Datasheet

LT1762 Series
13
Rev B
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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
dielectrics used are specified with EIA temperature char-
acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but they tend to have strong voltage
and temperature coefficients as shown in Figure4 and
Figure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
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. 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
APPLICATIONS INFORMATION
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
3 10
1762 F03
2 4 5
6
7 8
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
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
1762 F05
–25 0
50 100
125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
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