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LTC3251-1.2 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3251-1.2 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LTC3251/ LTC3251-1.2/LTC3251-1.5 32511215fb Figure 5. 10nH Inductor Used for Additional Input Noise Reduction OPERATIO (Refer to Block Diagram) Figure 4. 10nH Inductor Used for Additional Output Noise Reduction VOUT 10nH (TRACE INDUCTANCE) 1 µF 10 µF 3251 F04 VOUT LTC3251 GND VIN 10nH (TRACE INDUCTANCE) 1 µF 3251 F05 VIN SUPPLY LTC3251 GND Further output noise reduction can be achieved by filtering the LTC3251 output through a very small series inductor as shown in Figure 4. A 10nH inductor will reject the fast output transients caused by the blanking period. The 10nH inductor can be fabricated on the PC board with about 1cm (0.4") of 1mm wide PC board trace. Flying Capacitor Selection Warning: A polarized capacitor such as tantalum or alumi- num should never be used for the flying capacitors since their voltages can reverse upon start-up of the LTC3251. Ceramic capacitors should always be used for the flying capacitors. The flying capacitors control 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 0.4 µF of capacitance over operating temperature with a 2V bias (see Ceramic Capacitor Selection Guidelines). If only 200mA or less of output current is required for the application, the flying capacitor minimum can be reduced to 0.15 µF. 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 consider- able capacitance over that range (60% to 80% loss typ). Z5U and Y5V capacitors may also have a very strong voltage coefficient, causing them to lose an additional 60% or more of their capacitance when the rated voltage is applied. Therefore, when comparing different capaci- tors, 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 4.7 µF, 10V, Y5V ceramic capacitor in an 0805 case may not provide any more capacitance than a 1 µF,10V,X5RorX7R available in the same 0805 case. In fact, over bias and VIN Capacitor Selection The dual phase architecture used by the LTC3251 family makes input noise filtering much less demanding than conventional charge pump regulators. The input current should be continuous at about IOUT/2. The blanking period described in the VOUT section also effects the input. For this reason it is recommended that a low ESR, 1 µF (0.4µF min) or greater ceramic capacitor be used for CIN (see Ceramic Capacitor Selection Guidelines section). In cases where the supply impedance is high, heavy output transients can cause significant input transients. These input transients feed back to the output which slows the output transient recovery and increases overshoot and output impedance. This effect can generally be avoided by using low impedance supplies and short supply connec- tions. If this is not possible, a ≥4.7µF capacitor is recom- mended for the input capacitor. Aluminum and tantalum capacitors are not recommended because of their high ESR. Further input noise reduction can be achieved by filtering the input through a very small series inductor as shown in Figure 5. A 10nH inductor will reject the fast input tran- sients caused by the blanking period, thereby presenting a nearly constant load to the input supply. For economy, the 10nH inductor can be fabricated on the PC board with about 1cm (0.4") of 1mm wide PC board trace. |
Similar Part No. - LTC3251-1.2_15 |
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Similar Description - LTC3251-1.2_15 |
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