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LT3092MPST Datasheet(PDF) 8 Page - Linear Technology |
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LT3092MPST Datasheet(HTML) 8 Page - Linear Technology |
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8 / 22 page  LT3092 8 3092fb Introduction The LT3092 is a versatile IC that operates as a 2-terminal programmable current source with the addition of only two external resistors; no external bypass capacitors are needed for stability. The LT3092 is easy to use and has all the protection fea- tures expected in high performance products. Included are reverse-voltage protection, reverse-current protec- tion, short-circuit protection and thermal shutdown with hysteresis. The LT3092 operates with or without input and output capacitors. The simplest current source application requires only two discrete resistors to set a constant output current up to 200mA. A variety of analog tech- niques lend themselves to regulating and varying the current source value. The device utilizes a precision “0” TC 10μA reference cur- rent source to program output current. This 10μA current source connects to the noninverting input of a power operational amplifier. The power operational amplifier provides a low impedance buffered output of the voltage on the noninverting input. Many application areas exist in which operation without input and output capacitors is advantageous. A few of these applications include sensitive circuits that cannot endure surge currents under fault or overload conditions and intrinsic safety applications in which safety regulations limit energy storage devices that may spark or arc. Programming Output Current in 2-Terminal Current Source Mode Setting the LT3092 to operate as a 2-terminal current source is a simple matter. The 10μA reference current from 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 will help reject offset voltage, line regulation, and other errors without being excessively large). This voltage is then applied across a second resistor that connects from OUT to the first resistor. Figure 1 shows connections and formulas to calculate a basic current source configuration. APPLICATIONS INFORMATION With a 10μA current source generating the reference that gains up to set output current, leakage paths to or from the SET pin can create errors in the reference and output currents. High quality insulation should be used (e.g., Teflon, Kel-F). The cleaning of all insulating surfaces to remove fluxes and other residues may 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. Guarding both sides of the circuit board is required. Bulk leakage reduction depends on the guard ring width. Ten nano- amperes of leakage into or out of the SET pin and its as- sociated circuitry creates a 0.1% reference current error. Leakages of this magnitude, coupled with other sources of leakage, can cause significant offset voltage and refer- ence current drift, especially over the possible operating temperature range. Figure 1. Using the LT3092 as a Current Source ImA VµA R I V R µA OUT SET SET OUT SET OUT ≥ = == 05 10 10 . • • R R R SET OUT IN SET OUT + – LT3092 10μA IOUT VSET RSET 3092 F01 + – ROUT Selecting RSET and ROUT In Figure 1, both resistors RSET and ROUT program the value of the output current. The question now arises: the ratio of these resistors is known, but what value should each resistor be? The first resistor to select is RSET.Thevalueselectedshould generate enough voltage to minimize the error caused by the offset between the SET and OUT pins. A reasonable starting level is 200mV of voltage across RSET (RSET equal to 20k). Resultant errors due to offset voltage are a few percent. The lower the voltage across RSET becomes, the higher the error term due to the offset. |
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