LT1763CS8 datasheet(13/16 Pages) LINER

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LT1763 Series

1763fa

APPLICATIONS INFORMATION

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 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.

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.

The resulting voltages produced can cause appreciable

amounts of noise, especially when a ceramic capacitor is

used for noise bypassing. 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.

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

differential: (IOUT)(VIN – VOUT), and

2. GND pin current multiplied by the input voltage:

(IGND)(VIN).

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.

Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor

LT1763-5

COUT = 10µF

CBYP = 0.01µf

ILOAD = 100mA

VOUT

500

µV/DIV

100ms/DIV

1763 F06

Figure 5. Ceramic Capacitor Temperature Characteristics

Figure 4. Ceramic Capacitor DC Bias Characteristics

DC BIAS VOLTAGE (V)

1763 F04

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

TEMPERATURE (

°C)

–50

–20

–40

–60

–80

–100

1763 F05

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

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