lt3092 Linear Technology Corporation, lt3092 Datasheet - Page 6

• Manufacturer Part Number: lt3092
• Description: 200ma Two-terminal Programmable Current Source
• Manufacturer: Linear Technology Corporation

LT3092
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 data
sheet’s 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). If a clean, tight PCB layout exists as previ-
ously described, no additional capacitors are necessary.
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 fl oating in a signal
APPLICATIONS INFORMATION
6
Figure 2. Adding Series Resistor Decouples
and Dampens Long Line Reactances
LT3092
SET
R
SET
10μA
+
–
IN
R
R
LONG LINE
REACTANCE/INDUCTANCE
LONG LINE
REACTANCE/INDUCTANCE
SERIES
SERIES
OUT
R
OUT
3092 F02
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 suffi ciently 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 suffi cient, a frequency compensation network may be
necessary. Several options may be considered.
Figure 3 depicts the simplest frequency compensation
network as a single capacitor connected across the two
terminals of the current source. In this case, either a
capacitor with a value less than 1000pF , or greater than
1uF (ESR < 0.5Ω), may stabilize the circuit. Some ap-
plications may use the small value capacitor to stand
off DC voltage, but allow the transfer of data down a
signal line.
LT3092
Figure 3. Compensation Across From Input to
Output of Current Source Provides Stability
SET
R
SET
10μA
+
–
IN
OUT
R
OUT
3092 F03
C
COMP
OR
R
COMP
C
COMP
3092p