TC1300 Datasheet, PDF (9/16 Pages) Microchip Technology – 300 mA CMOS LDO with Shutdown, Bypass and Independent Delayed Reset Function

4.0 THERMAL CONSIDERATIONS
4.1 Thermal Shutdown
Integrated thermal protection circuitry shuts the regula-
tor off when the 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 prima-
rily a function of input and output voltage, and output
current. The following equation is used to calculate
worst case actual power dissipation:
EQUATION
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, P_DMAX, is a
function of the maximum ambient temperature (T_AMAX),
the maximum recommended die temperature (125°C)
and the thermal resistance from junction-to-air (θ_JA).
The MSOP-8 package has a θ_JA of approximately
200°C/Watt when mounted on a FR4 dielectric copper
clad PC board.
EQUATION
P_DMAX
=
-⁽--^T---^J---^M----^A---^X-----–----^T----^A---^M----^A---^X----⁾
θ_JA
TC1300
The worst case actual power dissipation equation can
be used in conjunction with the LDO maximum allow-
able power dissipation equation to ensure regulator
thermal operation is within limits. For example:
Given:
Find:
V_INMAX = 4.1V
V_OUTMIN = 3.0V -2.5%
I_LOADMAX = 200 mA
T_JMAX
= 125°C
T_AMAX
= 55°C
θ_JA
= 200°C/W
EQUATION:
ACTUAL POWER
DISSIPATION
P_D ≈ (V_INMAX – V_OUTMIN)I_LOADMAX
= [(4.1) – (3.0 × .975) ]200 × 10^–3
= 220 mW
EQUATION:
MAXIMUM ALLOWABLE
POWER DISSIPATION
P_DMAX
=
-⁽--^T---^J---^M----^A---^X-----–----^T----^A---^M----^A---^X----⁾
θ_JA
= ⁽---¹---²---⁵----–-----⁵---⁵---⁾-
200
= 350 mW
In this example, the TC1300 dissipates a maximum of
only 220 mW; below the allowable limit of 350 mW. In a
similar manner, the maximum actual power dissipation
equation and the maximum allowable power dissipa-
tion equation can be used to calculate maximum cur-
rent and/or input voltage limits. For example, the
maximum allowable V_IN is found by substituting the
maximum allowable power dissipation of 350 mW into
the actual power dissipation equation, from which
V_INMAX = 4.97V.
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 θ_JA and, therefore,
increase the maximum allowable power dissipation
limit.
 2002 Microchip Technology Inc.
DS21385C-page 9