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LT1761 Series

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 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

Figure 4. Ceramic Capacitor Temperature Characteristics

Figure 3. Ceramic Capacitor DC Bias Characteristics

TEMPERATURE (

°C)

–50

–20

–40

–60

–80

–100

1761 F04

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

DC BIAS VOLTAGE (V)

1761 F03

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

APPLICATIO S I FOR ATIO

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, especially when a ceramic

capacitor is used for noise bypassing. A ceramic capaci-

tor produced Figure 5’s trace in response to light tapping

from a pencil. Similar vibration induced behavior can

masquerade as increased output voltage noise.

100ms/DIV

1761 F05

VOUT

500

µV/DIV

Figure 5. Noise Resulting from Tapping on a Ceramic Capacitor

LT1761-5

COUT = 10µF

CBYP = 0.01µF

ILOAD = 100mA

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 ground pin current can be found by examining the

GND Pin Current curves in the Typical Performance Char-

acteristics section. Power dissipation will be equal to the

sum of the two components listed above.

The LT1761 series regulators have internal thermal limit-

ing designed to protect the device during overload condi-

tions. 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

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