LT1962 Linear Technology, LT1962 Datasheet - Page 6

• Manufacturer Part Number: LT1962
• Description: 300mA/ Low Noise/ Micropower LDO Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1962 Series
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, 4.7 F of output capaci-
tor is recommended. With a 1000pF bypass capacitor or
larger, a 6.8 F 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 10 F Y5V capacitor can exhibit an
effective value as low as 1 F to 2 F 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,
6
Figure 3. Ceramic Capacitor DC Bias Characteristics
–100
–20
–40
–60
–80
20
0
0
2
U
4
DC BIAS VOLTAGE (V)
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10 F
6
U
8
Y5V
X5R
10
12
W
14
1962 F04
16
U
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.
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:
Figure 5. Noise Resulting from Tapping on a Ceramic Capacitor
500 V/DIV
Figure 4. Ceramic Capacitor Temperature Characteristics
V
OUT
–100
–20
–40
–60
–80
40
20
0
LT1962-5
C
C
I
–50
LOAD
OUT
BYP
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10 F
= 10 F
= 0.01 f
= 100mA
–25
0
TEMPERATURE ( C)
100ms/DIV
25
50
Y5V
75
X5R
100
1962 F04
125
1962 F05