TC1014/TC1015/TC1185
Equation 5-1 can be used in conjunction with
Equation 5-2 to ensure regulator thermal operation is
within limits. For example:
5.0
5.1
THERMAL CONSIDERATIONS
Thermal Shutdown
Given:
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.
VINMAX
VOUTMIN
ILOADMAX
TJMAX
=
=
=
=
=
3.0V +10%
2.7V – 2.5%
40 mA
125°C
5.2
Power Dissipation
TAMAX
55°C
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:
Find:
1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
EQUATION 5-1:
PD ≈ (VINMAX – VOUTMIN)ILOADMAX
= [(3.0 x 1.1) – (2.7 x .975)]40 x 10–3
= 26.7 mW
PD ≈ (VINMAX – VOUTMIN)ILOADMAX
Where:
PD = Worst-case actual power
dissipation
Maximum allowable power dissipation:
(TJMAX – TAMAX
PDMAX = -------------------------------------------
θJA
)
VINMAX = Maximum voltage on VIN
VOUTMIN = Minimum regulator output voltage
ILOADMAX = Maximum output (load) current
(125 – 55)
= -------------------------
220
= 318 mW
The
maximum
allowable
power
dissipation
(Equation 5-2) is a function of the maximum ambient
temperature (TAMAX), the maximum allowable die
temperature (TJMAX) and the thermal resistance from
junction-to-air (θJA). The 5-pin SOT-23 package has a
In this example, the TC1014 dissipates a maximum of
26.7 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.
θJA of approximately 220°C/Watt.
EQUATION 5-2:
5.3
Layout Considerations
(TJMAX – TAMAX
)
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.
PDMAX = -------------------------------------------
θJA
Where all terms are previously defined.
© 2007 Microchip Technology Inc.
DS21335E-page 13
MICROCHIP [ MICROCHIP ]
