TC1262-3.3VDBTR Datasheet, PDF (4/12 Pages) Microchip Technology – 500mA Fixed Output CMOS LDO

TC1262
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
(TJ_MAX) 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 and Table 4-2 show various values of θ_JA for
the TC1262 packages.
TABLE 4-1:
THERMAL RESISTANCE
GUIDELINES FOR TC1262 IN
SOT-223 PACKAGE
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θ_JA)
2500 sq mm 2500 sq mm 2500 sq mm 45°C/W
1000 sq mm 2500 sq mm 2500 sq mm 45°C/W
225 sq mm 2500 sq mm 2500 sq mm 53°C/W
100 sq mm 2500 sq mm 2500 sq mm 59°C/W
1000 sq mm 1000 sq mm 1000 sq mm 52°C/W
1000 sq mm 0 sq mm 1000 sq mm 55°C/W
*Tab of device attached to topside copper
TABLE 4-2:
THERMAL RESISTANCE
GUIDELINES FOR TC1262 IN
3-PIN DDPAK/TO-220
PACKAGE
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(θ_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
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 = 2.7V ± 0.5%
I_LOADMAX = 275mA
T_JMAX = 125°C
T_AMAX = 95°C
θ_JA
= 59°C/W (SOT-223)
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) – (2.7 x .995)]275 x 10^–3
= 260mW
Maximum allowable power dissipation:
P_DMAX = (T_JMAX – T_AMAX)
θ_JA
= (125 – 95)
59
= 508mW
In this example, the TC1262 dissipates a maximum of
260mW; below the allowable limit of 508mW. 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 sustituting the maximum allowable
power dissipation of 508mW into Equation 4-1, from
which V_INMAX = 4.6V.
DS21373B-page 4
© 2002 Microchip Technology Inc.