Datasheet LTM8021 (Analog Devices) - 9

LTM8021
applicaTions inForMaTion Capacitor Selection Considerations
If this audible noise is unacceptable, use a high performance The C electrolytic capacitor at the output. This output capacitor IN and COUT capacitor values in Table 1 are the minimum recommended values for the associated oper- can be a parallel combination of a 1µF ceramic capacitor ating conditions. Applying capacitor values below those and a low cost electrolytic capacitor. indicated in Table 1 is not recommended, and may result A final precaution regarding ceramic capacitors con- in undesirable operation. Using larger values is generally cerns the maximum input voltage rating of the LTM8021. acceptable, and can yield improved dynamic response or A ceramic input capacitor combined with trace or cable fault recovery, if it is necessary. Again, it is incumbent inductance forms a high Q (under damped) tank circuit. upon the user to verify proper operation over the intended If the LTM8021 circuit is plugged into a live supply, the system’s line, load and environmental conditions. input voltage can ring to twice its nominal value, possi- Ceramic capacitors are small, robust and have very low bly exceeding the device’s rating. This situation is easily ESR. However, not all ceramic capacitors are suitable. avoided; see the Hot-Plugging Safely section. X5R and X7R types are stable over temperature and ap-
Minimum Input Voltage
plied voltage and give dependable service. Other types, including Y5V and Z5U have very large temperature and The LTM8021 is a step-down converter, so a minimum voltage coefficients of capacitance. In an application cir- amount of headroom is required to keep the output in cuit they may have only a small fraction of their nominal regulation. For most applications at full load, the input capacitance resulting in much higher output voltage ripple must be about 1.5V above the desired output. In addition, than expected. it takes more input voltage to turn on than is required for continuous operation. This is shown in Figure 1. Ceramic capacitors are also piezoelectric. At light loads, the LTM8021 skips switching cycles in order to maintain regulation. The resulting bursts of current can excite a ceramic capacitor at audio frequencies, generating audible noise. 8 6.0 VOUT = 5V VOUT = 3.3V 5.5 7 TO START TO START 5.0 6 RUN/SS ENABLED 4.5 RUN/SS 5 TO RUN 4.0 ENABLED 3.5 4 TO RUN INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3.0 3 2.5 2 2.0 0.001 0.01 0.1 1 0.001 0.01 0.1 1 LOAD CURRENT (A) LOAD CURRENT (A) 8021 F01
Figure 1. The LTM8021 Requires More Voltage to Start Than to Run
8021fd For more information www.linear.com/LTM8021 9 Document Outline Features Description Applications 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 Package Photo Typical Application Related Parts