© 2009 Microchip Technology Inc. DS21663D-page 13
TC2054/2055/2186
5.0 THERMAL CONSIDERATIONS
5.1 Thermal Shutdown
Integrated thermal protection circuitry shuts the
regulator off when the die temperature exceeds
approximately 160°C. The regulator remains off until
the die temperature cools to approximatley 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.
Equation 5-1 is used to calculate worst case 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 (125 °C) and the thermal resistance from
junction-to-air (θ
JA
). The 5-Pin SOT-23A package has
a θ
JA
of approximately 220°C/Watt when mounted on a
typical two layer FR4 dielectric copper clad PC board.
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 TC2054 dissipates a maximum of
only 26.7 mW; far 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
---------------------------------------=
Given:
V
INMAX
= 3.0V +10%
V
OUTMIN
= 2.7V – 2.5%
I
LOADMAX
=40mA
T
AMAX
= +55°C
Find:
1. Actual power dissipation
2. Maximum allowable dissipation
P
D
V
INMAX
V
OUTMIN
–()I
LOADMAX
=
3.0 1.1
×
()2.7 0.975
×
()–[]40 10
3–
×
=
26.7mW=
P
DMAX
T
JMAX
T
AMAX
–
θ
JA
---------------------------------------=
125 55–
220
-------------------- -=
318mW=
