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LT3470ITS8 Datasheet(PDF) 10 Page - Linear Integrated Systems |
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LT3470ITS8 Datasheet(HTML) 10 Page - Linear Integrated Systems |
10 / 16 page 10 LT3470 3470f Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3470. Not all ceramic capacitors are suitable. X5R and X7R types are stable over temperature and applied voltage and give dependable service. Other types, including Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application circuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected. Ceramic capacitors are piezoelectric. The LT3470’s switch- ing frequency depends on the load current, and at light loads the LT3470 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3470 operates at a lower current limit during BurstMode opera- tion, the noise is typically very quiet to a casual ear. If this audible noise is unacceptable, use a high performance electrolytic capacitor at the output. The input capacitor can be a parallel combination of a 2.2µF ceramic capacitor and a low cost electrolytic capacitor. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3470. A ce- ramic input capacitor combined with trace or cable induc- tance forms a high quality (under damped) tank circuit. If the LT3470 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3470’s rating. This situation is easily avoided; see the Hot Plugging Safely section. BOOST and BIAS Pin Considerations Capacitor C3 and the internal boost Schottky diode (see Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22µF capacitor will work well. Figure 2 shows two ways to arrange the boost circuit. The BOOST pin must be more than 2.5V above the SW pin for best efficiency. For outputs of 3.3V and above, the standard circuit (Figure 2a) is best. For outputs between 2.5V and 3V, use a 0.47µF. For lower output voltages the boost diode can be tied to the input Table 3. Capacitor Vendors Vendor Phone URL Part Series Comments Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, POSCAP Tantalum Murata (404) 436-1300 www.murata.com Ceramic AVX www.avxcorp.com Ceramic, Tantalum TPS Series Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic VIN BOOST LT3470 (2a) (2b) SW 36 5 7 4 C3 0.22µF VIN VOUT VBOOST – VSW ≅ VOUT MAX VBOOST ≅ VIN + VOUT BIAS GND VIN BOOST LT3470 SW BIAS 36 5 7 4 C3 0.22µF VIN VOUT 3470 F02 VBOOST – VSW ≅ VIN MAX VBOOST ≅ 2VIN GND Figure 2. Two Circuits for Generating the Boost Voltage APPLICATIO S I FOR ATIO |
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