LT1761 Series
W U U
U
APPLICATIO S I FOR ATIO
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 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.
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.
LT1761-5
C
OUT = 10µF
CBYP = 0.01µF
ILOAD = 100mA
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
VOUT
500µV/DIV
100ms/DIV
1761 F05
20
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 5. Noise Resulting from Tapping on a Ceramic Capacitor
0
X5R
Thermal Considerations
–20
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:
–40
–60
Y5V
–80
1. Output current multiplied by the input/output voltage
differential: (IOUT)(VIN – VOUT), and
–100
0
8
12 14
2
4
6
10
16
DC BIAS VOLTAGE (V)
2. GND pin current multiplied by the input voltage:
(IGND)(VIN).
1761 F03
Figure 3. Ceramic Capacitor DC Bias Characteristics
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.
40
20
X5R
0
–20
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
allsourcesofthermalresistancefromjunctiontoambient.
Additional heat sources mounted nearby must also be
considered.
–40
Y5V
–60
–80
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
–100
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
1761 F04
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
Figure 4. Ceramic Capacitor Temperature Characteristics
14
Linear [ Linear ]
