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LT3070 Datasheet(PDF) 11 Page - Linear Integrated Systems |
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LT3070 Datasheet(HTML) 11 Page - Linear Integrated Systems |
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11 / 28 page  LT3089 11 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 20 1.2 24.3 1.5 30.1 1.8 35.7 2.5 49.9 3.3 66.5 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 IN SET OUT + – LT3089 50µA IOUT VSET RSET 3089 F03 + – ROUT |
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