TC1301A Datasheet, PDF (18/28 Pages) Microchip Technology – Dual LDO with Microcontroller RESET Function

TC1301A/B
The maximum power dissipation capability for a
package can be calculated given the junction to
ambient thermal resistance and the maximum ambient
temperature for the application. The following equation
can be used to determine the package maximum
internal power dissipation.
EQUATION 6-4:
P_D(MAX)
=
(_---T_{----J--(--M----A---X----)---–----}T_{----A---(--M----A---X---)---})
Rθ_JA
P_D(MAX) = Maximum device power dissipation.
T_J(MAX) = Maximum continuous junction
temperature.
T_A(MAX) = Maximum ambient temperature.
Rθ_JA = Thermal resistance from junction-to-
ambient.
EQUATION 6-5:
T_J(RISE) = P_D(MAX) × Rθ_JA
T_J(RISE) = Rise in device junction temperature
over the ambient temperature.
P_D(MAX) = Maximum device power dissipation.
Rθ_JA = Thermal resistance from junction-to-
ambient.
EQUATION 6-6:
T_J = T_J(RISE) + T_A
T_J = Junction Temperature.
T_J(RISE)= Rise in device junction temperature
over the ambient temperature.
T_A = Ambient Temperature.
6.3 Typical Application
Internal power dissipation, junction temperature rise,
junction temperature and maximum power dissipation
are calculated in the following example. The power
dissipation as a result of ground current is small
enough to be neglected.
6.3.1 POWER DISSIPATION EXAMPLE
Package
Package Type 3X3DFN8
=
Input Voltage
V_IN = 2.7V to 4.2V
LDO Output Voltages and Currents
V_OUT1 = 2.8V
I_OUT1 = 300 mA
V_OUT2 = 1.8V
I_OUT2 = 150 mA
DS21798B-page 18
Maximum Ambient Temperature
T_A(MAX) = 50°C
Internal Power Dissipation
Internal power dissipation is the sum of the power
dissipation for each LDO pass device.
P_LDO1(MAX) = (V_IN(MAX) - V_OUT1(MIN)) x
I_OUT1(MAX)
P_LDO1 = (4.2V - (0.975 x 2.8V)) x 300 mA
P_LDO1 = 441.0 milliWatts
P_LDO2 = (4.2V - (0.975 X 1.8V)) x 150 mA
P_LDO2 = 366.8 milliWatts
P_TOTAL = P_LDO1 + P_LDO2
P_TOTAL= 807.8 milliWatts
Device Junction Temperature Rise
The internal junction temperature rise is a function of
internal power dissipation and the thermal resistance
from junction to ambient for the application. The
thermal resistance from junction to ambient (Rθ_JA) is
derived from an EIA/JEDEC standard for measuring
thermal resistance for small surface-mount packages.
The EIA/JEDEC specification is JESD51-7, “High
Effective Thermal Conductivity Test Board for Leaded
Surface Mount Packages”. The standard describes the
test method and board specifications for measuring the
thermal resistance from junction to ambient. The actual
thermal resistance for a particular application can vary
depending on many factors such as copper area and
thickness. Refer to AN792, “A Method To Determine
How Much Power a SOT32 Can Dissapate in Your
Application” (DS00792), for more information regarding
this subject.
T_J(RISE) = P_TOTAL x Rq_JA
T_JRISE = 807.8 milliWatts x 41.0° C/W
T_JRISE = 33.1°C
Junction Temperature Estimate
To estimate the internal junction temperature, the
calculated temperature rise is added to the ambient or
offset temperature. For this example, the worst-case
junction temperature is estimated below:
T_J = T_JRISE + T_A(MAX)
T_J = 83.1°C
Maximum Package Power Dissipation at 50°C
Ambient Temperature
3X3DFN8 (41° C/W Rθ_JA)
P_D(MAX) = (125°C - 50°C) / 41° C/W
P_D(MAX) = 1.83 Watts
MSOP8 (208° C/W Rθ_JA)
P_D(MAX) = (125°C - 50°C) / 208° C/W
P_D(MAX) = 0.360 Watts
© 2005 Microchip Technology Inc.