7

LT3014B

3014bf

Output Capacitance and Transient Response

The LT3014B is designed to be stable with a wide range of

output capacitors. The ESR of the output capacitor affects

stability, most notably with small capacitors. A minimum

output capacitor of 0.47

µF with an ESR of 3Ω or less is

recommended to prevent oscillations. The LT3014B 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 LT3014B, will increase the

effective output capacitor value.

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 Figures 2 and 3.

When used with a 5V regulator, a 16V 10

µF Y5V capacitor

can exhibit an effective value as low as 1

µF 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

APPLICATIO S I FOR ATIO

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.

Voltage and temperature coefficients are not the only

sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates

voltage across its terminals due to mechanical stress,

similar to the way a piezoelectric accelerometer or micro-

phone works. For a ceramic capacitor the stress can be

induced by vibrations in the system or thermal transients.

Thermal Considerations

The power handling capability of the device will be limited

by the maximum rated junction temperature (125

°C). The

power dissipated by the device will be made up of two

components:

1. Output current multiplied by the input/output voltage

2. GND pin current multiplied by the input voltage:

The GND pin current can be found by examining the GND

Pin Current curves in the Typical Performance Character-

istics. Power dissipation will be equal to the sum of the two

components listed above.

DC BIAS VOLTAGE (V)

3014 F02

20

0

–20

–40

–60

–80

–100

0

4

8

10

26

12

14

X5R

Y5V

16

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

Figure 2. Ceramic Capacitor DC Bias Characteristics

TEMPERATURE (

°C)

–50

40

20

0

–20

–40

–60

–80

–100

25

75

3014 F03

–25

0

50

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

Figure 3. Ceramic Capacitor Temperature Characteristics