TC1017_13 Datasheet, PDF (12/26 Pages) Microchip Technology – 150 mA, Tiny CMOS LDO With Shutdown

TC1017
5.0 THERMAL CONSIDERATIONS
5.1 Thermal Shutdown
Integrated thermal protection circuitry shuts the
regulator off when the die temperature exceeds
approximately 160°C. The regulator remains off until
the die temperature drops to approximately 150°C.
5.2 Power Dissipation: SC-70
The TC1017 is available in the SC-70 package. The
thermal resistance for the SC-70 package is
approximately 450°C/W when the copper area used in
the PCB layout is similar to the JEDEC J51-7 high ther-
mal conductivity standard or semi-G42-88 standard.
For applications with a larger or thicker copper area,
the thermal resistance can be lowered. See AN792, “A
Method to Determine How Much Power a SOT-23 Can
Dissipate in an Application” (DS00792), for a method to
determine the thermal resistance for a particular appli-
cation.
The TC1017 power dissipation capability is dependant
upon several variables: input voltage, output voltage,
load current, ambient temperature and maximum
junction temperature. The absolute maximum steady-
state junction temperature is rated at +125°C. The
power dissipation within the device is equal to:
EQUATION 5-1:
P_D = V_IN – V_OUT  I_LOAD + V_IN  I_GND
The V_IN x I_GND term is typically very small when
compared to the (V_IN–V_OUT) x I_LOAD term, simplifying
the power dissipation within the LDO to be:
EQUATION 5-2:
P_D = V_IN – V_OUT  I_LOAD
To determine the maximum power dissipation
capability, the following equation is used:
EQUATION 5-3:
Where:
P_DMAX = -^--T----^J--^_--^M----^A---R^X----_^–---_J--T_A---^A---^_--^M----^A---^X----^
T_{J_MAX} = the maximum junction
temperature allowed
T_{A_MAX} = the maximum ambient
temperature
R_JA = the thermal resistance from
junction to air
Given the following example:
Find:
V_IN = 3.0V to 4.1V
V_OUT = 2.85V ±2.5%
I_LOAD = 120 mA (output current)
T_A = 55°C (max. desired ambient)
1. Internal power dissipation:
P_DMAX = V_{IN_MAX} – V_{OUT_MIN}  I_LOAD
= 4.1V – 2.85  0.975  120mA
= 158.5mW
2. Maximum allowable ambient temperature:
T_{A_MAX} = T_{J_MAX} – P_DMAX  R_JA
= 125C – 158.5mW  450C/W
= 125C – 71C
= 54C
3. Maximum allowable power dissipation at
desired ambient:
P_D = ^T----J--^_--^M-R---^A-_--^X--_J--_A-^–----T----^A-
= -1---2---54---5^---0C---_--^–-C----5/--W-5---^---C--
= 155mW
In this example, the TC1017 dissipates approximately
158.5 mW and the junction temperature is raised 71°C
over the ambient. The absolute maximum power
dissipation is 155 mW when given a maximum ambient
temperature of 55°C.
Input voltage, output voltage or load current limits can
also be determined by substituting known values in the
power dissipation equations.
Figure 5-1 and Figure 5-2 depict typical maximum
power dissipation versus ambient temperature, as well
as typical maximum current versus ambient tempera-
ture, with a 1V input voltage to output voltage
differential, respectively.
400
350
300
250
200
150
100
50
0
-40 -15 10 35 60 85 110
Ambient Temperature (°C)
FIGURE 5-1:
Power Dissipation vs.
Ambient Temperature (SC-70 package).
DS21813F-page 12
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