lt3082ets8 Linear Technology Corporation, lt3082ets8 Datasheet - Page 10

• Manufacturer Part Number: lt3082ets8
• Description: 200ma Single Resistor Low Dropout Linear Regulator
• Manufacturer: Linear Technology Corporation

LT3082
APPLICATIONS INFORMATION
electrics, each with different behavior across temperature
and applied voltage. The most common dielectrics used
are specifi ed 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
coeffi cients, 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 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. In a
ceramic capacitor, the stress can be induced by vibrations
in the system or thermal transients.
10
Figure 3. Ceramic Capacitor DC Bias Characteristics
–100
–20
–40
–60
–80
20
0
0
2
4
DC BIAS VOLTAGE (V)
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
6
8
Y5V
X5R
10
12
14
3082 F03
16
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-specifi c 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 modifi cations/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 fl owing 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
Figure 4. Ceramic Capacitor Temperature Characteristics
–100
–20
–40
–60
–80
40
20
0
–50
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
–25
0
TEMPERATURE (°C)
25
50
Y5V
75
X5R
100
3082 F04
125
3082f