Datasheet

12
LT1762 Series
1762fa
APPLICATIONS INFORMATION
WUU
U
Output Capacitance and Transient Response
The LT1762 regulators are designed to be stable with a
wide range of output capacitors. The ESR of the output
capacitor affects stability, most notably with small capaci-
tors. A minimum output capacitor of 2.2µF with an ESR of
3Ω or less is recommended to prevent oscillations. The
LT1762-X is a micropower device and output transient
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 LT1762-X, will
increase the effective output capacitor value. With larger
capacitors used to bypass the reference (for low noise
operation), larger values of output capacitors are needed.
For 100pF of bypass capacitance, 3.3µF of output capaci-
tor is recommended. With a 330pF bypass capacitor or
larger, a 4.7µF output capacitor is recommended. The
shaded region of Figure 3 defines the range over which the
LT1762 regulators are stable. The minimum ESR needed
is defined by the amount of bypass capacitance used,
while the maximum ESR is 3Ω.
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 di-
electrics used are specified with EIA temperature charac-
teristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V
dielectrics are good for providing high capacitances in a
Figure 5. Ceramic Capacitor Temperature Characteristics
Figure 4. Ceramic Capacitor DC Bias Characteristics
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
310
1762 F03
245
6
78
9
STABLE REGION
C
BYP
= 330pF
C
BYP
≥ 3300pF
C
BYP
= 100pF
C
BYP
= 0
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
1762 F04
20
0
–20
–40
–60
–80
–100
0
4
8
10
26
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
small package, but they tend to have strong voltage and
temperature coefficients as shown in Figures 4 and 5.
When used with a 5V regulator, a 16V 10µF Y5V capacitor
can exhibit an effective value as low as 1µF to 2µF for the
DC bias voltage applied and over the operating tempera-
ture 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. Care still must be exercised
when using X5R and X7R capacitors; the X5R and X7R
codes only specify operating temperature range and maxi-
mum capacitance change over temperature. Capacitance
change due to DC bias with X5R and X7R capacitors is
better than Y5V and Z5U capacitors, but can still be
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
1762 F05
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF