Datasheet

LT3085
11
3085fb
APPLICATIONS INFORMATION
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3085 F02
20
0
–20
–40
–60
–80
–100
0
4
8
10
26
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
3085 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
in a small package, but they tend to have strong voltage
and temperature coeffi cients 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
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 signifi cant 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 verifi ed.
Voltage and temperature coeffi cients 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,
ceramic capacitor the stress can be induced by vibrations
in the system or thermal transients.
Paralleling Devices
LT3085’s may be paralleled with other LT308X devices 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 using a small piece
of PC trace as a ballast resistor to equalize the currents.
PC trace resistance in milliohms/inch is shown in Table
1. Only a tiny area is needed for ballasting.
Table 1. PC Board Trace Resistance
WEIGHT (oz) 10 mil WIDTH 20 mil WIDTH
1 54.3 27.1
2 27.1 13.6
Trace resistance is measured in mΩ/in