LTC3250 Datasheet, PDF (9/12 Pages) Linear Technology – High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter

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OPERATIO (Refer to Simplified Block Diagram)
Thermal Management
For higher input voltages and maximum output current
there can be substantial power dissipation in the
LTC3250-1.5/LTC3250-1.2. If the junction temperature
increases above approximately 160°C the thermal shut-
down circuitry will automatically deactivate the output. To
reduce the maximum junction temperature, a good ther-
mal connection to the PC board is recommended. Con-
necting the GND pin (Pin 2) to a ground plane, and
maintaining a solid ground plane under the device can
reduce the thermal resistance of the package and PC board
considerably.
Derating Power at Higher Temperatures
To prevent an overtemperature condition in high power
applications Figure 3 should be used to determine the
maximum combination of ambient temperature and power
LTC3250-1.5/LTC3250-1.2
dissipation. The power dissipated in the LTC3250-1.5/
LTC3250-1.2 should always fall under the line shown (i.e.
within the safe region) for a given ambient temperature.
The power dissipated in the LTC3250-1.5/LTC3250-1.2 is
given by the expression:
P_D
=
 _VIN
 2
–

V_{OUT } I_OUT
This derating curve assumes a maximum thermal resis-
tance, θ_JA , of 175°C/W for the 6-pin ThinSOT-23. This
thermal resistances can be achieved from a printed circuit
board layout with a solid ground plane (2000mm²)on at
least one layer with a good thermal connection to the
ground pin of the LTC3250-1.5/LTC3250-1.2. Operation
outside of this curve will cause the junction temperature to
exceed 140°C which may trigger the thermal shutdown
circuitry and ultimately reduce the life of the device.
1.2
θ_JA = 175°C/W
T_J = 140°C
1.0
0.8
0.6
0.4
0.2
0
–50
–25 0 25 50 75 100
AMBIENT TEMPERATURE (°C)
3250 • F03
Figure 3. Maximum Power Dissipation vs Ambient Temperature
3250fa
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