Datasheet LT3082 (Analog Devices) - 10

LT3082
APPLICATIONS INFORMATION
electrics, each with different behavior across temperature
Stability and Input Capacitance
and applied voltage. The most common dielectrics used Low ESR, ceramic input bypass capacitors are acceptable are specifi ed with EIA temperature characteristic codes of for applications without long input leads. However, applica- Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are tions connecting a power supply to an LT3082 circuit’s IN good for providing high capacitances in a small package, and GND pins with long input wires combined with a low but they tend to have strong voltage and temperature ESR, ceramic input capacitors are prone to voltage spikes, coeffi cients, as shown in Figures 3 and 4. When used with reliability concerns and application-specifi c board oscil- a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an lations. The input wire inductance found in many battery effective value as low as 1μF to 2μF for the DC bias voltage powered applications, combined with the low ESR ceramic applied and over the operating temperature range. The X5R input capacitor, forms a high-Q LC resonant tank circuit. In and X7R dielectrics result in more stable characteristics some instances this resonant frequency beats against the and are more suitable for use as the output capacitor. output current dependent LDO bandwidth and interferes The X7R type has better stability across temperature, with proper operation. Simple circuit modifi cations/solu- while the X5R is less expensive and is available in higher values. Care still must be exercised when using X5R and tions are then required. This behavior is not indicative of LT3082 instability, but is a common ceramic input bypass X7R capacitors. The X5R and X7R codes only specify capacitor application issue. operating temperature range and maximum capacitance change over temperature. Capacitance change due to DC The self-inductance, or isolated inductance, of a wire is bias with X5R and X7R capacitors is better than with Y5V directly proportional to its length. Wire diameter is not a and Z5U capacitors, but can still be signifi cant enough to major factor on its self-inductance. For example, the self- drop capacitor values below appropriate levels. Capacitor inductance of a 2-AWG isolated wire (diameter = 0.26") is DC bias characteristics tend to improve as component about half the self-inductance of a 30-AWG wire (diameter case size increases, but expected capacitance at operating = 0.01"). One foot of 30-AWG wire has about 465nH of voltage should be verifi ed. self-inductance. Voltage and temperature coeffi cients are not the only One of two ways reduces a wire’s self-inductance. One sources of problems. Some ceramic capacitors have a method divides the current fl owing towards the LT3082 piezoelectric response. A piezoelectric device generates between two parallel conductors. In this case, the farther voltage across its terminals due to mechanical stress. In a apart the wires are from each other, the more the self-in- ceramic capacitor, the stress can be induced by vibrations ductance is reduced; up to a 50% reduction when placed in the system or thermal transients. a few inches apart. Splitting the wires basically connects 20 40 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF 20 0 X5R 0 X5R –20 ALUE (%) –20 –40 –40 Y5V –60 CHANGE IN V –60 Y5V 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) 3082 F03 3082 F04
Figure 3. Ceramic Capacitor DC Bias Characteristics Figure 4. Ceramic Capacitor Temperature Characteristics
3082f 10