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TC1265
DS21376C-page 8
2004 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
(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 (
θ
JA
).
maximum
allowable
power
dissipation
EQUATION 5-2:
Table 5-1 and Table 5-2 show various values of
θ
JA
for
the TC1265 package types.
TABLE 5-1:
THERMAL RESISTANCE
GUIDELINES FOR TC1265 IN
8-PIN SOIC PACKAGE
TABLE 5-2:
THERMAL RESISTANCE
GUIDELINES FOR TC1265 IN
5-PIN DDPAK/TO-220
PACKAGE
Equation 5-1 can be used in conjunction with
Equation 5-2 to ensure regulator thermal operation is
within limits. For example:
Find:
1.
2.
Actual power dissipation:
≈
Actual power dissipation
Maximum allowable dissipation
Maximum allowable power dissipation:
In this example, the TC1265 dissipates a maximum of
260 mW, below the allowable limit of 500 mW. In a
similar manner, Equation 5-1 and Equation 5-2 can be
used to calculate maximum current and/or input
voltage limits. For example, the maximum allowable
V
IN
is found by substituting the maximum allowable
power dissipation of 500 mW into Equation 5-1, from
which V
INMAX
= 4.6V.
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θ
JA
)
60°C/W
60°C/W
68°C/W
2500 sq mm
1000 sq mm
225 sq mm
100 sq mm
*
Pin 2 is ground. Device is mounted on the top-side.
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
74°C/W
Where:
V
INMAX
V
OUTMIN
I
LOADMAX
P
D
= Worst-case actual power dissipation
= Maximum voltage on V
IN
= Minimum regulator output voltage
= Maximum output (load) current
P
D
V
INMAX
V
OUTMIN
–
(
)
I
LOADMAX
=
Where:
V
INMAX
V
OUTMIN
I
LOADMAX
P
D
= Worst-case actual power dissipation
= Maximum voltage on V
IN
= Minimum regulator output voltage
= Maximum output (load) current
P
DMAX
T
------------------–
T
θ
JA
=
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θ
JA
)
25°C/W
27°C/W
35°C/W
2500 sq mm
1000 sq mm
125 sq mm
*
Tab of device attached to top-side copper
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
2500 sq mm
Given:
V
INMAX
= 3.3V ± 10%
V
OUTMIN
= 2.7V ± 0.5%
I
LOADMAX
= 275 mA
T
JMAX
= 125°C
T
AMAX
= 95°C
θ
JA
= 60°C/W (SOIC)
P
D
P
D
P
D
V
INMAX
3.3
(
260 mW
V
OUTMIN
2.7
(
–
–
(
)
I
LOADMAX
.995
)
275
1.1
×
)
×
10
3
–
×
=
=
P
DMAX
T
--------------------------------------
T
JA
)
60
500 mW
–
=
P
DMAX
P
DMAX
-------------–
(
=
=