lt3060 Linear Technology Corporation, lt3060 Datasheet - Page 13

• Manufacturer Part Number: lt3060
• Description: 45v Vin, Micropower, Low Noise, 100ma Low Dropout, Linear Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
voltages set for greater than 24V, use a minimum output
capacitor of 4.7μF. The LT3060 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
LT3060, 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 tempera-
ture 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 4 and 5. 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.
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, similar to the way
a piezoelectric accelerometer or microphone 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 Figure 6 in response to light
tapping from a pencil. Similar vibration induced behavior
can masquerade as increased output voltage noise.
Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor
Figure 5. Ceramic Capacitor Temperature Characteristics
Figure 4. Ceramic Capacitor DC Bias Characteristics
500μV/DIV
–100
–100
V
–20
–40
–60
–80
–20
–40
–60
–80
OUT
40
20
20
0
0
–50
0
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
–25
2
4
DC BIAS VOLTAGE (V)
0
TEMPERATURE (°C)
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
6
25
4ms/DIV
8
Y5V
X5R
50
10
Y5V
V
C
C
I
LOAD
OUT
OUT
REF/BYP
75
12
X5R
= 0.6V
= 10μF
= 100mA
100
= 10nF
14
3060 F06
3060 F04
3060 F05
LT3060
125
16
13
3060f