8

LTC1844 Series

1844ia

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 strong voltage and tem-

perature coefficients as shown in Figures 4 and 5. When

used with a 5V regulator, a 10

µF Y5V capacitor can exhibit

an effective value as low as 1

µF 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.

Additionally, 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 microphone works. For a

ceramic capacitor the stress can be induced by vibrations

in the system or thermal transients. The resulting voltages

produced can cause appreciable amounts of noise, espe-

cially when a ceramic capacitor is used for noise bypass-

ing. A ceramic capacitor produced Figure 6’s trace in

response to light tapping from a pencil. Similar vibration-

induced behavior can masquerade as increased output

voltage noise.

Figure 4. Ceramic Capacitor DC Bias Characteristics

Figure 5. Ceramic Capacitor Temperature Characteristics

500

µV/DIV

100ms/DIV

1844 F06

Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor

LTC1844-2.8

DC BIAS VOLTAGE (V)

1844 F04

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

TEMPERATURE (

°C)

–50

40

20

0

–20

–40

–60

–80

–100

25

75

1844 F05

–25

0

50

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

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:

The LTC1844 series regulators have internal thermal lim-

iting 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 consider-

ation to all sources of thermal resistance from junction to

ambient. Additional heat sources mounted nearby must

also be considered.