Datasheet
© 2007 Microchip Technology Inc. DS21335E-page 13
TC1014/TC1015/TC1185
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:
The maximum allowable power dissipation
(Equation 5-2) is a function of the maximum ambient
temperature (T
A
MAX
), the maximum allowable die
temperature (T
JMAX
) and the thermal resistance from
junction-to-air (θ
JA
). The 5-pin SOT-23 package has a
θ
JA
of approximately 220°C/Watt.
EQUATION 5-2:
Equation 5-1 can be used in conjunction with
Equation 5-2 to ensure regulator thermal operation is
within limits. For example:
Given:
Find:
1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
P
D
≈ (V
INMAX
– V
OUTMIN
)I
LOADMAX
= [(3.0 x 1.1) – (2.7 x .975)]40 x 10
–3
= 26.7 mW
Maximum allowable power dissipation:
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.
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 θ
JA
and therefore
increase the maximum allowable power dissipation
limit.
P
D
V
INMAX
V
OUTMIN
–()I
LOADMAX
≈
Where:
P
D
= Worst-case actual power
dissipation
V
INMAX
= Maximum voltage on V
IN
V
OUTMIN
= Minimum regulator output voltage
I
LOADMAX
= Maximum output (load) current
Where all terms are previously defined.
P
DMAX
T
JMAX
T
AMAX
–()
θ
JA
--------------------------------------------=
V
INMAX
=3.0V +10%
V
OUTMIN
= 2.7V – 2.5%
I
LOADMAX
=40mA
T
JMAX
=125°C
T
AMAX
=55°C
P
DMAX
T
JMAX
T
AMAX
–()
θ
JA
--------------------------------------------=
125 55–()
220
-------------------------=
318 mW=