Datasheet

LT3080-1
10
30801fb
temperature and applied voltage. The most common
dielectrics used are specified with EIA temperature char-
acteristic 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 2 and 3.
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 tempera-
ture 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 avail-
able 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. Capaci-
tor 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,
similar to the way a piezoelectric microphone works. For a
ceramic capacitor the stress can be induced by vibrations
in the system or thermal transients.
Paralleling Devices
LT3080-1’s may be directly paralleled to obtain higher
output current. The SET pins are tied together and the
IN pins are tied together. This is the same whether it’s in
three terminal mode or has separate input supplies. The
outputs are connected in common; the internal ballast
resistor equalizes the currents.
The worst-case offset between the SET pin and the output
of only ±2 millivolts allows very small ballast resistors
to be used. As shown in Figure 4, the two devices have
internal ballast resistors, which at full output current gives
better than 90 percent equalized sharing of the current.
The internal resistance of 25 milliohms (per device) only
adds about 25 millivolts of output regulation drop at an
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
30801 F02
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 2. Ceramic Capacitor DC Bias Characteristics
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
3080 F03
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 3. Ceramic Capacitor Temperature Characteristics
Figure 4. Parallel Devices
applicaTions inFormaTion
+
–
LT3080-1
V
IN
V
CONTROL
OUT
SET
25mΩ
+
–
LT3080-1
V
IN
V
IN
4.8V TO 28V
V
OUT
3.3V
2.2A
V
CONTROL
OUT
10µF
1µF
SET
165k
30801 F04
25mΩ