LT3050EDDB LINER [Linear Technology], LT3050EDDB Datasheet - Page 15

• Manufacturer Part Number: LT3050EDDB
• Description: 100mA, Low Noise Linear Regulator With Precision Current Limit And Diagnostic Functions
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Table 1 shows 1% resistor divider values for some common
output voltages with a resistor divider current of 5μA.
Table 1. Output Voltage Resistor Divider Valves
Bypass Capacitance and Output Voltage Noise
The LT3050 regulator provides low output voltage noise
over the 10Hz to 100kHz bandwidth while operating at
full load with the addition of a bypass capacitor from the
REF/BYP pin to GND. A good quality, low leakage capacitor
is recommended. This capacitor bypasses the reference of
the regulator, providing a low frequency noise pole for the
internal reference. The noise pole provided by this bypass
capacitor decreases the output voltage noise to as low
as 30μV
when the output voltage is 0.6V. For higher output voltages
(generated by using a resistor divider), the output voltage
noise increases proportionately.
Higher values of output voltage noise are often measured
if care is not exercised with regard to circuit layout and
testing. Crosstalk from nearby traces induces unwanted
noise onto the LT3050’s output. Power supply ripple
rejection must also be considered. The LT3050 regulator
does not have unlimited power supply rejection and passes
a small portion of the input noise through to the output.
During start-up, the internal reference will soft-start the
reference if a bypass capacitor is present. Regulator start-
up time is directly proportional to the size of the bypass
capacitor, slowing to 5.5ms with a 10nF bypass capacitor
and 2.2μF output capacitor.
V
OUT
RMS
1.2
1.5
1.8
2.5
3.3
3
5
(V)
with the addition of a 10nF bypass capacitor
R1 (kΩ)
118
121
124
115
124
124
115
R2 (kΩ)
118
182
249
365
499
562
845
Output Capacitance and Transient Response
The LT3050 regulator is stable with a wide range of output
capacitors. The ESR of the output capacitor affects stability,
most notably with small capacitors. Use a minimum output
capacitor of 2.2μF to prevent oscillations. The LT3050 is a
micropower device and output load transient response is
a function of output capacitance. Larger values of output
capacitance decrease the peak deviations and provide
improved transient response for larger load current
changes. Bypass capacitors, used to decouple individual
components powered by the LT3050, increase the effective
output capacitor value. For applications with large load
current transients, a low ESR ceramic capacitor in parallel
with a bulk tantalum capacitor often provides an optimally
damped response.
Give extra consideration to the use of ceramic capacitors.
Manufacturers make ceramic capacitors with a variety of
dielectrics, each with different behavior across temperature
and applied voltage. The most common dielectrics are
specifi ed with EIA temperature characteristic codes of
Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics
provide high C-V products in a small package at low cost,
but exhibit 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 yield much more stable characteristics and are
more suitable for use as the output capacitor.
The X7R type works over a wider temperature range and
has better temperature stability, 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.
LT3050
15
3050f