Datasheet

LT3090
14
3090fa
For more information www.linear.com/LT3090
applicaTions inForMaTion
decrease peak output deviations during a load transient.
The LT3090 requires a minimum 10µA load current to
maintain stability under all operating conditions.
Give extra consideration to the use of ceramic capacitors.
Ceramic capacitors are manufactured with a variety of di
-
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 capacitance in small packages, but
they 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 over
the operating temperature range.
Figure 3. Ceramic Capacitor DC Bias Characteristics
Figure 4. Ceramic Capacitor Temperature Characteristics
The X5R and X7R dielectrics result in more stable character-
istics, and are thus more suitable for use as the regulator’s
output
capacitor. The X7R dielectric has better stability
across temperature, while X5R is less expensive and is
available in higher values. Nonetheless, care must
still be
exercised when using X5R and X7R capacitors. The X5R
and X7R codes only specify operating temperature range
and the maximum capacitance change over temperature.
While capacitance change due to DC bias for X5R and
X7R is better than Y5V and Z5U dielectrics, it can still be
significant enough to drop capacitance below sufficient
levels. Capacitor DC bias characteristics tend to improve
as component case size increases, but verification of
expected capacitance at the operating voltage is highly
recommended.
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates a
voltage across its terminals due to mechanical stress upon
it, similar to how a piezoelectric microphone works. For a
ceramic capacitor the stress can be induced by mechanical
vibrations within the system or due to thermal transients.
Output Noise Analysis
The LT3090 offers many advantages with respect to noise
performance. Traditional linear regulators have several
sources of noise. The most critical noise sources for an
LDO are its voltage reference, the error amplifier, the noise
of the resistors in the divider network setting output volt
-
age and the noise gain created by this resistor divider.
Many low noise regulators pin out the voltage reference
to allow for bypassing and noise reduction of the refer
-
ence. Unlike other linear regulators, the LT3090 does not
use a traditional voltage reference, but instead it uses a
50µA current source reference. That current operates
with typical noise current levels of 31.6pA/√Hz (10nA
RMS
over a 10Hz to 100kHz bandwidth). The voltage noise
equals the noise current multiplied by the resistor value.
The resistor itself generates spot noise equal to √4KTR
(whereby K = Boltzmann’s constant, 1.38 • 10
–23
J/K and
T is the absolute temperature) which is RMS summed with
the reference current noise.
DC BIAS VOLTAGE (V)
0
CHANGE IN VALUE (%)
–20
0
20
6 10
3090 F03
–40
–60
2 4
8 12 14 16
–80
–100
BOTH CAPACITORS ARE 16V
1210 CASE SIZE, 10µF
Y5V
X5R
TEMPERATURE (°C)
–50
CHANGE IN VALUE (%)
–20
0
40
20
25 75
3090 F04
–40
–60
–25 0
50 100 125
–80
–100
BOTH CAPACITORS ARE 16V
1210 CASE SIZE, 10µF
Y5V
X5R
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