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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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LTC3246 데이터시트(HTML) 13 Page - Analog Devices |
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LTC3246 데이터시트(HTML) 13 Page - Analog Devices |
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LTC3246 13 3246fa For more information www.linear.com/LTC3246 VIN Capacitor Selection ThefinitechargetransferarchitectureusedbytheLTC3246 makes input noise filtering much less demanding than the sharp current spikes of conventional regulated charge pumps. Depending on the mode of operation, the input current of the LTC3246 can step from about 1A to 0A on a cycle-by-cycle basis. Low ESR will reduce the voltage steps caused by changing input current, while the ab- solute capacitor value will determine the level of ripple. The total amount and type of capacitance necessary for input bypassing is very dependent on the applied source impedance as well as existing bypassing already on the VIN node. For optimal input noise and ripple reduction, it is recommended that a low ESR ceramic capacitor be used for CIN bypassing. An electrolytic or tantalum capacitor may be used in parallel with the ceramic capacitor on CIN to increase the total capacitance, but, due to the higher ESR, it is not recommended that an electrolytic or tan- talum capacitor be used alone for input bypassing. The LTC3246 will operate with capacitors less than 1µF, but, depending on the source impedance, input noise can feed through to the output causing degraded performance. For best performance 1µF or greater total capacitance is suggested for CIN. Flying Capacitor Selection Ceramic capacitors should always be used for the flying capacitor. The flying capacitor controls the strength of the charge pump. In order to achieve the rated output current, it is necessary for the flying capacitor to have at least 1µF of capacitance over operating temperature with a bias voltage equal to the programmed VOUT (see Ceramic Capacitor Selection Guidelines). If only 100mA or less of output current is required for the application, the flying capacitor minimum can be reduced to 0.2µF. The voltage rating of the ceramic capacitor should be VOUT +1V or greater. Ceramic Capacitor Selection Guidelines Capacitors of different materials lose their capacitance with higher temperature and voltage at different rates. For example, a ceramic capacitor made of X5R or X7R material will retain most of its capacitance from –40°C to 85°C, whereas a Z5U or Y5V style capacitor will lose considerable capacitance over that range (60% to 80% loss typical). Z5U and Y5V capacitors may also have a very strong voltage coefficient, causing them to lose an additional60%ormoreoftheircapacitancewhentherated voltage is applied. Therefore, when comparing different capacitors, it is often more appropriate to compare the amount of achievable capacitance for a given case size rather than discussing the specified capacitance value. For example, over rated voltage and temperature conditions, a 4.7µF, 10V, Y5V ceramic capacitor in an 0805 case may not provide any more capacitance than a 1µF, 10V, X5R or X7R available in the same 0805 case. In fact, over bias and temperature range, the 1µF, 10V, X5R or X7R will provide more capacitance than the 4.7µF, 10V, Y5V. The capacitor manufacturer’s data sheet should be consulted to determine what value of capacitor is needed to ensure minimum capacitance values are met over operating temperature and bias voltage. Below is a list of ceramic capacitor manufacturers and how to contact them: MANUFACTURER WEBSITE AVX www.avxcorp.com Kemet www.kemet.com Murata www.murata Taiyo Yuden www.t-yuden.com TDK www.tdk.com Wurth Elektronik www.we-online.com BIAS Pin and Capacitor Selection TheBIASpinoftheLTC3246isa5Voutputthatisgenerated by an internal Low Drop-Out (LDO) regulator supplied by VIN. The BIAS voltage is used as a supply for the internal low voltage circuitry. A capacitor on the BIAS pin is neces- sary to stabilize the LDO output and minimize ripple during transient conditions. A low ESR ceramic capacitor with a minimum capacitance of 2µF over temperature with 5V bias should be used. Since the BIAS voltage comes from an LDO, the BIAS voltage will drop with VIN as VIN goes below 5V. This is normal and expected operation. The BIAS pin voltage is for internal circuitry only and should not be loaded externally. APPLICATIONS INFORMATION |
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