TC1301B Microchip Technology, TC1301B Datasheet - Page 18

• Manufacturer Part Number: TC1301B
• Description: Dual LDO
• Manufacturer: Microchip Technology

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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:
EQUATION 6-5:
EQUATION 6-6:
6.3
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
DS21798B-page 18
Package
Input Voltage
LDO Output Voltages and Currents
P
T
T
R
T
P
R
T
T
T
Package Type
A(MAX)
A
J(MAX)
J(RISE)
J
J(RISE)
D(MAX)
D(MAX)
JA
JA
=
V
V
= Junction Temperature.
= Rise in device junction temperature
= Ambient Temperature.
Typical Application
= Rise in device junction temperature
= Maximum device power dissipation.
= Thermal resistance from junction-to-
I
I
P
= Maximum device power dissipation.
= Maximum continuous junction
= Maximum ambient temperature.
= Thermal resistance from junction-to-
OUT1
OUT1
OUT2
OUT2
D MAX
over the ambient temperature.
ambient.
over the ambient temperature.
V
POWER DISSIPATION EXAMPLE
T
temperature.
ambient.
IN
J RISE
= 2.7V to 4.2V
= 2.8V
= 300 mA
= 1.8V
= 150 mA
T
J
3X3DFN8
=
=
=
---------------------------------------------------
T
T
J RISE
P
J MAX
D MAX
R
+
–
JA
T
T
A
A MAX
R
JA
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 (R
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.
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:
Maximum Package Power Dissipation at 50°C
Ambient Temperature
Maximum Ambient Temperature
Internal Power Dissipation
Internal power dissipation is the sum of the power
dissipation for each LDO pass device.
3X3DFN8 (41° C/W R
MSOP8 (208° C/W R
P
LDO1(MAX)
P
P
P
P
T
T
T
D(MAX)
D(MAX)
D(MAX)
D(MAX)
J(RISE)
T
P
JRISE
JRISE
P
A(MAX)
P
P
P
P
TOTAL
TOTAL
LDO1
LDO1
LDO2
LDO2
T
T
J
J
= P
= 807.8 milliWatts x 41.0
= 33.1
= T
= 83.1°C
= (125°C - 50°C) / 41° C/W
= 1.83 Watts
= (125°C - 50°C) / 208° C/W
= 0.360 Watts
= 50°C
= (V
= (4.2V - (0.975 x 2.8V)) x 300 mA
= 441.0 milliWatts
= (4.2V - (0.975 X 1.8V)) x 150 mA
= 366.8 milliWatts
= P
= 807.8 milliWatts
JRISE
TOTAL
I
OUT1(MAX)
LDO1
IN(MAX)
°
C
JA
+ T
© 2005 Microchip Technology Inc.
JA
x Rq
+ P
)
)
A(MAX)
- V
LDO2
JA
OUT1(MIN)
°
) x
C/W
JA
) is