TC1108
DS21357C-page 4 2002-2012 Microchip Technology Inc.
4.0 THERMAL CONSIDERATIONS
4.1 Thermal Shutdown
Integrated thermal protection circuitry shuts the
regulator off when 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
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:
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:
Table 4-1 shows various values of
JA
for the TC1108
versus board copper area.
TABLE 4-1: THERMAL RESISTANCE
GUIDELINES FOR TC1108
NOTE: *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
JA
= 59°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) – (2.7 x .995)]275 x 10
–3
= 260mW
Maximum allowable power dissipation:
In this example, the TC1108 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.
Copper
Area
(Topside)*
Copper
Area
(Backside)
Board
Area
Thermal
Resistance
(
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
Where:
P
D
(V
INMAX
– V
OUTMIN
)I
LOADMAX
V
INMAX
V
OUTMIN
I
LOADMAX
= Worst case actual power dissipation
= Minimum regulator output voltage
= Maximum output (load) current
= Maximum voltage on V
IN
P
D
P
DMAX
= (T
JMAX
– T
AMAX
)
JA
Where all terms are previously defined.
P
DMAX
= (T
JMAX
– T
AMAX
)
JA
= (125 – 95)
59
= 508mW
