Datasheet LTM8025 (Analog Devices) - 10

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APPLICATIONS INFORMATION
For most applications, the design process is straight for- Ceramic capacitors are small, robust and have very low ward, summarized as follows: ESR. However, not all ceramic capacitors are suitable. X5R 1. Look at Table 1 and find the row that has the desired and X7R types are stable over temperature and applied input range and output voltage. voltage and give dependable service. Other types, includ- ing Y5V and Z5U have very large temperature and voltage 2. Apply the recommended CIN, COUT, RADJ and RT values. coefficients of capacitance. In an application circuit they 3. Connect BIAS as indicated. may have only a small fraction of their nominal capaci- tance resulting in much higher output voltage ripple than While these component combinations have been tested expected. for proper operation, it is incumbent upon the user to verify proper operation over the intended system’s line, Ceramic capacitors are also piezoelectric. In Burst Mode load and environmental conditions. Bear in mind that the operation, the LTM8025’s switching frequency depends maximum output current is limited by junction tempera- on the load current, and can excite a ceramic capacitor turetemperature, the relationship between the input and at audio frequencies, generating audible noise. Since the output voltage magnitude and polarity and other factors. LTM8025 operates at a lower current limit during Burst Please refer to the graphs in the Typical Performance Mode operation, the noise is typically very quiet to a Characteristics section for guidance. casual ear. The maximum frequency (and attendant R If this audible noise is unacceptable, use a high perfor- T value) at which the LTM8025 should be allowed to switch is given mance electrolytic capacitor at the output. It may also be in Table 1 in the f a parallel combination of a ceramic capacitor and a low MAX column, while the recommended frequency (and R cost electrolytic capacitor. T value) for optimal efficiency over the given input condition is given in the fOPTIMAL column. A final precaution regarding ceramic capacitors concerns There are additional conditions that must be satisfied if the maximum input voltage rating of the LTM8025. A the synchronization function is used. Please refer to the ceramic input capacitor combined with trace or cable Synchronization section for details. inductance forms a high Q (under damped) tank circuit. If the LTM8025 circuit is plugged into a live supply, the
Capacitor Selection Considerations
input voltage can ring to twice its nominal value, possi- The CIN and COUT capacitor values in Table 1 are the mini- bly exceeding the device’s rating. This situation is easily mum recommended values for the associated operating avoided; see the Hot-Plugging Safely section. conditions. Applying capacitor values below those indi- cated in Table 1 is not recommended, and may result in
Frequency Selection
undesirable operation. Using larger values is generally The LTM8025 uses a constant frequency PWM architec- acceptable, and can yield improved dynamic response, ture that can be programmed to switch from 200kHz to if it is necessary. Again, it is incumbent upon the user to 2.4MHz by using a resistor tied from the RT pin to ground. verify proper operation over the intended system’s line, Table 2 provides a list of RT resistor values and their resul- load and environmental conditions. tant frequencies. Rev. D 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Package Photography Typical Application Related Parts