LT3083MPDF datasheet(11/28 Pages) AD

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LT3083

Rev. B

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APPLICATIONS INFORMATION

Table 1 lists many common output voltages and the clos-

est standard 1% resistor values used to generate that

output voltage.

Regulation of the output voltage requires a minimum

load current of 1mA. For a true zero voltage output

operation, return this 1mA load current to a negative

supply voltage.

Table 1. 1% Resistors for Common Output Voltages

VOUT (V)

RSET (k)

1.2

24.3

1.5

30.1

1.8

35.7

2.5

49.9

3.3

66.5

100

With the lower level current used to generate the reference

voltage, leakage paths to or from the SET pin can create

errors in the reference and output voltages. High quality

insulation should be used (e.g., Teflon, Kel-F); cleaning

of all insulating surfaces to remove fluxes and other resi-

dues will probably be required. Surface coating may be

necessary to provide a moisture barrier in high humidity

environments.

Minimize board leakage by encircling the SET pin and

circuitry with a guard ring operated at a potential close

to itself. Tie the guard ring to the OUT pin. Guard rings

on both sides of the circuit board are required. Bulk leak-

age reduction depends on the guard ring width. 50nA

of leakage into or out of the SET pin and its associated

circuitry creates a 0.1% reference voltage error. Leakages

of this magnitude, coupled with other sources of leakage,

can cause significant offset voltage and reference drift,

especially over the possible operating temperature range.

Figure 2 depicts an example of a guard ring layout.

If guard ring techniques are used, this bootstraps any

stray capacitance at the SET pin. Since the SET pin is

a 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 capacitors at full load current. The easiest way

to remedy this is to bypass the SET pin with a small

amount of capacitance from SET to ground, 10pF to

20pF is sufficient.

Stability and Input Capacitance

Typical minimum input capacitance is 10µF for IN and

2.2µF for VCONTROL. These amounts of capacitance work

well using low ESR ceramic capacitors when placed close

to the LT3083 and the circuit is located in close proximity

to the power source. Higher values of input capacitance

may be necessary to maintain stability depending on the

application.

Oscillating regulator circuits are often viewed as a problem

of phase margin and inadequate stability with the output

capacitor used. More and more frequently, the problem

is not the regulator operating without sufficient output

capacitance, but instead with too little input capacitance.

The entire circuit must be analyzed and debugged as a

whole; conditions relating to the input of the regulator

cannot be ignored.

The LT3083 input presents a high impedance to its power

source: the output voltage and load current are indepen-

dent of input voltage variations. To maintain stability of the

regulator circuit as a whole, the LT3083 must be powered

from a low impedance supply. When using short supply

lines or powering directly from a large switching supply,

there is no issue—hundreds or thousands of microfarads

of capacitance are available through a low impedance.

Figure 2. Guard Ring Layout Example for DF Package

3083 F02

SET PIN

GND

OUT

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