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LT3682 Datasheet(PDF) 11 Page - Linear Technology |
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LT3682 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3645 11 3645f APPLICATIONS INFORMATION Now the required on time has decreased below the mini- mum on time of 100ns. Instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the part skips a few pulses so that the aver- age inductor current meets and does not exceed the load current requirement. The LT3645 is robust enough to survive prolonged opera- tion under these conditions as long as the peak inductor current does not exceed 1.2A. Inductor saturation due to high current may further limit performance in this operating region. Inductor Selection and Maximum Output Current Choose the inductor value according to: L = 2.2 •(VOUT + VD)/ƒ where VD is the forward voltage drop of the catch diode (~0.4V), f is the switching frequency in MHz and L is in μH. With this value, there will be no subharmonic oscilla- tion for applications with 50% or greater duty cycle. For robust operation in fault conditions, the saturation current should be above 1.5A. To keep efficiency high, the series resistance (DCR) should be less than 0.1Ω. Table 1 lists several inductor vendors. If the buck load current is less than 500mA, then a lower valued inductor can be used. Catch Diode Depending on load current, a 500mA to 1A Schottky diode is recommended for the catch diode, D1. The diode must have a reverse voltage rating equal to or greater than the overvoltage lockout voltage (38.5V). The ON Semiconduc- tor MBRA140T3 and Central Semiconductor CMMSH1-40 are good choices, as they are rated for 1A continuous forward current and a maximum reverse voltage of 40V. Input Filter Network Bypass VIN with a 1μF or higher ceramic capacitor of X7R or X5R type. Y5V types have poor performance over tem- perature and applied voltage and should not be used. A 1μF ceramic capacitor is adequate to bypass the LT3645 and will easily handle the ripple current. However, if the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance might be necessary. This can be provided with a low performance (high ESR) electrolytic capacitor in parallel with the ceramic device. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3645 input and to force this very high frequency switching 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 LT3645 and catch diode (see the PCB layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3645. A ceramic input capacitor combined with trace or cable inductance forms a high quality (underdamped) tank cir- cuit. If the LT3645 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3645’s voltage rating. This situation can easily be avoided. For more details, see Linear Technology Application Note 88. Table 1. Inductor Vendors Vendor URL Part Series Inductance Range (μH) Size (mm) Sumida www.sumida.com CDRH4D28 CDRH5D28 CDRH8D28 1.2 to 4.7 2.5 to 10 2.5 to 33 4.5 × 4.5 5.5 × 5.5 8.3 × 8.3 Toko www.toko.com A916CY D585LC 2 to 12 1.1 to 39 6.3 × 6.2 8.1 × 8.0 Würth Elektronik www.we-online.com WE-TPC(M) WE-PD2(M) WE-PD(S) 1 to 10 2.2 to 22 1 to 27 4.8 × 4.8 5.2 × 5.8 7.3 × 7.3 |
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