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LT1963EFE-3.3 Datasheet(PDF) 15 Page - Linear Integrated Systems |
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LT1963EFE-3.3 Datasheet(HTML) 15 Page - Linear Integrated Systems |
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15 / 28 page  LT1963A Series 15 1963aff For more information www.linear.com/LT1963A POSCAP capacitors are used. The output voltage is at the worst case value of 1.2V. Trace A, is with a 10µF ceramic output capacitor and shows significant ringing with a peak amplitude of 25mV. For Trace B, a 22µF/45mΩ POSCAP is added in parallel with the 10µF ceramic. The output is well damped and settles to within 10mV in less than 20µs. ForTraceC,a100µF/35mΩPOSCAPisconnectedinparallel withthe10µFceramiccapacitor.Inthiscasethepeakoutput deviation is less than 20mV and the output settles in about 10µs. For improved transient response the value of the bulk capacitor (tantalum or aluminum electrolytic) should be greater than twice the value of the ceramic capacitor. Tantalum and Polytantalum Capacitors There is a variety of tantalum capacitor types available, with a wide range of ESR specifications. Older types have ESRspecificationsinthehundredsofmΩtoseveralOhms. Some newer types of polytantalum with multi-electrodes have maximum ESR specifications as low as 5mΩ. In gen- eral the lower the ESR specification, the larger the size and the higher the price. Polytantalum capacitors have better surge capability than older types and generally lower ESR. Some types such as the Sanyo TPE and TPB series have ESR specifications in the 20mΩ to 50mΩ range, which provide near optimum transient response. Aluminum Electrolytic Capacitors Aluminumelectrolyticcapacitorscanalsobeusedwiththe LT1963A.Thesecapacitorscanalsobeusedinconjunction with ceramic capacitors. These tend to be the cheapest and lowest performance type of capacitors. Care must be used in selecting these capacitors as some types can have ESR which can easily exceed the 3Ω maximum value. Ceramic Capacitors Extra consideration must be given to the use of ceramic capacitors. Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior over temperature and applied voltage. The most common dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U and Table 1. Capacitor Minimum ESR VOUT 10µF 22µF 47µF 100µF 1.2V 20mΩ 15mΩ 10mΩ 5mΩ 1.5V 20mΩ 15mΩ 10mΩ 5mΩ 1.8V 15mΩ 10mΩ 10mΩ 5mΩ 2.5V 5mΩ 5mΩ 5mΩ 5mΩ 3.3V 0mΩ 0mΩ 0mΩ 5mΩ ≥5V 0mΩ 0mΩ 0mΩ 0mΩ Figures 3 through 8 show the effect of ESR on the transient response of the regulator. These scope photos show the transientresponsefortheLT1963Aatthreedifferentoutput voltageswithvariouscapacitorsandvariousvaluesofESR. The output load conditions are the same for all traces. In all cases there is a DC load of 500mA. The load steps up to 1A at the first transition and steps back to 500mA at the second transition. At the worst case point of 1.2VOUT with 10µF COUT (Figure 3), a minimum amount of ESR is required. While 20mΩ is enough to eliminate most of the ringing, a value closer to 50mΩ provides a more optimum response. At 2.5V output with 10µF COUT (Figure 4) the output rings at the transitions with 0Ω ESR but still settles to within 10mV in 20µs after the 0.5A load step. Once again a small value of ESR will provide a more optimum response. At 5VOUT with 10µF COUT (Figure 5) the response is well damped with 0Ω ESR. With a COUT of 100µF at 0Ω ESR and an output of 1.2V (Figure 6), the output rings although the amplitude is only 20mVp-p. With COUT of 100µF it takes only 5mΩ to 20mΩ of ESR to provide good damping at 1.2V output. Performanceat2.5Vand5Voutputwith100µFCOUTshows similar characteristics to the 10µF case (see Figures 7-8). At2.5VOUT5mΩto20mΩcanimprovetransientresponse. At 5VOUT the response is well damped with 0Ω ESR. CapacitortypeswithinherentlyhigherESRcanbecombined with 0mΩ ESR ceramic capacitors to achieve both good high frequency bypassing and fast settling time. Figure 9 illustrates the improvement in transient response that can be seen when a parallel combination of ceramic and applicaTions inForMaTion |
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