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LTC1751-5 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1751-5 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC3245 11 3245f For more information www.linear.com/LTC3245 applicaTions inForMaTion strength. The value of COUT directly controls the amount of output ripple for a given load current when operating in constant frequency mode. Increasing the size of COUT will reduce the output ripple. To reduce output noise and ripple, it is suggested that a low ESR (equivalent series resistance < 0.1Ω) ceramic capacitor (10μF or greater) be used for COUT. Tantalum and aluminum capacitors can be used in parallel with a ceramic capacitor to increase the total capacitance but are not recommended to be used alone because of their high ESR. BoththestyleandvalueofCOUTcansignificantlyaffectthe stability of the LTC3245. As shown in the Block Diagram, the device uses a control loop to adjust the strength of the charge pump to match the current required at the output. The error signal of this loop is stored directly on the output chargestoragecapacitor.Thechargestoragecapacitoralso serves to form the dominant pole for the control loop. To prevent ringing or instability it is important for the output capacitor to maintain at least 4μF of capacitance over all conditions (see Ceramic Capacitor Selection Guidelines). Likewise excessive ESR on the output capacitor will tend to degrade the loop stability of the LTC3245. The closed loopoutputresistanceofthedeviceisdesignedtobe0.3Ω for a 5V output and 0.2Ω for a 3.3V output. For a 250mA load current change, the output voltage will change by about 1.5%V. If the output capacitor has more ESR than the closed loop impedance, the closed loop frequency response will cease to roll off in a simple 1-pole fashion andpoorloadtransientresponseorinstabilitycouldresult. Ceramic capacitors typically have exceptional ESR perfor- mance, and combined with a tight board layout, should yield excellent stability and load transient performance. VIN Capacitor Selection The constant frequency architecture used by the LTC3245 makesinputnoisefilteringmuchlessdemandingthanwith conventional regulated charge pumps. Depending on the mode of operation the input current of the LTC3245 can vary from IOUT to 0mA on a cycle-by-cycle basis. Low ESR will reduce the voltage steps caused by changing input current, while the absolute capacitor value will determine thelevelofripple.Thetotalamountandtypeofcapacitance necessary for input bypassing is very dependant 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 ce- ramic capacitor on CIN to increase the total capacitance, but due to the higher ESR it is not recommended that an electrolytic or tantalum capacitor be used alone for input bypassing. The LTC3245 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 Warning: A polarized capacitor such as tantalum or alumi- num should never be used for the flying capacitors since the voltage can reverse upon start-up of the LTC3245. 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 0.4μF 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 0.15μF. 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 |
Similar Part No. - LTC1751-5 |
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Similar Description - LTC1751-5 |
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