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LT1764 Datasheet(PDF) 11 Page - Linear Technology |
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LT1764 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 26 page  LT3088 11 3088fb For more information www.linear.com/LT3088 added in series with the SET pin of the LT3088 and uses the same existing resistor divider. This technique can be used to easily satisfy the LT3088’s 2mA minimum load current requirement. Configuring the LT3088 as a Current Source Setting the LT3088 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 connects from OUT to the first resistor. Figure 6 shows connections and formulas to calculate a basic current source configuration. Again, the lower current levels used in the LT3088 neces- sitate attention to board leakages as error sources (see the Programming Linear Regulator Output Voltage section). Selecting RSET and ROUT in Current Source Applications In Figure 6, 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? ThefirstresistortoselectisRSET.Thevalueselectedshould generate enough voltage to minimize the error caused by the offset between the SET and OUT pins. A reasonable startinglevelis~200mVofvoltageacrossRSET(RSETequal to 4.02k). 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. From this point, selecting ROUT is easy, as it is a straight- forwardcalculationfromRSET.Takenote,however,resistor errors must be accounted for as well. While larger voltage drops across RSET minimize the error due to offset, they also increase the required operating headroom. Obtainingthebesttemperaturecoefficientdoesnotrequire the use of expensive resistors with low ppm temperature coefficients.Instead,sincetheoutputcurrentoftheLT3088 is determined by the ratio of RSET to ROUT, those resis- tors should have matching temperature characteristics. Less expensive resistors made from the same material provide matching temperature coefficients. See resistor manufacturers’ data sheets for more details. Higher output currents necessitate the use of higher watt- age resistors for ROUT. There may be a difference between the resistors used for ROUT and RSET. A better method to maintain consistency in resistors is to use multiple resis- tors in parallel to create ROUT, allowing the same wattage and type of resistor as RSET. Stability and Input Capacitance The LT3088 does not require an input capacitor to main- tain stability. Input capacitors are recommended in linear regulator configurations to provide a low impedance input source to the LT3088. If using an input capacitor, low ESR, ceramic input bypass capacitors are acceptable for applications without long input leads. However, applica- tions connecting a power supply to an LT3088 circuit’s IN and GND pins with long input wires combined with low ESR, ceramic input capacitors are prone to voltage spikes, reliability concerns and application-specific board oscillations. The input wire inductance found in many Figure 6. Using the LT3088 as a Current Source IN SET OUT + – LT3088 50µA IOUT VSET RSET 3088 F06 + – ROUT IOUT ≥ 2mA VSET = 50µA •RSET IOUT = VSET ROUT = 50µA •RSET ROUT |
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