LT1761ES5-2.8 LINER [Linear Technology], LT1761ES5-2.8 Datasheet - Page 14

• Manufacturer Part Number: LT1761ES5-2.8
• Description: 100mA, Low Noise, LDO Micropower Regulators in SOT-23
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LT1761 Series
dielectrics 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 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,
similar to the way a piezoelectric accelerometer or
14
Figure 4. Ceramic Capacitor Temperature Characteristics
Figure 3. Ceramic Capacitor DC Bias Characteristics
–100
–100
–20
–40
–60
–80
–20
–40
–60
–80
20
40
20
0
0
–50
0
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10 F
–25
2
U
4
DC BIAS VOLTAGE (V)
0
TEMPERATURE ( C)
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10 F
6
25
U
8
Y5V
X5R
50
10
Y5V
75
12
W
X5R
100
14
1761 F03
1761 F04
16
125
U
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, especially when a ceramic
capacitor is used for noise bypassing. A ceramic capaci-
tor 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:
1. Output current multiplied by the input/output voltage
2. GND pin current multiplied by the input voltage:
The ground pin current can be found by examining the
GND Pin Current curves in the Typical Performance Char-
acteristics section. Power dissipation will be equal to the
sum of the two components listed above.
The LT1761 series regulators have internal thermal limit-
ing designed to protect the device during overload condi-
tions. For continuous normal conditions, the maximum
junction temperature rating of 125 C must not be
exceeded. It is important to give careful consideration to
all sources of thermal resistance from junction to ambient.
Additional heat sources mounted nearby must also be
considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
Figure 5. Noise Resulting from Tapping on a Ceramic Capacitor
differential: (I
(I
GND
500 V/DIV
)(V
V
OUT
IN
).
LT1761-5
C
C
I
LOAD
OUT
BYP
OUT
= 0.01 F
= 10 F
= 100mA
)(V
IN
100ms/DIV
– V
OUT
), and
1761 F05