LT1962 Series
APPLICATIO S I FOR ATIO
and provide improved transient response for larger load
current changes. Bypass capacitors, used to decouple
individual components powered by the LT1962, will in-
crease the effective output capacitor value. With larger
capacitors used to bypass the reference (for low noise
operation), larger values of output capacitance are needed.
For 100pF of bypass capacitance, 4.7µF of output capaci-
tor is recommended. With a 1000pF bypass capacitor or
larger, a 6.8µF output capacitor is recommended.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitance in
a small package, but exhibit strong voltage and tempera-
ture coefficients as shown in Figures 3 and 4. 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.
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,
CHANGE IN VALUE (%)
20
0
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
X5R
CHANGE IN VALUE (%)
–20
100ms/DIV
–40
–60
Y5V
–80
1962 F05
–100
0
2
4
8
6
10 12
DC BIAS VOLTAGE (V)
14
16
1962 F04
Figure 3. Ceramic Capacitor DC Bias Characteristics
6
U
40
20
0
–20
–40
–60
–80
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
50
25
75
0
TEMPERATURE (°C)
100
125
Y5V
X5R
–100
–50 –25
1962 F04
W
U U
Figure 4. Ceramic Capacitor Temperature Characteristics
similar to the way a piezoelectric accelerometer or micro-
phone 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, especially when a ceramic capacitor is
used for noise bypassing. A ceramic capacitor produced
Figure 5’s trace in response to light tapping from a pencil.
Similar vibration induced behavior can masquerade as
increased output voltage noise.
LT1962-5
C
OUT
= 10µF
C
BYP
= 0.01µf
I
LOAD
= 100mA
V
OUT
500µV/DIV
Figure 5. 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 made up of two
components:
LINER [ LINEAR TECHNOLOGY ]
