LT3012B_15 datasheet(9/16 Pages) LINER

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LT3012B

3012bf

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

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.

Current Limit and Safe Operating Area Protection

Like many IC power regulators, the LT3012B has safe

operating area protection. The safe operating area protec-

tion decreases the current limit as the input voltage

increases and keeps the power transistor in a safe operat-

ing region. The protection is designed to provide some

output current at all values of input voltage up to the device

breakdown (see curve of Current Limit vs Input Voltage in

the Typical Performance Characteristics).

The LT3012B is limited for operating conditions by maxi-

mum junction temperature. While operating at maximum

input voltage, the output current range must be limited;

when operating at maximum output current, the input

voltage range must be limited. Device specifications will

not apply for all possible combinations of input voltage

and output current. Operating the LT3012B beyond the

maximum junction temperature rating may impair the life

of the device.

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.

APPLICATIO S I FOR ATIO

Figure 2. Ceramic Capacitor DC Bias Characterics

Figure 3. Ceramic Capacitor Temperature Characterics

DC BIAS VOLTAGE (V)

3012B F02

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

TEMPERATURE (

°C)

–50

–20

–40

–60

–80

–100

3012B F03

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

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