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LTM8032IVPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTM8032IVPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTM8032 10 8032fa APPLICATIONS INFORMATION may have only a small fraction of their nominal capaci- tance resulting in much higher output voltage ripple than expected. Ceramic capacitors are also piezoelectric. In Burst Mode operation, the LTM8032’s switching frequency depends on the load current, and can excite a ceramic capacitor at audio frequencies, generating audible noise. Since the LTM8032 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this audible noise is unacceptable, use a high performance electrolytic capacitor at the output. The input capacitor can be a parallel combination of a 2.2μF ceramic capacitor and a low cost electrolytic capacitor. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LTM8032. A ceramic input capacitor combined with trace or cable inductance forms a high Q (under damped) tank circuit. If the LTM8032 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. Electromagnetic Compliance The LTM8032 is compliant with the radiated emissions requirements of EN55022 class B. Graphs of the LTM8032’s EMC performance are given in the Typical Performance Characteristics section. Further data, operating conditions and test setup are detailed in an EMI Test report available from Linear Technology. Frequency Selection The LTM8032 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the RT pin to ground. Table 2 provides a list of RT resistor values and their resultant frequencies. 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 LTM8032 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 LTM8032 if the output is overloaded or short-circuited. A frequency that is too low can result in a final design that has too much output ripple or too large of an output cap. The maximum frequency (and attendant RT value) at which the LTM8032 should be allowed to switch is given in Table 1 in the fMAX column, while the recommended frequency (and RT value) for optimal efficiency over the given input condition is given in the fOPTIMAL column. There are additional conditions that must be satisfied if the synchronization function is used. Please refer to the Synchronization section for details. BIAS Pin Considerations The BIAS pin is used to provide drive power for the internal power switching stage and operate internal circuitry. For proper operation, it must be powered by at least 2.8V. If the output voltage is programmed to be 2.8V or higher, simply tie BIAS to AUX. If VOUT is less than 2.8V, BIAS can be tied to VIN or some other voltage source. In all cases, ensure that the maximum voltage at the BIAS pin is both less than 25V and the sum of VIN and BIAS is less Table 2. Switching Frequency vs RT Value SWITCHING FREQUECNY (MHz) RT VALUE (kΩ) 0.2 187 0.3 124 0.4 88.7 0.5 69.8 0.6 54.9 0.7 44.2 0.8 39.2 0.9 34 1.0 29.4 1.2 23.7 1.4 19.1 1.5 16.2 1.8 13.3 2 11.5 2.2 9.76 2.4 8.66 |
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