TC1268 Datasheet, PDF (6/12 Pages) Microchip Technology – 500mA Fixed Output, Fast Response CMOS LDO with Shutdown

TC1268
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 (θ_JA).
EQUATION 4-2:
P_DMAX = (T_JMAX – T_AMAX)
θ_JA
Where all terms are previously defined.
Table 4-1 shows various values of θ_JA for the TC1268
package.
TABLE 4-1:
THERMAL RESISTANCE
GUIDELINES FOR TC1268 IN
8-PIN SOIC PACKAGE
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θ_JA)
2500 sq mm 2500 sq mm 2500 sq mm 60°C/W
1000 sq mm 2500 sq mm 2500 sq mm 60°C/W
225 sq mm 2500 sq mm 2500 sq mm 68°C/W
100 sq mm 2500 sq mm 2500 sq mm 74°C/W
*Pin 2 is ground. Device is mounted on topside.
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 = 3.3V ± 10%
V_OUTMIN = 2.5V ± 0.5%
I_LOADMAX = 275mA
T_JMAX = 125°C
T_AMAX = 95°C
θ_JA
= 60°C/W
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
Actual power dissipation:
P_D ≈ (V_INMAX – V_OUTMIN)I_LOADMAX
= [(3.3 x 1.1) – (2.5 x .995)]275 x 10^-3
= 314mW
Maximum allowable power dissipation:
P_DMAX = (T_JMAX – T_AMAX)
θ_JA
= (125 – 95)
60
= 500mW
In this example, the TC1268 dissipates a maximum of
314mW; below the allowable limit of 500mW. In a
similar manner, Equation 4-1 and Equation 4-2 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 500mW into Equation 4-1, from
which V_INMAX = 3.94V.
DS21379B-page 6
© 2002 Microchip Technology Inc.