Datasheet LTM8022 (Analog Devices) - 9

LTM8022
APPLICATIONS INFORMATION
For most applications, the design process is straight Mode operation, the noise is typically very quiet to a forward, summarized as follows: casual ear. 1. In Table 1, fi nd the row that has the desired input voltage If this audible noise is unacceptable, use a high per- range and output voltage. formance electrolytic capacitor at the output. The input capacitor can be a parallel combination of a 2.2μF ceramic 2. Apply the recommended CIN, COUT, RADJ and RT values. capacitor and a low cost electrolytic capacitor. 3. Connect BIAS as indicated. A fi nal precaution regarding ceramic capacitors concerns While these component combinations have been tested for the maximum input voltage rating of the LTM8022. A proper operation, it is incumbent upon the user to verify ceramic input capacitor combined with trace or cable proper operation over the intended system’s line, load and inductance forms a high Q (under damped) tank circuit. environmental conditions. If the LTM8022 circuit is plugged into a live supply, the If the desired output voltage is not listed in Table 1, set the input voltage can ring to twice its nominal value, possibly output by applying an R exceeding the device’s rating. This situation is easily ADJ resistor whose value is given by the equation R avoided; see the Hot-Plugging Safely section. ADJ = 394.21/(VOUT – 0.79), where RADJ is in kΩ and VOUT is in volts. Verify the LTM8022’s operation over the system’s intended line, load and environmental
Frequency Selection
conditions. The LTM8022 uses a constant-frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz
Capacitor Selection Considerations
by using a resistor tied from the RT pin to ground. Table 2 The C provides a list of R IN and COUT capacitor values in Table 1 are the T resistor values and their resultant minimum recommended values for the associated oper- frequencies. ating conditions. Applying capacitor values below those
Table 2. Switching Frequency vs RT Value
indicated in Table 1 is not recommended, and may result
SWITCHING FREQUENCY (MHz) R
in undesirable operation. Using larger values is generally
T VALUE (k
Ω
)
0.2 187 acceptable, and can yield improved dynamic response, if 0.3 121 it is necessary. Again, it is incumbent upon the user to 0.4 88.7 verify proper operation over the intended system’s line, 0.5 68.1 0.6 56.2 load and environmental conditions. 0.7 46.4 0.8 40.2 Ceramic capacitors are small, robust and have very low ESR. 0.9 34 However, not all ceramic capacitors are suitable. X5R and 1.0 29.4 1.2 23.7 X7R types are stable over temperature and applied voltage 1.4 19.1 and give dependable service. Other types, including Y5V and 1.6 16.2 Z5U, have very large temperature and voltage coeffi cients 1.8 13.3 2.0 11.5 of capacitance. In an application circuit they may have only 2.2 9.76 a small fraction of their nominal capacitance, resulting in 2.4 8.66 much higher output voltage ripple than expected.
Operating Frequency Tradeoffs
Ceramic capacitors are also piezoelectric. In Burst Mode It is recommended that the user apply the optimal RT operation, the LTM8022’s switching frequency depends value given in Table 1 for the input and output operating on the load current, and can excite a ceramic capacitor condition. System level or other considerations, however, at audio frequencies, generating audible noise. Since the may necessitate another operating frequency. While the LTM8022 operates at a lower current limit during Burst LTM8022 is fl exible enough to accommodate a wide range 8022fd 9