Datasheet LT3012B (Analog Devices) - 10

LT3012B
U U W U APPLICATIO S I FOR ATIO
The LT3012B has internal thermal limiting designed to Continuous operation at large input/output voltage differ- protect the device during overload conditions. For con- entials and maximum load current is not practical due to tinuous normal conditions the maximum junction tem- thermal limitations. Transient operation at high input/ perature rating of 125°C must not be exceeded. It is output differentials is possible. The approximate thermal important to give careful consideration to all sources of time constant for a 2500sq mm 3/32" FR-4 board with thermal resistance from junction to ambient. Additional maximum topside and backside area for one ounce copper heat sources mounted nearby must also be considered. is 3 seconds. This time constant will increase as more thermal mass is added (i.e. vias, larger board, and other For surface mount devices, heat sinking is accomplished components). by using the heat spreading capabilities of the PC board and its copper traces. Copper board stiffeners and plated For an application with transient high power peaks, aver- through-holes can also be used to spread the heat gener- age power dissipation can be used for junction tempera- ated by power devices. ture calculations as long as the pulse period is significantly less than the thermal time constant of the device and The following tables list thermal resistance for several board. different board sizes and copper areas. All measurements were taken in still air on 3/32" FR-4 board with one ounce
Calculating Junction Temperature
copper. Example 1: Given an output voltage of 5V, an input voltage
Table 1. DFN Measured Thermal Resistance
range of 24V to 30V, an output current range of 0mA to
COPPER AREA THERMAL RESISTANCE
50mA, and a maximum ambient temperature of 50°C,
TOPSIDE BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
what will the maximum junction temperature be? 2500 sq mm 2500 sq mm 2500 sq mm 40°C/W 1000 sq mm 2500 sq mm 2500 sq mm 45°C/W The power dissipated by the device will be equal to: 225 sq mm 2500 sq mm 2500 sq mm 50°C/W IOUT(MAX) • (VIN(MAX) – VOUT) + (IGND • VIN(MAX)) 100 sq mm 2500 sq mm 2500 sq mm 62°C/W where:
Table 2. TSSOP Measured Thermal Resistance
IOUT(MAX) = 50mA
COPPER AREA THERMAL RESISTANCE
VIN(MAX) = 30V
TOPSIDE BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq mm 2500 sq mm 2500 sq mm 40°C/W IGND at (IOUT = 50mA, VIN = 30V) = 1mA 1000 sq mm 2500 sq mm 2500 sq mm 45°C/W So: 225 sq mm 2500 sq mm 2500 sq mm 50°C/W P = 50mA 100 sq mm 2500 sq mm 2500 sq mm 62°C/W • (30V – 5V) + (1mA • 30V) = 1.28W The thermal resistance will be in the range of 40°C/W to The thermal resistance junction-to-case (θJC), measured 62°C/W depending on the copper area. So the junction at the exposed pad on the back of the die, is 16°C/W. temperature rise above ambient will be approximately equal to: 1.31W • 50°C/W = 65.5°C 3012bf 10