Datasheet LT3080-1 (Analog Devices) - 9

LT3080-1
applicaTions inFormaTion
IN LT3080-1 probably be required. Surface coating may be necessary to VCONTROL provide a moisture barrier in high humidity environments. Board leakage can be minimized by encircling the SET + + + pin and circuitry with a guard ring operated at a potential VIN VCONTROL – 25mΩ close to itself; the guard ring should be tied to the OUT OUT VOUT pin. Guarding both sides of the circuit board is required. SET C Bulk leakage reduction depends on the guard ring width. OUT RSET CSET Ten nanoamperes of leakage into or out of the SET pin and 30801 F01 associated circuitry creates a 0.1% error in the reference voltage. Leakages of this magnitude, coupled with other
Figure 1. Basic Adjustable Regulator
sources of leakage, can cause significant offset voltage and reference drift, especially over the possible operating and a lower voltage supply for the collector. This increases temperature range. efficiency and reduces dissipation. To further spread the heat, a resistor can be inserted in series with the collector
If guardring techniques are used, this bootstraps any
to move some of the heat out of the IC and spread it on
stray capacitance at the SET pin. Since the SET pin is
the PC board.
a high impedance node, unwanted signals may couple into the SET pin and cause erratic behavior. This will be
The LT3080-1 can be operated in two modes. Three terminal
most noticeable when operating with minimum output
mode has the control pin connected to the power input pin
capacitors at full load current. The easiest way to remedy
which gives a limitation of 1.35V dropout. Alternatively,
this is to bypass the SET pin with a small amount of ca-
the “control” pin can be tied to a higher voltage and the
pacitance from SET to ground, 10pF to 20pF is sufficient.
power IN pin to a lower voltage giving 300mV dropout on the IN pin and minimizing the power dissipation. This
Stability and Output Capacitance
allows for a 1.1A supply regulating from 2.5VIN to 1.8VOUT or 1.8V The LT3080-1 requires an output capacitor for stability. IN to 1.2VOUT with low dissipation. It is designed to be stable with most low ESR capacitors
Output Voltage
(typically ceramic, tantalum or low ESR electrolytic). A minimum output capacitor of 2.2µF with an ESR of 0.5Ω The LT3080-1 generates a 10µA reference current that or less is recommended to prevent oscillations. Larger flows out of the SET pin. Connecting a resistor from SET values of output capacitance decrease peak deviations to ground generates a voltage that becomes the reference and provide improved transient response for larger load point for the error amplifier (see Figure 1). The reference current changes. Bypass capacitors, used to decouple voltage is a straight multiplication of the SET pin current individual components powered by the LT3080-1, increase and the value of the resistor. Any voltage can be generated the effective output capacitor value. and there is no minimum output voltage for the regulator. A minimum load current of 1mA is required to maintain For improvement in transient performance, place a capaci- regulation regardless of output voltage. For true zero voltage tor across the voltage setting resistor. Capacitors up to output operation, this 1mA load current must be returned 1µF can be used. This bypass capacitor reduces system to a negative supply voltage. noise as well, but start-up time is proportional to the time constant of the voltage setting resistor (RSET in Figure 1) With the low level current used to generate the reference and SET pin bypass capacitor. voltage, leakage paths to or from the SET pin can create Extra consideration must be given to the use of ceramic errors in the reference and output voltages. High quality capacitors. Ceramic capacitors are manufactured with a insulation should be used (e.g., Teflon, Kel-F); cleaning of all variety of dielectrics, each with different behavior across insulating surfaces to remove fluxes and other residues will 30801fc For more information www.linear.com/LT3080-1 9 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 Typical Applications Package Description Revision History Typical Application Related Parts