TC1264_06 Datasheet, PDF (7/18 Pages) Microchip Technology – 800 mA Fixed-Output CMOS LDO with Shutdown

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:
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
The maximum allowable power dissipation
(Equation 5-2) is a function of the maximum ambient
temperature (T_AMAX), the maximum allowable die tem-
perature (T_JMAX) and the thermal resistance from junc-
tion-to-air (θ_JA).
EQUATION 5-2:
P_DMAX = (T_JMAX – T_AMAX)
θ_JA
Where all terms are previously defined.
Table 5-1 and Table 5-2 show various values of θ_JA for
the TC1264 packages.
TABLE 5-1:
THERMAL RESISTANCE
GUIDELINES FOR TC1264 IN
SOT-223 PACKAGE
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θ_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
225 sq mm 2500 sq mm 2500 sq mm 53°C/W
100 sq mm 2500 sq mm 2500 sq mm 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
TC1264
TABLE 5-2:
THERMAL RESISTANCE
GUIDELINES FOR TC1264 IN
3-PIN DDPAK/TO-220
PACKAGE
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θ_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
125 sq mm 2500 sq mm 2500 sq mm 35°C/W
* Tab of device attached to topside copper
Equation 5-1 can be used in conjunction with
Equation 5-2 to ensure regulator thermal operation is
within limits. For example:
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 = 59°C/W (SOT-223)
Find:
1. Actual power dissipation.
2. Maximum allowable dissipation
Actual power dissipation:
P_D ≈ (V_INMAX – V_OUTMIN)I_LOADMAX
P_D = (3.3 × 1.1) – (2.7 × .995)275 × 10^–3
P_D = 260 mW
Maximum allowable power dissipation:
P_DMAX
=
T_{----J--M-----A---X----–-----}T_{---A----M----A---X-}
θ_JA
P_DMAX
=
(_{---1---2---5----–-----9---5---})_-
59
P_DMAX = 508 mW
In this example, the TC1264 dissipates a maximum of
260 mW; 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 508mW into Equation 5-1, from
which V_INMAX = 4.6V.
© 2006 Microchip Technology Inc.
DS21375C-page 7