TC1269_13 Datasheet, PDF (5/12 Pages) Microchip Technology – 300mA CMOS LDO with Shutdown and VREF Bypass

4.0 THERMAL CONSIDERATIONS
4.1 Thermal Shutdown
Integrated thermal protection circuitry shuts the
regulator off when die temperature exceeds 150°C.
The regulator remains off until the die temperature
drops to approximately 140°C.
4.2 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 4-1:
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 4-2) 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 4-2:
P_DMAX = (T_JMAX – T_AMAX)
_JA
Where all terms are previously defined.
TC1269
Equation 4-1 can be used in conjunction with
Equation 4-2 to ensure regulator thermal operation is
within limits. For example:
Given:
V_INMAX
V_OUTMIN
I_LOAD
T_AMAX
= 3.0V ± 10%
= 2.7V – 2.5%
= 250mA
= 55°C
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
P_D  (V_INMAX – V_OUTMIN) I_LOADMAX
= [(3.0 x 1.1) – (2.7 x .975)]250 x 10^-3
= 167mW
Maximum allowable power dissipation:
P_DMAX = (T_JMAX – T_AMAX)
_JA
= (125 – 55)
200
= 350mW
In this example, the TC1269 dissipates a maximum of
167mW; below the allowable limit of 350mW. In a
similar manner, Equation 4-1 and Equation 4-2 can be
used to calculate maximum current and/or input
voltage limits.
4.3 Layout Considerations
The primary path of heat conduction out of the package
is via the package leads. Therefore, layouts having a
ground plane, wide traces at the pads, and wide power
supply bus lines combine to lower _JA and, therefore,
increase the maximum allowable power dissipation
limit.
 2002-2012 Microchip Technology Inc.
DS21380C-page 5