Datasheet

LT3055
18
3055f
For more information www.linear.com/LT3055
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.
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 accelerometer or micro-
phone works. For a ceramic capacitor, the stress is induced
by vibrations in the system or thermal transients. The
resulting voltages produced cause appreciable amounts
of noise. A ceramic capacitor produced the trace in
APPLICATIONS INFORMATION
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3055 F07
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
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
3055 F08
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 7. Ceramic Capacitor DC Bias Characteristics
Figure 8. Ceramic Capacitor Temperature Characteristics
V
OUT
1mV/DIV
10ms/DIV
3055 F09
V
OUT
= 5V
C
OUT
= 10µF
C
REF/BYP
= 10nF
Figure 9. Noise Resulting from Tapping On a Ceramic Capacitor
Figure 9 in response to light tapping from a pencil. Similar
vibration induced behavior can masquerade as increased
output voltage noise.
Stability and Input Capacitance
Low ESR, ceramic input bypass capacitors are acceptable
for applications without long input leads. However, appli-
tions connecting a power supply to an LT3055 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
oscillations.
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
LT3055 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 approximately
465nH of self-inductance.
Two methods can reduce wire self-inductance. One method
divides the current flowing towards the LT3055 between