LT3081
16
3081fc
applicaTions inForMaTion
In some extreme cases, capacitors or series RC networks
may be required on both the LT3081’s input and output to
stabilize the circuit. Figure 6 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. 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. Pay careful attention to using low
ESR input capacitors with long input lines (see the Stabil-
ity and Input Capacitance section for more information).
Using Ceramic Capacitors
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 7 and 8. 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.
Figure 8. Ceramic Capacitor DC Bias Characteristics
Figure 7. Ceramic Capacitor Temperature Characteristics
Figure 6. Input and/or Output Capacitors May
Be Used for Compensation
3081 F06
IN
SET
OUT
+
–
LT3081
50µA
I
OUT
R
SET
R
OUT
C
OUT
OR
V
IN
C
OUT
R
OUT
C
IN
R
IN
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25
75
3081 F07
–25
0
50
100
125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
DC BIAS VOLTAGE (V)
CHANGE IN V
ALUE (%)
3081 F08
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
