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LT3082 Datasheet PDF : 20 Pages
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LT3082
APPLICATIONS INFORMATION
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 3 and 4. 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 with 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.
Stability and Input Capacitance
Low ESR, ceramic input bypass capacitors are acceptable
for applications without long input leads. However, applica-
tions connecting a power supply to an LT3082 circuit’s IN
and GND pins with long input wires combined with a low
ESR, ceramic input capacitors are prone to voltage spikes,
reliability concerns and application-specific board oscil-
lations. The input wire inductance found in many battery
powered applications, combined with the low ESR ceramic
input capacitor, forms a high-Q LC resonant tank circuit. In
some instances this resonant frequency beats against the
output current dependent LDO bandwidth and interferes
with proper operation. Simple circuit modifications/solu-
tions are then required. This behavior is not indicative of
LT3082 instability, but is a common ceramic input bypass
capacitor application issue.
The self-inductance, or isolated inductance, of a wire is
directly proportional to its length. Wire diameter is not a
major factor on its self-inductance. For example, the self-
inductance of a 2-AWG isolated wire (diameter = 0.26") is
about half the self-inductance of a 30-AWG wire (diameter
= 0.01"). One foot of 30-AWG wire has about 465nH of
self-inductance.
One of two ways reduces a wire’s self-inductance. One
method divides the current flowing towards the LT3082
between two parallel conductors. In this case, the farther
apart the wires are from each other, the more the self-in-
ductance is reduced; up to a 50% reduction when placed
a few inches apart. Splitting the wires basically connects
20
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
0
X5R
–20
–40
–60
Y5V
–80
–100
0 2 4 6 8 10 12 14 16
DC BIAS VOLTAGE (V)
3082 F03
Figure 3. Ceramic Capacitor DC Bias Characteristics
10
40
20
0
X5R
–20
–40
Y5V
–60
–80 BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
–100
–50 –25 0 25 50 75
TEMPERATURE (°C)
100 125
3082 F04
Figure 4. Ceramic Capacitor Temperature Characteristics
3082f
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