Datasheet

10
LT3027
3027fa
APPLICATIO S I FOR ATIO
WUUU
Output Capacitance and Transient Response
The LT3027 regulator is designed to be stable with a wide
range of output capacitors. The ESR of the output capaci-
tor affects stability, most notably with small
capacitors. A minimum output capacitor of 1µF with an
ESR of 3Ω or less is recommended to prevent oscilla-
tions. The LT3027 is a micropower device and output
transient response will be a function of output capaci-
tance. 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
LT3027, 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, 2.2µF of
output capacitor is recommended. With a 330pF bypass
capacitor or larger, a 3.3µF output capacitor is recom-
mended. The shaded region of Figure 2 defines the region
over which the LT3027 regulator is stable. The minimum
ESR needed is defined by the amount of bypass capaci-
tance 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
small package, but they tend to have strong voltage and
temperature coefficients as shown in Figures 3 and 4.
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
significant enough to drop capacitor values below appro-
priate levels. Capacitor DC bias characteristics tend to
improve as component case size increases, but expected
capacitance at operating voltage should be verified.
Figure 2. 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
3027 F02
245
6
78
9
STABLE REGION
C
BYP
= 330pF
C
BYP
= 100pF
C
BYP
= 0
C
BYP
> 3300pF
Figure 4. Ceramic Capacitor Temperature Characteristics
Figure 3. Ceramic Capacitor DC Bias Characteristics
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
3027 F04
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3027 F03
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