Datasheet LTM8031 (Analog Devices) - 10

LTM8031
applicaTions inForMaTion
may have only a small fraction of their nominal capaci-
Table 2. Switching Frequency vs RT Value
tance resulting in much higher output voltage ripple than
SWITCHING FREQUENCY (MHz) RT VALUE (kΩ)
expected. Ceramic capacitors are also piezoelectric. In 0.2 187 Burst Mode operation, the LTM8031’s switching frequency 0.3 124 depends on the load current, and can excite a ceramic 0.4 88.7 capacitor at audio frequencies, generating audible noise. 0.5 69.8 Since the LTM8031 operates at a lower current limit during 0.6 56.2 Burst Mode operation, the noise is typically very quiet to a 0.7 47.5 casual ear. If this audible noise is unacceptable, use a high 0.8 39.2 performance electrolytic capacitor at the output. The input 0.9 34 capacitor can be a parallel combination of a 1µF ceramic 1.0 28.0 capacitor and a low cost electrolytic capacitor. 1.2 23.7 1.4 19.1 A final precaution regarding ceramic capacitors concerns 1.5 16.2 the maximum input voltage rating of the LTM8031. A 1.8 13.3 ceramic input capacitor combined with trace or cable 2 11.5 inductance forms a high Q (under damped) tank circuit. 2.2 9.76 If the LTM8031 circuit is plugged into a live supply, the 2.4 8.66 input voltage can ring to twice its nominal value, possi- bly exceeding the device’s rating. This situation is easily
Operating Frequency Trade-Offs
avoided; see the Hot-Plugging Safely section. It is recommended that the user apply the optimal RT value given in Table 1 for the input and output operating
Electromagnetic Compliance
condition. System level or other considerations, however, The LTM8031 is compliant with the radiated emissions may necessitate another operating frequency. While the requirements of EN55022 class B. Graphs of the LTM8031’s LTM8031 is flexible enough to accommodate a wide range EMC performance are given in the Typical Performance of operating frequencies, a haphazardly chosen one may Characteristics section. Further data, operating conditions result in undesirable operation under certain operating or and test setup are detailed in an EMI Test report available fault conditions. A frequency that is too high can reduce from Linear Technology. efficiency, generate excessive heat or even damage the LTM8031 if the output is overloaded or short-circuited. A
Frequency Selection
frequency that is too low can result in a final design that has too much output ripple or unnecessarily large output The LTM8031 uses a constant frequency PWM architecture capacitor. The maximum frequency (and attendant R that can be programmed to switch from 200kHz to 2.4MHz T value) at which the LTM8031 should be allowed to switch is given by using a resistor tied from the RT pin to ground. Table 2 in Table 1 in the f provides a list of R MAX column, while the recommended T resistor values and their resultant frequency (and R frequencies. T 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. 8031fb 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical application Related Parts