LTC3025

8

3025fb

Output Capacitance and Transient Response

The LTC3025 is designed to be stable with a wide range of

ceramic 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 0.05Ω

or less is recommended to ensure stability. The LTC3025

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. Note that bypass capacitors used to decouple

individual components powered by the LTC3025 will

increase the effective output capacitor value. High ESR

tantalum and electrolytic capacitors may be used, but

a low ESR ceramic capacitor must be in parallel at the

output. There is no minimum ESR or maximum capacitor

size requirements.

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 Z5U, Y5V, X5R and X7R. The Z5U and

Y5V dielectrics are good for providing high capacitances

in a small package, but exhibit large voltage and tem-

perature coefficients as shown in Figures 3 and 4. When

used with a 2V regulator, a 1μF Y5V capacitor can lose as

much as 75% of its intial capacitance over the operating

temperature range. The X5R and X7R dielectrics result in

more stable characteristics and are usually 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. In all cases, the output

capacitance should never drop below 0.4μF, or instability

or degraded performance may occur.

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 the output current

multiplied by the input/output voltage differential:

Note that the BIAS current is less than 300μA even under

heavy loads, so its power consumption can be ignored

for thermal calculations.

The LTC3025 has internal thermal limiting designed to

protect the device during momentary overload conditions.

For continuous normal conditions, the maximum junction

temperature rating of 125°C must not be exceeded. It is

important to give careful consideration to all sources of

thermal resistance from junction to ambient. Additional

heat sources mounted nearby must also be considered.

For surface mount devices, heat sinking is accomplished

by using the heat-spreading capabilities of the PC board

and its copper traces. Copper board stiffeners and plated

through holes can also be used to spread the heat gener-

ated by power devices.

APPLICATIONS INFORMATION

DC BIAS VOLTAGE (V)

3025 F03

20

0

–20

–40

–60

–80

–100

0

4

8

10

26

X5R

Y5V

BOTH CAPACITORS ARE 1μF,

10V, 0603 CASE SIZE

TEMPERATURE (°C)

–50

–100

–80

–60

–40

–20

X5R

Y5V

20

–25

025

50

3025 F04

75

0

BOTH CAPACITORS ARE 1μF,

10V, 0603 CASE SIZE

Figure 3. Ceramic Capacitor DC Bias Characteristics

Figure 4. Ceramic Capacitor Temperature Characteristics