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LT3990EDDB Datasheet(PDF) 11 Page - Linear Technology |
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LT3990EDDB Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3990 11 3990p APPLICATIONS INFORMATION the ripple current. Note that larger input capacitance is required when a lower switching frequency is used (due to longer on-times). If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor. Step-down regulators draw current from the input sup- ply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3990 and to force this very high frequency switch- ing current into a tight local loop, minimizing EMI. A 1μF capacitor is capable of this task, but only if it is placed close to the LT3990 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3990. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3990 circuit is plugged into a live supply, the input volt- age can ring to twice its nominal value, possibly exceeding the LT3990’s voltage rating. This situation is easily avoided (see the Hot Plugging Safely section). Output Capacitor and Output Ripple The output capacitor has two essential functions. It stores energy in order to satisfy transient loads and stabilize the LT3990’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: C Vf OUT OUT SW = 50 • where fSW is in MHz and COUT is the recommended output capacitance in μF. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if combined with a phase lead capacitor (typically 22pF) between the output and the feedback pin. A lower value of output capacitor can be used to save space and cost but transient performance will suffer. The second function is that the output capacitor, along with the inductor, filters the square wave generated by the LT3990 to produce the DC output. In this role it determines the output ripple, so low impedance (at the switching frequency) is important. The output ripple decreases with increasing output capacitance, down to approximately 1mV. See Figure 1. Note that a larger phase lead capacitor should be used with a large output capacitor. COUT (μF) 0 0 2 6 8 10 40 80 100 18 3990 F01 4 20 60 12 14 16 FRONT PAGE APPLICATION CLEAD = 47pF FOR COUT ≥ 47μF VIN = 24V VIN = 12V When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor or one with a higher voltage rating may be required. Table 3 lists several capacitor vendors. Table 3. Recommended Ceramic Capacitor Vendors MANUFACTURER WEBSITE AVX www.avxcorp.com Murata www.murata.com Taiyo Yuden www.t-yuden.com Vishay Siliconix www.vishay.com TDK www.tdk.com Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3990 due to their piezoelectric nature. When in Burst Mode operation, the LT3990’s switching frequency depends on the load current, and at very light loads the LT3990 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3990 Figure 1. Worst-Case Output Ripple Across Full Load Range |
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