Datasheet LT3027 (Analog Devices) - 10

LT3027
U U W U APPLICATIO S I FOR ATIO Output Capacitance and Transient Response
When used with a 5V regulator, a 16V 10µF Y5V capacitor can exhibit an effective value as low as 1µF to 2µF for the The LT3027 regulator is designed to be stable with a wide DC bias voltage applied and over the operating tempera- range of output capacitors. The ESR of the output capaci- ture range. The X5R and X7R dielectrics result in more tor affects stability, most notably with small stable characteristics and are more suitable for use as the capacitors. A minimum output capacitor of 1µF with an output capacitor. The X7R type has better stability across ESR of 3Ω or less is recommended to prevent oscilla- temperature, while the X5R is less expensive and is tions. The LT3027 is a micropower device and output available in higher values. Care still must be exercised transient response will be a function of output capaci- when using X5R and X7R capacitors; the X5R and X7R tance. Larger values of output capacitance decrease the codes only specify operating temperature range and maxi- peak deviations and provide improved transient response mum capacitance change over temperature. Capacitance for larger load current changes. Bypass capacitors, used change due to DC bias with X5R and X7R capacitors is to decouple individual components powered by the better than Y5V and Z5U capacitors, but can still be LT3027, will increase the effective output capacitor value. significant enough to drop capacitor values below appro- With larger capacitors used to bypass the reference (for priate levels. Capacitor DC bias characteristics tend to low noise operation), larger values of output capacitors improve as component case size increases, but expected are needed. For 100pF of bypass capacitance, 2.2µF of capacitance at operating voltage should be verified. output capacitor is recommended. With a 330pF bypass capacitor or larger, a 3.3µF output capacitor is recom- 20 BOTH CAPACITORS ARE 16V, mended. The shaded region of Figure 2 defines the region 1210 CASE SIZE, 10µF 0 over which the LT3027 regulator is stable. The minimum X5R ESR needed is defined by the amount of bypass capaci- –20 tance used, while the maximum ESR is 3Ω. –40 Extra consideration must be given to the use of ceramic –60 capacitors. Ceramic capacitors are manufactured with a CHANGE IN VALUE (%) Y5V variety of dielectrics, each with different behavior across –80 temperature and applied voltage. The most common di- electrics used are specified with EIA temperature charac- –100 0 2 4 6 8 10 12 14 16 teristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V DC BIAS VOLTAGE (V) dielectrics are good for providing high capacitances in a 3027 F03
Figure 3. Ceramic Capacitor DC Bias Characteristics
small package, but they tend to have strong voltage and temperature coefficients as shown in Figures 3 and 4. 40 4.0 20 3.5 X5R 3.0 0 STABLE REGION 2.5 –20 ) Ω 2.0 –40 ESR ( Y5V 1.5 CBYP = 0 –60 C CHANGE IN VALUE (%) BYP = 100pF 1.0 CBYP = 330pF C –80 BOTH CAPACITORS ARE 16V, BYP > 3300pF 0.5 1210 CASE SIZE, 10µF –100 0 –50 –25 0 25 50 75 100 125 1 2 3 4 5 6 7 8 9 10 TEMPERATURE (°C) OUTPUT CAPACITANCE (µF) 3027 F04 3027 F02
Figure 2. Stability Figure 4. Ceramic Capacitor Temperature Characteristics
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