LT3027 datasheet(10/16 Pages) LINER

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LT3027

3027f

APPLICATIO S I FOR ATIO

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

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.

Figure 2. Stability

OUTPUT CAPACITANCE (µF)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

310

3027 F02

6 78 9

STABLE REGION

CBYP = 330pF

CBYP = 100pF

CBYP = 0

CBYP > 3300pF

Figure 4. Ceramic Capacitor Temperature Characteristics

Figure 3. Ceramic Capacitor DC Bias Characteristics

TEMPERATURE (°C)

–50

–20

–40

–60

–80

–100

3027 F04

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

DC BIAS VOLTAGE (V)

3027 F03

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

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