2010 Microchip Technology Inc. DS21375D-page 7
TC1264
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
AMAX
), the maximum allowable die
temperature (T
JMAX
) and the thermal resistance from
junction-to-air (
JA
).
EQUATION 5-2:
Table 5-1 and Tabl e 5- 2 show various values of
JA
for
the TC1264 packages.
TABLE 5-1: THERMAL RESISTANCE
GUIDELINES FOR TC1264 IN
SOT-223 PACKAGE
TABLE 5-2: THERMAL RESISTANCE
GUIDELINES FOR TC1264 IN
3-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. Actual power dissipation.
2. Maximum allowable dissipation.
Actual power dissipation:
Maximum allowable power dissipation:
In this example, the TC1264 dissipates a maximum of
260 mW, which is below the allowable limit of 508 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 508 mW into Equation 5-1, from
which V
INMAX
= 4.6V.
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
JA
)
2500 sq mm 2500 sq mm 2500 sq mm 45°C/W
1000 sq mm 2500 sq mm 2500 sq mm 45°C/W
225sqmm 2500sqmm 2500sqmm 53°C/W
100sqmm 2500sqmm 2500sqmm 59°C/W
1000 sq mm 1000 sq mm 1000 sq mm 52°C/W
1000 sq mm 0 sq mm 1000 sq mm 55°C/W
* Tab of device attached to topside copper
Where:
V
INMAX
V
OUTMIN
I
LOADMAX
P
D
= Worst-case actual power dissipation
= Minimum regulator output voltage
= Maximum output (load) current
= Maximum voltage on V
IN
P
D
V
INMAX
V
OUTMIN
–I
LOADMAX
=
P
DMAX
= (T
JMAX
– T
AMAX
)
JA
Where all terms are previously defined.
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
JA
)
2500 sq mm 2500 sq mm 2500 sq mm 25°C/W
1000 sq mm 2500 sq mm 2500 sq mm 27°C/W
125sqmm 2500sqmm 2500sqmm 35°C/W
* Tab of device attached to topside copper
Given:
V
INMAX
= 3.3V ± 10%
V
OUTMIN
= 2.7V ± 0.5%
I
LOADMAX
=275mA
T
JMAX
= 125°C
T
AMAX
=95°C
JA
= 59°C/W (SOT-223)
P
D
V
INMAX
V
OUTMIN
–I
LOADMAX
P
D
3.3 1.12.7 .995– 275 10
3–
=
P
D
260 mW=
P
DMAX
T
JMAX
T
AMAX
–
JA
---------------------------------------
=
P
DMAX
125 95–
59
-------------------------=
P
DMAX
508 mW=
