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LT3085IMS8E-TRPBF Datasheet(PDF) 9 Page - Linear Technology |
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LT3085IMS8E-TRPBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  LT3085 9 3085fa The LT3085 can be operated in two modes. Three terminal mode has the control pin connected to the power input pin which gives a limitation of 1.35V dropout. Alternatively, the “control” pin can be tied to a higher voltage and the power IN pin to a lower voltage giving 275mV dropout on the IN pin and minimizing the power dissipation. This allows for a 500mA supply regulating from 2.5VIN to 1.8VOUT or 1.8VIN to 1.2VOUT with low dissipation. Setting Output Voltage The LT3085 generates a 10μA reference current that flows out of the SET pin. Connecting a resistor from SET to ground generates a voltage that becomes the reference point for the error amplifier (see Figure 1). The reference voltage is a straight multiplication of the SET pin current and the value of the resistor. Any voltage can be generated and there is no minimum output voltage for the regulator. Table 1 lists many common output voltages and standard 1% resistor values used to generate that output voltage. A minimum load current of 1mA is required to maintain regulation regardless of output voltage. For true zero voltage output operation, this 1mA load current must be returned to a negative supply voltage. APPLICATIONS INFORMATION With the low 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. Table 1. 1% Resistors for Common Output Voltages VOUT RSET 1V 100k 1.2V 121k 1.5V 150k 1.8V 182k 2.5V 249k 3.3V 332k 5V 499k Board leakage can be minimized by encircling the SET pin and circuitry with a guardring operated at a potential close to itself; the guardring should be tied to the OUT pin. Guarding both sides of the circuit board is required. Bulk leakage reduction depends on the guard ring width. Ten nanoamperes of leakage into or out of the SET pin and associated circuitry creates a 0.1% error in the reference voltage. Leakages of this magnitude, coupled with other sources of leakage, can cause significant offset voltage and reference drift, especially over a wide temperature range. If guardring 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. Figure 1. Basic Adjustable Regulator + – LT3085 10μA IN VCONTROL VCONTROL OUT 3085 F01 SET COU RSET VOU CSET + VIN + VOUT = RSET • 10μA |
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