TC1265 View Datasheet(PDF) - Microchip Technology
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TC1265 Datasheet PDF : 18 Pages
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TC1265
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:
PD = (VINMAX – VOUTMIN)ILOADMAX
Where:
PD = Worst-case actual power dissipation
VINMAX = Maximum voltage on VIN
VOUTMIN = Minimum regulator output voltage
ILOADMAX = Maximum output (load) current
The maximum allowable power dissipation
(Equation 5-2) is a function of the maximum ambient
temperature (TAMAX), the maximum allowable die
temperature (TJMAX) and the thermal resistance from
junction-to-air (θJA).
EQUATION 5-2:
Where:
PDMAX
=
T----J--M-----A---X----–-----T---A----M----A---X-
θJA
PD = Worst-case actual power dissipation
VINMAX = Maximum voltage on VIN
VOUTMIN = Minimum regulator output voltage
ILOADMAX = Maximum output (load) current
Table 5-1 and Table 5-2 show various values of θJA for
the TC1265 package types.
TABLE 5-1:
THERMAL RESISTANCE
GUIDELINES FOR TC1265 IN
8-PIN SOIC PACKAGE
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θJA)
2500 sq mm 2500 sq mm 2500 sq mm 60°C/W
1000 sq mm 2500 sq mm 2500 sq mm 60°C/W
225 sq mm 2500 sq mm 2500 sq mm 68°C/W
100 sq mm 2500 sq mm 2500 sq mm 74°C/W
* Pin 2 is ground. Device is mounted on the top-side.
TABLE 5-2:
THERMAL RESISTANCE
GUIDELINES FOR TC1265 IN
5-PIN DDPAK/TO-220
PACKAGE
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θJA)
2500 sq mm 2500 sq mm 2500 sq mm 25°C/W
1000 sq mm 2500 sq mm 2500 sq mm 27°C/W
125 sq mm 2500 sq mm 2500 sq mm 35°C/W
* Tab of device attached to top-side copper
Equation 5-1 can be used in conjunction with
Equation 5-2 to ensure regulator thermal operation is
within limits. For example:
Given:
VINMAX = 3.3V ± 10%
VOUTMIN = 2.7V ± 0.5%
ILOADMAX = 275 mA
TJMAX = 125°C
TAMAX = 95°C
θJA = 60°C/W (SOIC)
Find:
1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
PD ≈ (VINMAX – VOUTMIN)ILOADMAX
PD = (3.3 × 1.1) – (2.7 × .995)275 × 10–3
PD = 260 mW
Maximum allowable power dissipation:
PDMAX
=
T----J--M-----A---X----–-----T---A----M----A---X-
θJA
PDMAX
=
(---1---2---5----–-----9---5---)-
60
PDMAX = 500 mW
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
VIN is found by substituting the maximum allowable
power dissipation of 500 mW into Equation 5-1, from
which VINMAX = 4.6V.
DS21376D-page 8
© 2006 Microchip Technology Inc.
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