5.0 THERMAL CONSIDERATIONS
5.1 Thermal Shutdown
Integrated thermal protection circuitry shuts the
regulator off when die temperature exceeds 160°C.
The regulator remains off until the die temperature
drops to approximately 150°C.
5.2 Power Dissipation
The amount of power the regulator dissipates is
primarily a function of input and output voltage, and
output current. The following equation is used to
calculate worst-case actual power dissipation:
EQUATION 5-1:
PD ≈ (VINMAX – VOUTMIN)ILOADMAX
Where:
PD = Worst-case actual power dissipation
VINMAX = Maximum voltage on VIN
VOUTMIN = Minimum regulator output voltage
ILOADMAX = Maximum output (load) current
The maximum allowable power dissipation
(Equation 5-2) is a function of the maximum ambient
temperature (TAMAX), the maximum allowable die tem-
perature (TJMAX) and the thermal resistance from junc-
tion-to-air (θJA). The 6-Pin SOT-23 package has a θJA
of approximately 220°C/Watt.
EQUATION 5-2:
PDMAX = (TJMAX – TAMAX)
θJA
where all terms are previously defined.
TC1072/TC1073
Equation 5-1 can be used in conjunction with
Equation 5-2 to ensure regulator thermal operation is
within limits. For example:
Given:
VINMAX
= 3.0V ±5%
VOUTMIN
= 2.7V – 2.5%
ILOADMAX
= 40 mA
TJMAX
= 125°C
TAMAX
= 55°C
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
PD ≈ (VINMAX – VOUTMIN)ILOADMAX
= [(3.0 x 1.05) – (2.7 x 0.975)] x 40 x 10–3
= 20.7 mW
Maximum allowable power dissipation:
PDMAX = (TJMAX – TAMAX)
θJA
= (125 – 55)
220
= 318 mW
In this example, the TC1072 dissipates a maximum of
20.7 mW; below the allowable limit of 318 mW. In a
similar manner, Equation 5-1 and Equation 5-2 can be
used to calculate maximum current and/or input
voltage limits.
5.3 Layout Considerations
The primary path of heat conduction out of the package
is via the package leads. Therefore, layouts having a
ground plane, wide traces at the pads, and wide power
supply bus lines combine to lower θJA and therefore
increase the maximum allowable power dissipation
limit.
© 2007 Microchip Technology Inc.
DS21354D-page 13