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LT3092

200mA 2-Terminal Programmable Current Source

Linear Technology 

LT3092

APPLICATIONS INFORMATION

From this point, selecting ROUT is easy, as it is a straight-

forward calculation from RSET. Take note, 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.

Obtaining the best temperature coefficient does not require

the use of expensive resistors with low ppm temperature

coefficients. Instead, since the output current of the LT3092

is determined by the ratio of RSET to ROUT, those resistors

should have matching temperature characteristics. Less

expensive resistors made from the same material will

provide matching temperature coefficients. See resistor

manufacturers’ data sheets for more details.

Stability and Frequency Compensation

The LT3092 does not require input or output capacitors

for stability in many current-source applications. Clean,

tight PCB layouts provide a low reactance, well controlled

operating environment for the LT3092 without requiring

capacitors to frequency-compensate the circuit. The front

page Typical Application circuit illustrates the simplicity

of using the LT3092.

Some current source applications will use a capacitor

connected in parallel with the SET pin resistor to lower

the current source’s noise. This capacitor also provides a

soft-start function for the current source. This capacitor

connection is depicted in Figure 7 (see the Quieting the

Noise section).

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

following paragraphs discuss methods for stabilizing

the LT3092 for either this capacitance or other complex

impedances that may be presented to the device. Linear

Technology strongly recommends testing stability in situ

with final components before beginning production.

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 and output impedances that the LT3092 will encounter.

These impedances may include resistive, capacitive and

inductive components and may be complex distributed

networks. In addition, the current source’s value will dif-

fer between applications and its connection may be GND

referenced, power supply referenced or floating in a signal

line path. Linear Technology strongly recommends that

stability be tested in situ for any LT3092 application.

In LT3092 applications with long wires or PCB traces, the

inductive reactance may cause instability. In some cases,

adding series resistance to the input and output lines (as

shown in Figure 2) may sufficiently dampen these possible

high-Q lines and provide stability. The user must evaluate

the required resistor values against the design’s headroom

constraints. In general, operation at low output current

levels (< 5mA) automatically requires higher values of

programming resistors and may provide the necessary

damping without additional series impedance.

If the line impedances in series with the LT3092 are

complex enough such that series damping resistors are

not sufficient, a frequency compensation network may be

necessary. Several options may be considered.

LONG LINE

REACTANCE/INDUCTANCE

LT3092

RSERIES

IN

10μA

+

–

SET

RSET

OUT

ROUT

RSERIES

3092 F02

LONG LINE

REACTANCE/INDUCTANCE

Figure 2. Adding Series Resistor Decouples

and Dampens Long Line Reactances

3092fb

9

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