Datasheet

R
T
JA
=
165
o
C - T
A
P
INTERNAL
6P
COND
+ P
SWF
+ P
SWR
+ P
Q
+ P
BOOST
= P
INTERNAL
P
INTERNAL
= 322 mW
T
J
= (R
T
JC
x Power) + T
C
= 80
o
C/W x 322 mW + T
C
R
T
JA
=
T
J
- T
C
Power
R
T
JA
=
T
J
- T
A
Power
LMR12010
SNVS731A –SEPTEMBER 2011–REVISED SEPTEMBER 2011 www.ti.com
Thermal impedance from the silicon junction to the ambient air is defined as:
(39)
This impedance can vary depending on the thermal properties of the PCB. This includes PCB size, weight of
copper used to route traces and ground plane, and number of layers within the PCB. The type and number of
thermal vias can also make a large difference in the thermal impedance. Thermal vias are necessary in most
applications. They conduct heat from the surface of the PCB to the ground plane. Place two to four thermal vias
close to the ground pin of the device.
The datasheet specifies two different R
θJA
numbers for the Thin SOT-23–6 package. The two numbers show the
difference in thermal impedance for a four-layer board with 2oz. copper traces, vs. a four-layer board with 1oz.
copper. R
θJA
equals 120°C/W for 2oz. copper traces and GND plane, and 235°C/W for 1oz. copper traces and
GND plane.
Method 1:
To accurately measure the silicon temperature for a given application, two methods can be used. The first
method requires the user to know the thermal impedance of the silicon junction to case. (R
θJC
) is approximately
80°C/W for the Thin SOT-23-6 package. Knowing the internal dissipation from the efficiency calculation given
previously, and the case temperature, which can be empirically measured on the bench we have:
(40)
Therefore:
T
J
= (R
θJC
x P
LOSS
) + T
C
(41)
Design Example 2:
Operating Conditions
V
IN
5.0V P
OUT
2.5W
V
OUT
2.5V P
DIODE
151mW
I
OUT
1.0A P
IND
75mW
V
D
0.35V P
SWF
53mW
Freq 3MHz P
SWR
53mW
I
Q
1.5mA P
COND
187mW
T
RISE
8ns P
Q
7.5mW
T
FALL
8ns P
BOOST
21mW
R
DSON
330mΩ P
LOSS
548mW
IND
DCR
75mΩ
D 0.568
(42)
The second method can give a very accurate silicon junction temperature. The first step is to determine R
θJA
of
the application. The LMR12010 has over-temperature protection circuitry. When the silicon temperature reaches
165°C, the device stops switching. The protection circuitry has a hysteresis of 15°C. Once the silicon
temperature has decreased to approximately 150°C, the device will start to switch again. Knowing this, the R
θJA
for any PCB can be characterized during the early stages of the design by raising the ambient temperature in the
given application until the circuit enters thermal shutdown. If the SW-pin is monitored, it will be obvious when the
internal NFET stops switching indicating a junction temperature of 165°C. Knowing the internal power dissipation
from the above methods, the junction temperature and the ambient temperature, R
θJA
can be determined.
(43)
Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be
found.
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