TC1054_12 Datasheet, PDF (10/20 Pages) Microchip Technology – 50 mA, 100 mA and 150 mA CMOS LDOs with Shutdown and ERROR Output

TC1054/TC1055/TC1186
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:
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
temperature (T_JMAX) and the thermal resistance from
junction-to-air (_JA). The 5-Pin SOT-23 package has a
_JA of approximately 220°C/Watt.
EQUATION 5-2:
P_DMAX = -^--^T----^J--^M----^A----^X-_---–-_J--_A-^T----^A---^M----^A----^X---^
Where all terms are previously defined.
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
V_OUTMIN
I_LOADMAX
T_JMAX
T_AMAX
Find:
= 3.0V +5%
= 2.7V – 2.5%
= 40 mA
= +125°C
= +55°C
1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
P_D  V_INMAX – V_OUTMINI_LOADMAX
= 3.0  1.05 – 2.7  0.97540  10^-3
= 20.7mW
Maximum allowable power dissipation:
P_DMAX = -^--T----^J--^M----^A---^X--_---^–-_J--_A-T----^A---^M----^A----^X---^
= ^---1---2---52---2--^–-0---5---5----^
= 318mW
In this example, the TC1054 dissipates a maximum of
20.7 mW; 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. Layouts having a ground
plane, wide traces at the pads and wide power supply
bus lines, combine to lower θJA and increase the max-
imum allowable power dissipation limit.
DS21350E-page 10
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