Datasheet LT3080 (Analog Devices) - 10

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APPLICATIONS INFORMATION If guard-ring techniques are used, this bootstraps any
IN LT3080 V
stray capacitance at the SET pin. Since the SET pin is a
CONTROL
high impedance node, unwanted signals may couple into
+
the SET pin and cause erratic behavior. This will be most
+ +
noticeable when operating with minimum output capaci-
VIN VCONTROL – OUT
tors at full load current. The easiest way to remedy this is
VOUT SET
to bypass the SET pin with a small amount of capacitance
COUT
from SET to ground, 10pF to 20pF is sufficient.
RSET CSET 3080 F01
Stability and Output Capacitance Figure 1. Basic Adjustable Regulator
The LT3080 requires an output capacitor for stability. It is designed to be stable with most low ESR capacitors (typ-
Output Voltage
ically ceramic, tantalum or low ESR electrolytic). A min- The LT3080 generates a 10µA reference current that flows imum output capacitor of 2.2µF with an ESR of 0.5Ω out of the SET pin. Connecting a resistor from SET to or less is recommended to prevent oscillations. Larger ground generates a voltage that becomes the reference values of output capacitance decrease peak deviations point for the error amplifier (see Figure 1). The reference and provide improved transient response for larger load voltage is a straight multiplication of the SET pin current current changes. Bypass capacitors, used to decouple and the value of the resistor. Any voltage can be gen- individual components powered by the LT3080, increase erated and there is no minimum output voltage for the the effective output capacitor value. regulator. A minimum load current of 1mA is required to For improvement in transient performance, place a capac- maintain regulation regardless of output voltage. For true itor across the voltage setting resistor. Capacitors up to zero voltage output operation, this 1mA load current must 1µF can be used. This bypass capacitor reduces system be returned to a negative supply voltage. noise as well, but start-up time is proportional to the time With the low level current used to generate the refer- constant of the voltage setting resistor (RSET in Figure 1) ence voltage, leakage paths to or from the SET pin can and SET pin bypass capacitor. create errors in the reference and output voltages. High Extra consideration must be given to the use of ceramic quality insulation should be used (e.g., Teflon, Kel-F); capacitors. Ceramic capacitors are manufactured with a cleaning of all insulating surfaces to remove fluxes and variety of dielectrics, each with different behavior across other residues will probably be required. Surface coating temperature and applied voltage. The most common may be necessary to provide a moisture barrier in high dielectrics used are specified with EIA temperature char- humidity environments. acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances Board leakage can be minimized by encircling the SET in a small package, but they tend to have strong voltage pin and circuitry with a guard ring operated at a potential and temperature coefficients as shown in Figure 2 and close to itself; the guard ring should be tied to the OUT Figure 3. When used with a 5V regulator, a 16V 10µF Y5V pin. Guarding both sides of the circuit board is required. capacitor can exhibit an effective value as low as 1µF to Bulk leakage reduction depends on the guard ring width. 2µF for the DC bias voltage applied and over the operating Ten nanoamperes of leakage into or out of the SET pin and temperature range. The X5R and X7R dielectrics result in associated circuitry creates a 0.1% error in the reference more stable characteristics and are more suitable for use voltage. Leakages of this magnitude, coupled with other as the output capacitor. The X7R type has better stability sources of leakage, can cause significant offset voltage across temperature, while the X5R is less expensive and and reference drift, especially over the possible operating is available in higher values. Care still must be exercised temperature range. Rev. D 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Output Voltage Stability and Output Capacitance Paralleling Devices Thermal Performance Quieting the Noise Overload Recovery Load Regulation Thermal Considerations T Package, 5-Lead TO-220 Calculating Junction Temperature Reducing Power Dissipation Typical Applications Package Description Revision History Typical Application Related Parts