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LT3089

3089f

For more information www.linear.com/LT3089

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 3mA. For true zero voltage output operation,

return this 3mA load current to a negative output voltage.

Table 1. 1% Resistors for Common Output Voltages

VOUT (V)

RSET (kΩ)

1.2

24.3

1.5

30.1

1.8

35.7

2.5

49.9

3.3

66.5

100

Withthe50µAcurrentsourceusedtogeneratethereference

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 residues

is 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. Guarding both

sidesofthecircuitboardisrequired.Bulkleakagereduction

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 signifi-

cant offset voltage and reference drift, especially over the

possible operating temperature range. Figure 2 depicts an

example 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.

Configuring the LT3089 as a Current Source

Setting the LT3089 to operate as a 2-terminal current

sourceisasimplematter.The50µAreferencecurrentfrom

the SET pin is used with one resistor to generate a small

voltage, usually in the range of 100mV to 1V (200mV is a

level that rejects offset voltage, line regulation, and other

errors without being excessively large). This voltage is

then applied across a second resistor that connect from

OUT to the first resistor. Figure 3 shows connections and

formulas to calculate a basic current source configuration.

3089 F02

SET PIN

GND

OUT

Figure 2. Guard Ring Layout Example of DF Package

Figure 3. Using the LT3089 as a Current Source

IOUT ≥ 3mA

VSET = 50µA •RSET

IOUT =

VSET

ROUT

50µA •RSET

ROUT

SET

OUT

–

LT3089

50µA

IOUT

VSET

RSET

3089 F03

–

ROUT

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