Datasheet LT3092 (Analog Devices) - 9

LT3092
applicaTions inForMaTion Introduction
With a 10μA current source generating the reference that The LT3092 is a versatile IC that operates as a 2-terminal gains up to set output current, leakage paths to or from programmable current source with the addition of only the SET pin can create errors in the reference and output two external resistors; no external bypass capacitors are currents. High quality insulation should be used (e.g., needed for stability. Teflon, Kel-F). The cleaning of all insulating surfaces to remove fluxes and other residues may be required. Surface The LT3092 is easy to use and has all the protection fea- coating may be necessary to provide a moisture barrier tures expected in high performance products. Included in high humidity environments. are reverse-voltage protection, reverse-current protec- Minimize board leakage by encircling the SET pin and tion, short-circuit protection and thermal shutdown with circuitry with a guard ring operated at a potential close hysteresis. to itself; tie the guard ring to the OUT pin. Guarding The LT3092 operates with or without input and output both sides of the circuit board is required. Bulk leakage capacitors. The simplest current source application reduction depends on the guard ring width. Ten nano- requires only two discrete resistors to set a constant amperes of leakage into or out of the SET pin and its as- output current up to 200mA. A variety of analog tech- sociated circuitry creates a 0.1% reference current error. niques lend themselves to regulating and varying the Leakages of this magnitude, coupled with other sources current source value. of leakage, can cause significant offset voltage and refer- The device utilizes a precision “0” TC 10μA reference cur- ence current drift, especially over the possible operating rent source to program output current. This 10µA current temperature range. source connects to the noninverting input of a power operational amplifier. The power operational amplifier LT3092 IN provides a low impedance buffered output of the voltage 10µA on the noninverting input. IOUT ≥ 0.5mA + VSET = 10µA •RSET Many application areas exist in which operation without – I VSET 10µA •RSET OUT = = input and output capacitors is advantageous. A few of ROUT ROUT SET OUT these applications include sensitive circuits that cannot + V endure surge currents under fault or overload conditions SET RSET ROUT – and intrinsic safety applications in which safety regula- IOUT 3092 F01 tions limit energy storage devices that may spark or arc.
Figure 1. Using the LT3092 as a Current Source Programming Output Current in 2-Terminal Current Source Mode Selecting RSET and ROUT
Setting the LT3092 to operate as a 2-terminal current In Figure 1, both resistors RSET and ROUT program the source is a simple matter. The 10µA reference current from value of the output current. The question now arises: the the SET pin is used with one resistor to generate a small ratio of these resistors is known, but what value should voltage, usually in the range of 100mV to 1V (200mV is a each resistor be? level that will help reject offset voltage, line regulation, and The first resistor to select is R other errors without being excessively large). This voltage SET. The value selected should generate enough voltage to minimize the error caused by is then applied across a second resistor that connects from the offset between the SET and OUT pins. A reasonable OUT to the first resistor. Figure 1 shows connections and starting level is 200mV of voltage across R formulas to calculate a basic current source configuration. SET (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. 3092fc For more information www.linear.com/LT3092 9 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Applications Information Package Description Related Parts