LTC1844 Datasheet, PDF (8/12 Pages) Linear Technology – 150mA, Micropower, Low Noise, VLDO Linear Regulator

LTC1844 Series
APPLICATIONS INFORMATION
temperature and applied voltage. The most common di-
electrics used are Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but exhibit strong voltage and tem-
perature coefficients as shown in Figures 4 and 5. When
used with a 5V regulator, a 10µF Y5V capacitor can exhibit
an effective value as low as 1µF to 2µF 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.
Additionally, 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 can be induced by vibrations
in the system or thermal transients. The resulting voltages
produced can cause appreciable amounts of noise, espe-
cially when a ceramic capacitor is used for noise bypass-
ing. A ceramic capacitor produced Figure 6’s trace in
response to light tapping from a pencil. Similar vibration-
induced behavior can masquerade as increased output
voltage noise.
20
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
0
X5R
–20
–40
–60
Y5V
–80
–100
0 2 4 6 8 10 12 14 16
DC BIAS VOLTAGE (V)
1844 F04
Figure 4. Ceramic Capacitor DC Bias Characteristics
40
20
0
X5R
–20
–40
Y5V
–60
–80 _{BOTH CAPACITORS ARE 16V,}
1210 CASE SIZE, 10µF
–100
–50 –25 0 25 50 75
TEMPERATURE (°C)
100 125
1844 F05
Figure 5. Ceramic Capacitor Temperature Characteristics
LTC1844-2.8
C_OUT = 10µF
C_BYP = 0.01µF
I_LOAD = 100mA
V_OUT
500µV/DIV
100ms/DIV
1844 F06
Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). The
power dissipated by the device will be the output current
multiplied by the input/output voltage differential:
(I_OUT)(V_IN – V_OUT).
The LTC1844 series regulators have internal thermal lim-
iting designed to protect the device during momentary
overload conditions. For continuous normal conditions,
the maximum junction temperature rating of 125°C must
not be exceeded. It is important to give careful consider-
ation to all sources of thermal resistance from junction to
ambient. Additional heat sources mounted nearby must
also be considered.
1844ia
8