Datasheet LT3092 (Analog Devices) - 11

LT3092
applicaTions inForMaTion
Figure 3 depicts the simplest frequency compensation Technology strongly recommends testing stability in situ network as a single capacitor connected across the two for any LT3092 application across all operating conditions, terminals of the current source. In this case, either a especially ones that present complex impedance networks capacitor with a value less than 1000pF, or greater than at the input and output of the current source. 1µF (ESR < 0.5Ω), may stabilize the circuit. Some applica- If an application refers the bottom of the LT3092 current tions may use the small value capacitor to stand off DC source to GND, it may be necessary to bypass the top voltage, but allow the transfer of data down a signal line. of the current source with a capacitor to GND. In some For some applications, this capacitance range may be cases, this capacitor may already exist and no additional unacceptable or present a design constraint. One circuit capacitance is required. For example, if the LT3092 was example typifying this is an “intrinsically-safe” circuit in used as a variable current source on the output of a power which an overload or fault condition potentially allows supply, the output bypass capacitance would suffice to the capacitor’s stored energy to create a spark or arc. provide LT3092 stability. Other applications may require For applications in which a single capacitor is unacceptable, the addition of a bypass capacitor. Once again, the same Figure 3 alternately shows a series RC network connected capacitor value requirements previously mentioned apply across the two terminals of the current source. This network in that an upper bound of 1000pF exists for small values has two benefits. First, it limits the potential discharge of capacitance, and a lower bound of 1µF (ESR < 0.5Ω) current of the capacitor under a fault condition, preventing exists for large value capacitors. A series RC network may sparks or arcs. Second, it bridges the gap between the also be used as necessary, and depends on the application upper bound of 1000pF for small capacitors to the lower requirements. bound of 1µF for large capacitors such that almost any In some extreme cases, capacitors or series RC networks value capacitor can be used. This allows the user greater may be required on both the LT3092’s input and output to flexibility for frequency compensating the loop and fine stabilize the circuit. Figure 4 depicts a general application tuning the RC network for complex impedance networks. using input and output capacitor networks, rather than In many instances, a series RC network is the best solution an input-to-output capacitor. As the input of the current for stabilizing the application circuit. Typical resistor values source tends to be high impedance, placing a capacitor will range from 100Ω to about 5k, especially for capacitor on the input does not have the same effect as placing a values in between 1000pF and 1µF. Once again, Linear capacitor on the lower impedance output, and the same VIN LT3092 IN RIN LT3092 IN RCOMP C 10µA IN 10µA CCOMP OR + C + COMP – – SET OUT SET OUT R R SET ROUT SET ROUT 3092 F03 3092 F04 I R OUT OUT COUT OR COUT
Figure 3. Compensation From Input to Output of Current Source Provides Stability Figure 4. Input and/or Output Capacitors May Be Used for Compensation
3092fc For more information www.linear.com/LT3092 11 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Applications Information Package Description Related Parts