LT3012 datasheet(10/16 Pages) LINER

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LT3012

3012fd

APPLICATIONS INFORMATION

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, simi-

lar 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.

Current Limit and Safe Operating Area Protection

Like many IC power regulators, the LT3012 has safe oper-

ating area protection. The safe operating area protection

decreases the current limit as the input voltage increases

and keeps the power transistor in a safe operating 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 LT3012 is limited for operating conditions by maximum

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 LT3012 beyond the maximum junc-

tion 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 of (125°C for

LT3012E, or 140°C for LT3012HFE). 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.

The LT3012 has internal thermal limiting designed to pro-

tect the device during overload conditions. For continuous

normal conditions the maximum junction temperature

rating of 125°C (E-Grade) or 140°C (H-Grade)must not

be exceeded. It is important to give careful consideration

to all sources of thermal resistance from junction to ambi-

ent. 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.

The following tables list thermal resistance for several

different board sizes and copper areas. All measurements

were taken in still air on 3/32" FR-4 board with one ounce

copper.

Table 1. DFN Measured Thermal Resistance

COPPER AREA

TOPSIDE

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

2500 sq mm

40°C/W

1000 sq mm

2500 sq mm

45°C/W

225 sq mm

2500 sq mm

50°C/W

100 sq mm

2500 sq mm

62°C/W

Table 2. TSSOP Measured Thermal Resistance

COPPER AREA

TOPSIDE

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

2500 sq mm

40°C/W

1000 sq mm

2500 sq mm

45°C/W

225 sq mm

2500 sq mm

50°C/W

100 sq mm

2500 sq mm

62°C/W

The thermal resistance junction-to-case (θJC), measured

at the exposed pad on the back of the die, is 16°C/W.

Continuous operation at large input/output voltage dif-

ferentials and maximum load current is not practical

due to thermal limitations. Transient operation at high

input/output differentials is possible. The approximate

thermal time constant for a 2500sq mm 3/32" FR-4 board

with maximum topside and backside area for one ounce

copper is 3 seconds. This time constant will increase as

more thermal mass is added (i.e., vias, larger board, and

other components).

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