TC1305 Datasheet, PDF (7/18 Pages) Microchip Technology – Dual 150mA CMOS LDO With Select Mode™ Operation, Shutdown and Independent RESET Output

5.0 THERMAL CONSIDERATIONS
5.1 Thermal Shutdown
Integrated thermal protection circuitry shuts the
regulator off when die exceeds approximately 160°C.
The regulator remains off until the die temperature
drops to approximately 145°C.
Thermal shutdown is intended to protect the device
under transient accidental (fault) overload conditions.
Thermal Shutdown may not protect the LDO while
operating above junction temperatures of 125°C
continuously. Sufficient thermal evaluation of the
design needs to be conducted to ensure that the
junction temperature does not exceed 125°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_OUT1MIN)I_LOAD1MAX +
(V_INMAX – V_OUT2MIN)I_LOAD2MAX
Where:
P_D = Worst case actual power dissipation
V_INMAX = Maximum voltage on V_IN
V_OUT1MIN = Minimum regulator output voltage1
I_LOAD1MAX = Maximum output (load) current1
V_OUT2MIN = Minimum regulator output voltage2
I_LOAD2MAX = Maximum output (load) current2
The maximum allowable power dissipation
(Equation 5-2) is a function of the maximum ambient
temperature (T_AMAX), the maximum allowable die
temperature (125°C), and the thermal resistance from
junction-to-air (θ_JA). The MSOP-10 package has a θ_JA
of approximately 113°C/W when mounted on a four
layer FR4 dielectric copper clad PC board.
EQUATION 5-2:
P_DMAX = (T_JMAX – T_AMAX)
θ_JA
Where all terms are previously defined.
TC1305
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 = 3.8V ± 5%
V_OUT1MIN = 3.0V ± 2.5%
V_OUT2MIN = 3.0V ± 2.5%
I_LOAD1MAX = 120mA
I_LOAD2MAX = 120mA
T_JMAX = 125°C
T_AMAX = 55°C
θ_JA
= 113°C/W
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
P_D ≈ [(V_INMAX – V_OUT1MIN)] x I_LOAD1MAX
+ [(V_INMAX – V_OUT2MIN)] x I_LOAD2MAX
[(3.8 x 1.05) – (3.0 x .975)] x 120 x 10^-3
+ [(3.8 x 1.05) – (3.0 x .975)] x 120 x 10^-3
= 256mW
Maximum allowable power dissipation:
P_D = (T_JMAX – T_AMAX)
θ_JA
= (125 – 55)
113
= 620mW
In this example, the TC1305 dissipates a maximum of
256mW; below the allowable limit of 620mW. 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 620mW into Equation 5-1, from
which V_INMAX = 5.6V.
5.3 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 θ_JA and therefore
increase the maximum allowable power dissipation
limit.
© 2002 Microchip Technology Inc.
DS21526A-page 7