Datasheet LTM8020 (Analog Devices) - 8

LTM8020
APPLICATIONS INFORMATION
For most applications, the design process is straight frequencies, generating audible noise. Since the LTM8020 forward, summarized as follows: operates at a lower current limit during Burst Mode opera- 1. Look at Table 1 and find the row that has the desired tion, the noise is typically very quiet to a casual ear. input range and output voltage. If this audible noise is unacceptable, use a high performance 2. Apply the C electrolytic capacitor at the output. The input capacitor can IN, COUT, RADJ and BIAS connection indicated on that row. be a parallel combination of a 2.2µF ceramic capacitor and a low cost electrolytic capacitor. While these component combinations have been tested for proper operation, it is incumbent upon the user to verify A final precaution regarding ceramic capacitors concerns proper operation over the intended system’s line, load and the maximum input voltage rating of the LTM8020. A environmental conditions. ceramic input capacitor combined with trace or cable inductance forms a high Q (under damped) tank circuit. If an output voltage other than those listed in Table 1 is If the LTM8020 circuit is plugged into a live supply, the desired, use the equation RADJ = 623.75/(VOUT – 1.25), input voltage can ring to twice its nominal value, possi- where RADJ is in kW. As a starting point, use values for bly exceeding the device’s rating. This situation is easily CIN and COUT that correspond to the input voltage and avoided; see the Hot-Plugging Safely section. output voltage that most closely matches the intended application, and verify proper operation over the system’s
Shorted Input Protection
line, load and environmental conditions. Care needs to be taken in systems where the output will be
Capacitor Selection Considerations
held high when the input to the LTM8020 is absent. This may occur in battery charging applications or in battery The CIN and COUT capacitor values in Table 1 are the backup systems where a battery or some other supply minimum recommended values for the associated oper- is diode ORed with the LTM8020’s output. If the VIN pin ating conditions. Applying capacitor values below those is allowed to float and the SHDN pin is held high (either indicated in Table 1 is not recommended, and may result by a logic signal or because it is tied to VIN), then the in undesirable operation. An input system bulk capacitor LTM8020’s internal circuitry will pull its quiescent current is assumed. Using larger values is generally acceptable, from its output. This is fine if your system can tolerate a and can yield improved dynamic response, if it is neces- few milliamps in this state. If you ground the SHDN pin, sary. Again, it is incumbent upon the user to verify proper this quiescent current will drop to essentially zero. How- operation over the intended system’s line, load and envi- ever, if the VIN pin is grounded while the output is held ronmental conditions. high, then parasitic diodes inside the LTM8020 can pull Ceramic capacitors are small, robust and have very low large currents from the output through the internal power ESR. However, not all ceramic capacitors are suitable. switch, possibly damaging the device. Figure 2 shows a X5R and X7R types are stable over temperature and ap- circuit that will run only when the input voltage is present plied voltage and give dependable service. Other types, and that protects against a shorted or reversed input. including Y5V and Z5U have very large temperature and D1 voltage coefficients of capacitance. In an application cir- VIN VIN cuit they may have only a small fraction of their nominal LTM8020 100k capacitance resulting in much higher output voltage ripple SHDN than expected. 1M GND 8020 F02 Ceramic capacitors are also piezoelectric. The LTM8020’s switching frequency depends on the load current, and
Figure 2. Diode D1 Prevents a Shorted Input from Discharging
at light loads it can excite a ceramic capacitor at audio
a Backup Battery Tied to the Output, as Well as Protecting the LTM8020 from a Reversed Input
8020fe 8 For more information www.linear.com/LTM8020