TC1054/TC1055/TC1186
DS21350E-page 10 2002-2012 Microchip Technology Inc.
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 voltage, 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
AMAX
), the maximum allowable die
temperature (T
JMAX
) and the thermal resistance from
junction-to-air (
JA
). The 5-Pin SOT-23 package has a
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:
Actual power dissipation:
Maximum allowable power dissipation:
In this example, the TC1054 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. Layouts having a ground
plane, wide traces at the pads and wide power supply
bus lines, combine to lower θJA and increase the max-
imum 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
–
JA
--------------------------------------------=
Given:
V
INMAX
= 3.0V +5%
V
OUTMIN
= 2.7V – 2.5%
I
LOADMAX
= 40 mA
T
JMAX
= +125°C
T
AMAX
= +55°C
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
P
D
V
INMAX
V
OUTMIN
–I
LOADMAX
3.0 1.05
2.7 0.975
–40 10
-3
=
20.7mW=
P
DMAX
T
JMAX
T
AMAX
–
JA
--------------------------------------------=
125 55–
220
-------------------------=
318mW=
