TC1267_13 Datasheet, PDF (5/14 Pages) Microchip Technology – 400mA PCI LDO

4.0 THERMAL CONSIDERATIONS
4.1 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.
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.
Table 4-1 shows various values of _JA for the TC1267
packages.
TABLE 4-1:
THERMAL RESISTANCE
GUIDELINES FOR TC1267 IN
5-PIN DDPAK
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
_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.
TC1267
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 = 5V ± 5%
V_OUTMIN = 3.217V
I_LOADMAX = 400mA
T_JMAX = 95°C
T_AMAX = 70°C
_JA
= 27°C/W (DDPAK mounted on
1000 sq mm topside copper area)
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
P_D  (V_INMAX – V_OUTMIN)I_LOADMAX
= (5.25V - 3.217V) 400mA
= 813mW
Maximum allowable power dissipation:
P_DMAX = (T_JMAX – T_AMAX)
_JA
= (95 – 70)
27
= 423mW
In this example, the TC1267 dissipates a maximum of
813mW; below the allowable limit of 926mW.
 2001-2012 Microchip Technology Inc.
DS21378C-page 5