TC1017 Datasheet, PDF (12/22 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 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 printed circuit board layout is similar to the
JEDEC J51-7 high thermal conductivity standard or
semi-G42-88 standard. For applications with 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 application.
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:
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:
P_D = (V_IN – V_OUT) × I_LOAD
To determine the maximum power dissipation
capability, the following equation is used:
EQUATION:
Where:
P_DMAX = -⁽--T---^J--^_---^M----^A--R^X----_θ-^–---_J-T-_A---^A---^_--^M----^A---^X---⁾-
T_{J_MAX} = the maximum junction
temperature allowed
T_{A_MAX} = the maximum ambient
temperature
Rθ_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
DM
AX
×
R
θ_J
A
= (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----A--X-----}–_----T----A-
Rθ_JA
= -1--2---5----^°--C------^–-----5---5---^°---C--
450 ° C /W
= 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 and
typical maximum current versus ambient temperature,
with a one volt 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).
DS21813B-page 12
 2003 Microchip Technology Inc.