterized for the lowest operating temperature. If it is impractical to place a resistive load permanently across the output, a resistor and Zener diode may be used instead, as shown in Figure 2. While the minimum load resistor mentioned in the prior paragraph will always draw current while the LTM8067 output is powered, the series resistor-Zener diode combination will only draw current if the output is too high. When using this circuit, take care to ensure that the characteristics of the Zener diode are ap- propriatefortheintendedapplication’stemperaturerange. Figure 2: Use a Resistor and Zener Diode to Meet the Minimum Load Requirement Hot-Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of the LTM8067. However, these capaci- tors can cause problems if the LTM8067 is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion). The low loss ceramic capacitor combined with stray inductance in series with the power source forms an underdamped tank circuit, and the volt- LTM8067 VOUTP VOUTN 8067 F02 |
Nº de peça semelhante - LTM8067 |
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Descrição semelhante - LTM8067 |
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