Datasheet

LT1962 Series
13
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applicaTions inForMaTion
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 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.
The LT1962-X is a micropower device and output tran
-
sient response
will be a function of output capacitance.
Larger values of output capacitance decrease the peak
deviations and provide improved transient response for
larger load current changes. Bypass capacitors, used to
decouple individual components powered by the LT1962,
will increase 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
capacitor is recommended. With a 1000pF bypass capaci
-
tor or larger, a 6.8µF output capacitor is recommended.
The
shaded region of Figure 3 defines the range over
which the LT1962 regulators are stable. The minimum ESR
needed is defined by the amount of bypass capacitance
used, while the maximum ESR is 3Ω.
Figure 3. Stability
OUTPUT CAPACITANCE (µF)
1
ESR (Ω)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
3
10
1962 F03
2 4 5
6
7 8
9
STABLE REGION
C
BYP
= 330pF
C
BYP
≥ 1000pF
C
BYP
= 100pF
C
BYP
= 0
Figure 5. Ceramic Capacitor Temperature Characteristics
Figure 4. Ceramic Capacitor DC Bias Characteristics
Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
1962 F04
20
0
–20
–40
–60
–80
–100
0
4
8
10
2 6
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
1962 F05
–25 0
50 100
125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
100ms/DIV
1962 F06
V
OUT
500µV/DIV
LT1962-5
C
OUT
= 10µF
C
BYP
= 0.01µF
I
LOAD
= 100mA
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