LT3080 applicaTions inForMaTionIf guardring techniques are used, this bootstraps any IN LT3080 V stray capacitance at the SET pin. Since the SET pin is CONTROL a high impedance node, unwanted signals may couple + into the SET pin and cause erratic behavior. This will + + V be most noticeable when operating with minimum IN VCONTROL – OUT output capacitors at full load current. The easiest way VOUT SET to remedy this is to bypass the SET pin with a small COUT amount of capacitance from SET to ground, 10pF to RSET CSET 20pF is sufficient. 3080 F01 Figure 1. Basic Adjustable RegulatorStability and Output Capacitance The LT3080 requires an output capacitor for stability. It Output Voltage is designed to be stable with most low ESR capacitors The LT3080 generates a 10µA reference current that flows (typically ceramic, tantalum or low ESR electrolytic). out of the SET pin. Connecting a resistor from SET to A minimum output capacitor of 2.2µF with an ESR of 0.5Ω ground generates a voltage that becomes the reference or less is recommended to prevent oscillations. Larger point for the error amplifier (see Figure 1). The reference values of output capacitance decrease peak deviations voltage is a straight multiplication of the SET pin current and provide improved transient response for larger load and the value of the resistor. Any voltage can be generated current changes. Bypass capacitors, used to decouple and there is no minimum output voltage for the regulator. individual components powered by the LT3080, increase A minimum load current of 1mA is required to maintain the effective output capacitor value. regulation regardless of output voltage. For true zero For improvement in transient performance, place a capaci- voltage output operation, this 1mA load current must be tor across the voltage setting resistor. Capacitors up to returned to a negative supply voltage. 1µF can be used. This bypass capacitor reduces system With the low level current used to generate the reference noise as well, but start-up time is proportional to the time voltage, leakage paths to or from the SET pin can create constant of the voltage setting resistor (RSET in Figure 1) errors in the reference and output voltages. High quality and SET pin bypass capacitor. insulation should be used (e.g., Teflon, Kel-F); cleaning Extra consideration must be given to the use of ceramic of all insulating surfaces to remove fluxes and other resi- capacitors. Ceramic capacitors are manufactured with a dues will probably be required. Surface coating may be variety of dielectrics, each with different behavior across necessary to provide a moisture barrier in high humidity temperature and applied voltage. The most common environments. dielectrics used are specified with EIA temperature char- acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Board leakage can be minimized by encircling the SET Y5V dielectrics are good for providing high capacitances pin and circuitry with a guard ring operated at a potential in a small package, but they tend to have strong volt- close to itself; the guard ring should be tied to the OUT age and temperature coefficients as shown in Figures 2 pin. Guarding both sides of the circuit board is required. and 3. When used with a 5V regulator, a 16V 10µF Y5V Bulk leakage reduction depends on the guard ring width. capacitor can exhibit an effective value as low as 1µF to Ten nanoamperes of leakage into or out of the SET pin and 2µF for the DC bias voltage applied and over the operating associated circuitry creates a 0.1% error in the reference temperature range. The X5R and X7R dielectrics result in voltage. Leakages of this magnitude, coupled with other more stable characteristics and are more suitable for use sources of leakage, can cause significant offset voltage as the output capacitor. The X7R type has better stability and reference drift, especially over the possible operating across temperature, while the X5R is less expensive and is temperature range. 3080fc 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts
