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LT1934ES6 Datasheet(PDF) 9 Page - Linear Technology |
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LT1934ES6 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LT1934/LT1934-1 1934f capacitor is subject to large surge currents if the LT1934 circuit is connected to a low impedance supply, and that some electrolytic capacitors (in particular tantalum) must be specified for such use. Output Capacitor and Output Ripple The output capacitor filters the inductor’s ripple current and stores energy to satisfy the load current when the LT1934 is quiescent. In order to keep output voltage ripple low, the impedance of the capacitor must be low at the LT1934’s switching frequency. The capacitor’s equivalent series resistance (ESR) determines this impedance. Choose one with low ESR intended for use in switching regulators. The contribution to ripple voltage due to the ESR is approximately ILIM•ESR.ESRshouldbelessthan~150mΩ for the LT1934 and less than ~500m Ω for the LT1934-1. The value of the output capacitor must be large enough to accept the energy stored in the inductor without a large change in output voltage. Setting this voltage step equal to 1% of the output voltage, the output capacitor must be: COUT > 50 • L • (ILIM/VOUT)2 For example, an LT1934 producing 3.3V with L = 47 µH requires 33 µF.Thisvaluecanberelaxedifsmallcircuitsize is more important than low output ripple. Sanyo’s POSCAP series in B-case and C-case sizes pro- vides very good performance in a small package for the LT1934. Similar performance in traditional tantalum ca- pacitors requires a larger package (C- or D-case). The APPLICATIO S I FOR ATIO LT1934-1, with its lower switch current, can use a B-case tantalum capacitor. With a high quality capacitor filtering the ripple current from the inductor, the output voltage ripple is determined by the hysteresis and delay in the LT1934’s feedback comparator. This ripple can be reduced further by adding a small (typically 10pF) phase lead capacitor between the output and the feedback pin. Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT1934. Not all ceramic capacitors are suitable. X5R and X7R types are stable over temperature and applied voltage and give dependable service. Other types (Y5V and Z5U) have very large temperature and voltage coefficients of capacitance. In the application circuit they may have only a small fraction of their nominal capacitance and voltage ripple may be much larger than expected. Ceramic capacitors are piezoelectric. The LT1934’s switch- ing frequency depends on the load current, and at light loads the LT1934 can excite the ceramic capacitor at audio frequencies, generating audible noise. If this is unaccept- able, use a high performance electrolytic capacitor at the output. The input capacitor can be a parallel combination of a 2.2 µF ceramic capacitor and a low cost electrolytic capacitor. The level of noise produced by the LT1934-1 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 |
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