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LT3506EFE Datasheet(PDF) 11 Page - Linear Technology |
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LT3506EFE Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3506/LT3506A 3506afb Finally,theremustbeenoughcapacitanceforgoodtransient performance.Thelastequationgivesagoodstartingpoint. Alternatively, you can start with one of the designs in this data sheet and experiment to get the desired performance. This topic is covered more thoroughly in the section on loop compensation. For 5V and 3.3V outputs with greater than 1A output, a 22μF 6.3V ceramic capacitor (X5R or X7R) at the output results in very low output voltage ripple and good tran- sient response. For lower voltages, 22μF is adequate but increasing COUT will improve transient performance. For the LT3506A, 10μF of output capacitance is sufficient at VOUT between 3.3V and 5V. Other types and values can be used. The following discusses tradeoffs in output ripple and transient performance. The high performance (low ESR), small size and robust- ness of ceramic capacitors make them the preferred type for LT3506 applications. However, all ceramic capacitors are not the same. As mentioned above, many of the higher value capacitors use poor dielectrics with high temperature and voltage coefficients. In particular, Y5V and Z5U types lose a large fraction of their capacitance with applied voltage and temperature extremes. Because the loop stability and transient response depend on the value of COUT, you may not be able to tolerate this loss. Use X7R and X5R types. You can also use electrolytic capacitors. The ESRs of most aluminum electrolytics are too large to deliver low output ripple. Tantalum and newer, lower ESR organic electrolytic capacitors intended for power supply use are suitable, and the manufacturers will specify the ESR. The choice of capacitor value will be based on the ESR required for low ripple. Because the volume of the capacitor determines its ESR, both the size and the value will be larger than a ceramic capacitor that would give similar ripple perfor- mance. One benefit is that the larger capacitance may give bettertransientresponseforlargechangesinloadcurrent. Table 2 lists several capacitor vendors. Table 2. Low-ESR Surface Mount Capacitors VENDOR TYPE SERIES Taiyo-Yuden Ceramic AVX Ceramic Tantalum TPS Kemet Tantalum Tantalum Organic Aluminum Organic T491, T494, T495, T520 A700 Sanyo Tantalum or Aluminum Organic POSCAP Panasonic Aluminum Organic SP CAP TDK Ceramic Catch Diode The catch diode (D1 in Figure 2) must have a reverse volt- age rating greater than the maximum input voltage. The average current of the catch diode is given by: IDAVE=IOUT(1-DCMIN) A Schottky diode with a 1A average forward current rating will suffice for most applications. The ON Semiconductor MBRM120LT3 (20V) and MBRM130LT3 (30V) are good choices;theyhaveatinypackagewithgoodthermalproper- ties.Manyvendorshavesuitablesurfacemountversionsof the 1N5817 (20V) and 1N5818 (30V) 1A Schottky diodes such as the Microsemi UPS120. Applications with large step down ratios and high output currents may have more than 1A of average diode current. TheONSemiconductorMBRS230LT3orInternationalRec- tifier 20BQ030 (both 2A, 30V) would be good choices. BOOST Pin Considerations The capacitor and diode tied to the BOOST pin generate a voltage that is higher than the input voltage. In most cases a 0.1μF capacitor and fast switching diode (such as the CMDSH-3 or FMMD914) will work well. Figure 3 APPLICATIO S I FOR ATIO |
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