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LTM8056 Scheda tecnica(PDF) 12 Page - Linear Technology |
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LTM8056 Scheda tecnica(HTML) 12 Page - Linear Technology |
12 / 28 page LTM8056 12 8056fa For more information www.linear.com/LTM8056 APPLICATIONS INFORMATION Capacitor Selection Considerations The CIN and COUT capacitor values in Table 1 are the minimum recommended values for the associated oper- ating conditions. Applying capacitor values below those indicated in Table 1 is not recommended, and may result in undesirable operation. Using larger values is generally acceptable, and can yield improved dynamic response, if it is necessary. Again, it is incumbent upon the user to verify proper operation over the intended system’s line, load and environmental conditions. Ceramic capacitors are small, robust and have very low ESR. However, not all ceramic capacitors are suitable. X5R and X7R types are stable over temperature and ap- plied voltage and give dependable service. Other types, including Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application cir- cuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LTM8056. A ceramic input capacitor combined with trace or cable inductance forms a high Q (underdamped) tank circuit. If the LTM8056 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possi- bly exceeding the device’s rating. This situation is easily avoided; see the Hot-Plugging Safely section. Frequency Selection The LTM8056 uses a constant frequency PWM architec- ture that can be programmed to switch from 100kHz to 800kHz by tying a resistor from the RT pin to ground. Table 3 provides a list of RT resistor values and their re- sultant frequencies. Table 3. Switching Frequency vs RT Value FREQUENCY RT VALUE (kΩ) 100 453 200 147 300 84.5 400 59 500 45.3 600 36.5 700 29.4 800 24.9 An external resistor within the range stated in Table 3 from RT to GND is required. Even when synchronizing to an external clock. When synchronizing the switching of the LTM8056 to an external signal source, the frequency range is 200kHz to 700kHz. Operating Frequency Trade-Offs It is recommended that the user apply the optimal RT value given in Table 1 for the input and output operating condition. System level or other considerations, however, may necessitate another operating frequency. While the LTM8056 is flexible enough to accommodate a wide range of operating frequencies, a haphazardly chosen one may result in undesirable operation under certain operating or fault conditions. A frequency that is too high can reduce efficiency, generate excessive heat or even damage the Table 2. Electrolytic Caps Used in LTM8056 Testing DESCRIPTION MANUFACTURER PART NUMBER 100µF, 6V, 75mΩ, Tantalum C Case AVX TPSC107M006R0075 100µF, 16V, 100mΩ, Tantalum Y Case AVX TPSY107M016R0100 68µF, 16V, 200mΩ, Tantalum C Case AVX TPSC686M016R0200 47µF, 25V, 900mΩ, Tantalum D Case AVX TAJD476M025R 33µF, 35V, 300mΩ, Tantalum D Case AVX TPSD336M035R0300 10µF, 50V, 120mΩ, Aluminum 6.3 × 6mm case Suncon 50HVP10M 10µF, 63V, 120mΩ, Aluminum 6.3 × 5.8mm case Panasonic EEHZA1J100P |
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Descrizione simile - LTM8056 |
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