Datasheet LT3012 (Analog Devices) - 9

LT3012
APPLICATIONS INFORMATION
IN OUT V improved transient response for larger load current OUT LT3012 + R2 changes. Bypass capacitors, used to decouple individual VIN ADJ components powered by the LT3012, will increase the 3012 F01 GND R1 effective output capacitor value. Extra consideration must be given to the use of ceramic capacitors. Ceramic capacitors are manufactured with a R2 VOUT = 1.24V 1 + + (IADJ)(R2) R1 variety of dielectrics, each with different behavior across VADJ = 1.24V IADJ = 30nA AT 25°C temperature and applied voltage. The most common OUTPUT RANGE = 1.24V TO 60V dielectrics used are specifi ed with EIA temperature char-
Figure 1. Adjustable Operation
acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances The adjustable device is tested and specifi ed with the in a small package, but they tend to have strong voltage ADJ pin tied to the OUT pin and a 5μA DC load (unless and temperature coeffi cients as shown in Figures 2 and 3. otherwise specifi ed) for an output voltage of 1.24V. Speci- When used with a 5V regulator, a 16V 10μF Y5V capacitor fi cations for output voltages greater than 1.24V will be can exhibit an effective value as low as 1μF to 2μF for the proportional to the ratio of the desired output voltage to DC bias voltage applied and over the operating tempera- 1.24V; (VOUT/1.24V). For example, load regulation for an ture range. The X5R and X7R dielectrics result in more output current change of 1mA to 250mA is –7mV typical at stable characteristics and are more suitable for use as the VOUT = 1.24V. At VOUT = 12V, load regulation is: output capacitor. The X7R type has better stability across (12V/1.24V) • (–7mV) = –68mV temperature, while the X5R is less expensive and is avail- able in higher values. Care still must be exercised when
Output Capacitance and Transient Response
using X5R and X7R capacitors; the X5R and X7R codes The LT3012 is designed to be stable with a wide range of only specify operating temperature range and maximum output capacitors. The ESR of the output capacitor affects capacitance change over temperature. Capacitance change stability, most notably with small capacitors. A minimum due to DC bias with X5R and X7R capacitors is better than output capacitor of 3.3μF with an ESR of 3Ω or less is Y5V and Z5U capacitors, but can still be signifi cant enough recommended to prevent oscillations. The LT3012 is a to drop capacitor values below appropriate levels. Capaci- micropower device and output transient response will be tor DC bias characteristics tend to improve as component a function of output capacitance. Larger values of output case size increases, but expected capacitance at operating capacitance decrease the peak deviations and provide voltage should be verifi ed. 20 40 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF 20 0 X5R 0 –20 X5R –20 –40 –40 Y5V –60 CHANGE IN VALUE (%) Y5V –60 CHANGE IN VALUE (%) –80 –80 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF –100 –100 0 2 4 6 8 10 12 14 16 –50 –25 0 25 50 75 100 125 DC BIAS VOLTAGE (V) TEMPERATURE (°C) 3012 F02 3012 F03
Figure 2. Ceramic Capacitor DC Bias Characteristics Figure 3. Ceramic Capacitor Temperature Characteristics
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