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LT3092EST#PBF

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LT3092
10
3092fc
For more information www.linear.com/LT3092
applicaTions inForMaTion
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 T
echnology 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.
From this point, selecting R
OUT
is easy, as it is a straight-
forward calculation from R
SET
. Take note, however, resistor
errors must be accounted for as well. While larger voltage
drops across R
SET
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 R
SET
to R
OUT
, 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
Figure 2. Adding Series Resistor Decouples
and Dampens Long Line Reactances
IN
SET OUT
+
–
LT3092
10µA
R
SET
R
OUT
R
SERIES
R
SERIES
LONG LINE
REACTANCE/INDUCTANCE
3092 F02
LONG LINE
REACTANCE/INDUCTANCE
LT3092
11
3092fc
For more information www.linear.com/LT3092
applicaTions inForMaTion
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
1µF (ESR < 0.5Ω), may stabilize the circuit. Some applica
-
tions may use the small value capacitor to stand off DC
voltage, but allow the transfer of data down a signal line.
For some applications, this capacitance range may be
unacceptable or present a design constraint. One cir
cuit
example typifying this is an
“intrinsically-safe” circuit in
which an overload or fault condition potentially allows
the capacitor’s stored energy to create a spark or arc.
For applications in which a single capacitor is unacceptable,
Figure 3 alternately shows a series RC network connected
across the two terminals of the current source. This network
has two benefits. First, it limits the potential discharge
current of the capacitor under a fault condition, preventing
sparks or arcs. Second, it bridges the gap between the
upper bound of 1000pF for small capacitors to the lower
bound of 1µF for large capacitors such that almost any
value capacitor can be used. This allows the user greater
flexibility for frequency compensating the loop and fine
tuning the RC network for complex impedance networks.
In many instances, a series RC network is the best solution
for stabilizing the application circuit. Typical resistor values
will range from 100Ω to about 5k, especially for capacitor
values in between 1000pF and 1µF. Once again, Linear
Figure 4. Input and/or Output Capacitors May
Be Used for Compensation
Technology strongly recommends testing stability in situ
for any LT3092 application across all operating conditions,
especially ones that present complex impedance networks
at the input and output of the current source.
If an application refers the bottom of the LT3092 current
source to GND, it may be necessary to bypass the top
of the current source with a capacitor to GND. In some
cases, this capacitor may already exist and no additional
capacitance is required. For example, if the LT3092 was
used as a variable current source on the output of a power
supply, the output bypass capacitance would suffice to
provide LT3092 stability. Other applications may require
the addition of a bypass capacitor. Once again, the same
capacitor value requirements previously mentioned apply
in that an upper bound of 1000pF exists for small values
of capacitance, and a lower bound of 1µF (ESR < 0.5Ω)
exists for large value capacitors. A series RC network may
also be used as necessary, and depends on the application
requirements.
In some extreme cases, capacitors or series RC networks
may be required on both the LT3092’s input and output to
stabilize the circuit. Figure 4 depicts a general application
using input and output capacitor networks, rather than
an input-to-output capacitor. As the input of the current
source tends to be high impedance, placing a capacitor
on the input does not have the same effect as placing a
capacitor on the lower impedance output, and the same
3092 F04
IN
SET OUT
+
–
LT3092
10µA
I
OUT
R
SET
R
OUT
C
OUT
OR
V
IN
C
OUT
R
OUT
C
IN
R
IN
Figure 3. Compensation From Input to
Output of Current Source Provides Stability
3092 F03
IN
SET OUT
+
–
LT3092
10µA
C
COMP
OR
R
SET
R
OUT
R
COMP
C
COMP
LT3092
12
3092fc
For more information www.linear.com/LT3092
applicaTions inForMaTion
restrictions do not apply. Capacitors in the range of 0.1µF
to 1µF usually provide sufficient bypassing on the input,
and the value of input capacitance may be increased
without limit.
If an application uses GND referred capacitors on the input
or output (particularly the input), pay attention to the length
of the lines powering and returning ground from the circuit.
In the case where long power supply and return lines are
coupled with low ESR input capacitors, application-specific
voltage spikes, oscillations and reliability concerns may
be seen. This is not an issue with LT3092 stability, but
rather the low ESR capacitor forming a high-Q resonant
tank circuit with the inductance of the input wires. Adding
series resistance with the input of the LT3092, or with the
input capacitor, often solves this. Resistor values of 0.1Ω
to 1Ω are often sufficient to dampen this resonance.
Give extra consideration to the use of ceramic capacitors.
Ceramic capacitors are manufactured with a variety of di
-
electrics, each with different behavior across temperature
and applied voltage. The most common dielectrics used
are specified with EIA temperature characteristic codes of
Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are
good for providing high capacitances in a small package,
but they tend to have strong voltage and temperature
coefficients as shown in Figures 5 and 6. When used with
a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an
effective value as low as 1μF to 2μF for the DC bias voltage
applied and over the operating temperature range. The X5R
and X7R dielectrics result in more stable characteristics
and are more suitable for use as the output capacitor.
The X7R type has better stability across temperature,
while the X5R is less expensive and is available in higher
values. Care still must be exercised when using X5R and
X7R capacitors; the X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to DC
bias with X5R and X7R capacitors is better than Y5V and
Z5U capacitors, but can still be significant enough to drop
capacitor values below appropriate levels. Capacitor DC
bias characteristics tend to improve as component case
size increases, but expected capacitance at operating
voltage should be verified.
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress. In a
ceramic capacitor the stress can be induced by vibrations
in the system or thermal transients.
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3092 F05
20
0
–20
–40
–60
–80
–100
0
4
8
10
2 6
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 5. Ceramic Capacitor DC Bias Characteristics
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
3092 F06
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 6. Ceramic Capacitor Temperature Characteristics
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LT3092EST#PBF

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Buy LT3092EST#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Current & Power Monitors & Regulators 200mA Two Terminal Programmable Current Source
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