Datasheet LT3092 (Analog Devices) - 10

LT3092
applicaTions inForMaTion
From this point, selecting R inductive components and may be complex distributed OUT is easy, as it is a straight- forward calculation from R networks. In addition, the current source’s value will dif- SET. Take note, however, resistor errors must be accounted for as well. While larger voltage fer between applications and its connection may be GND drops across R referenced, power supply referenced or floating in a signal SET minimize the error due to offset, they also increase the required operating headroom. line path. Linear Technology strongly recommends that stability be tested in situ for any LT3092 application. Obtaining the best temperature coefficient does not require the use of expensive resistors with low ppm temperature In LT3092 applications with long wires or PCB traces, the coefficients. Instead, since the output current of the LT3092 inductive reactance may cause instability. In some cases, is determined by the ratio of R adding series resistance to the input and output lines (as SET to ROUT, those resistors should have matching temperature characteristics. Less shown in Figure 2) may sufficiently dampen these possible expensive resistors made from the same material will high-Q lines and provide stability. The user must evaluate provide matching temperature coefficients. See resistor the required resistor values against the design’s headroom manufacturers’ data sheets for more details. constraints. In general, operation at low output current levels (< 5mA) automatically requires higher values of
Stability and Frequency Compensation
programming resistors and may provide the necessary damping without additional series impedance. The LT3092 does not require input or output capacitors for stability in many current-source applications. Clean, If the line impedances in series with the LT3092 are tight PCB layouts provide a low reactance, well controlled complex enough such that series damping resistors are operating environment for the LT3092 without requiring not sufficient, a frequency compensation network may be capacitors to frequency-compensate the circuit. The front necessary. Several options may be considered. page Typical Application circuit illustrates the simplicity of using the LT3092. Some current source applications will use a capacitor LONG LINE REACTANCE/INDUCTANCE connected in parallel with the SET pin resistor to lower the current source’s noise. This capacitor also provides a RSERIES soft-start function for the current source. This capacitor LT3092 IN connection is depicted in Figure 7 (see the Quieting the Noise section). 10µA When operating with a capacitor across the SET pin resis- + tor, external compensation is usually required to maintain – stability and compensate for the introduced pole. The SET OUT following paragraphs discuss methods for stabilizing RSET ROUT the LT3092 for either this capacitance or other complex impedances that may be presented to the device. Linear R 3092 F02 SERIES Technology strongly recommends testing stability in situ with final components before beginning production. LONG LINE REACTANCE/INDUCTANCE Although the LT3092’s design strives to be stable without any capacitors over a wide variety of operating conditions, it is not possible to test for all possible combinations of input
Figure 2. Adding Series Resistor Decouples and Dampens Long Line Reactances
and output impedances that the LT3092 will encounter. These impedances may include resistive, capacitive and 3092fc 10 For more information www.linear.com/LT3092 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Applications Information Package Description Related Parts