Datasheet LT3012 (Analog Devices) - 10

LT3012
APPLICATIONS INFORMATION
Voltage and temperature coeffi cients are not the only normal conditions the maximum junction temperature sources of problems. Some ceramic capacitors have a rating of 125°C (E-Grade) or 140°C (H-Grade)must not piezoelectric response. A piezoelectric device generates be exceeded. It is important to give careful consideration voltage across its terminals due to mechanical stress, simi- to all sources of thermal resistance from junction to ambi- lar to the way a piezoelectric accelerometer or microphone ent. Additional heat sources mounted nearby must also works. For a ceramic capacitor the stress can be induced be considered. by vibrations in the system or thermal transients. For surface mount devices, heat sinking is accomplished by using the heat spreading capabilities of the PC board
Current Limit and Safe Operating Area Protection
and its copper traces. Copper board stiffeners and plated Like many IC power regulators, the LT3012 has safe oper- through-holes can also be used to spread the heat gener- ating area protection. The safe operating area protection ated by power devices. decreases the current limit as the input voltage increases The following tables list thermal resistance for several and keeps the power transistor in a safe operating region. different board sizes and copper areas. All measurements The protection is designed to provide some output current were taken in still air on 3/32" FR-4 board with one ounce at all values of input voltage up to the device breakdown copper. (see curve of Current Limit vs Input Voltage in the Typical Performance Characteristics).
Table 1. DFN Measured Thermal Resistance COPPER AREA THERMAL RESISTANCE
The LT3012 is limited for operating conditions by maximum
TOPSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
junction temperature. While operating at maximum input 2500 sq mm 2500 sq mm 40°C/W voltage, the output current range must be limited; when 1000 sq mm 2500 sq mm 45°C/W operating at maximum output current, the input voltage 225 sq mm 2500 sq mm 50°C/W range must be limited. Device specifi cations will not apply 100 sq mm 2500 sq mm 62°C/W for all possible combinations of input voltage and output current. Operating the LT3012 beyond the maximum junc-
Table 2. TSSOP Measured Thermal Resistance
tion temperature rating may impair the life of the device.
COPPER AREA THERMAL RESISTANCE TOPSIDE BOARD AREA (JUNCTION-TO-AMBIENT) Thermal Considerations
2500 sq mm 2500 sq mm 40°C/W 1000 sq mm 2500 sq mm 45°C/W The power handling capability of the device will be limited 225 sq mm 2500 sq mm 50°C/W by the maximum rated junction temperature of (125°C for 100 sq mm 2500 sq mm 62°C/W LT3012E, or 140°C for LT3012HFE). The power dissipated by the device will be made up of two components: The thermal resistance junction-to-case (θJC), measured 1. Output current multiplied by the input/output voltage at the exposed pad on the back of the die, is 16°C/W. differential: IOUT • (VIN – VOUT) and, Continuous operation at large input/output voltage dif- 2. GND pin current multiplied by the input voltage: ferentials and maximum load current is not practical IGND • VIN. due to thermal limitations. Transient operation at high The GND pin current can be found by examining the GND input/output differentials is possible. The approximate Pin Current curves in the Typical Performance Character- thermal time constant for a 2500sq mm 3/32" FR-4 board istics. Power dissipation will be equal to the sum of the with maximum topside and backside area for one ounce two components listed above. copper is 3 seconds. This time constant will increase as more thermal mass is added (i.e., vias, larger board, and The LT3012 has internal thermal limiting designed to pro- other components). tect the device during overload conditions. For continuous 3012fd 10