Datasheet

R
T
JA
=
165
o
C - T
A
P
INTERNAL
LM27341, LM27342, LM27341-Q1, LM27342-Q1
www.ti.com SNVS497E –NOVEMBER 2008–REVISED APRIL 2013
METHOD 2 EXAMPLE
The operating conditions are the same as the previous Efficiency Calculation:
V
IN
= 12V V
OUT
= 3.3V I
OUT
= 2A
f
SW
= 2 MHz V
D1
= 0.5V R
DCR
= 20 mΩ
Internal Power Losses are:
P
COND
= I
OUT
2
x R
DSON
x D
= 2
2
x 0.15Ω x 0.314
= 188 mW (52)
P
SW
= (V
IN
x I
OUT
x f
SW
x t
FALL
)
= (12V x 2A x 2 MHz x 10ns)
= 480 mW (53)
P
Q
= I
Q
x V
IN
= 1.5 mA x 12V
= 29 mW (54)
P
BOOST
= I
BOOST
x V
BOOST
= 7 mA x 4.5V
= 37 mW (55)
P
INTERNAL
= P
COND
+ P
SW
+ P
Q
+ P
BOOST
= 733 mW (56)
The junction temperature can now be estimated as:
T
J
= (R
θJC
x P
INTERNAL
) + T
C
(57)
A Texas Instruments MSOP-PowerPad evaluation board was used to determine the T
J
of the
LM27341/LM27342. The four layer PCB is constructed using FR4 with 2oz copper traces. There is a ground
plane on the internal layer directly beneath the device, and a ground plane on the bottom layer. The ground
plane is accessed by fourteen 10 mil vias. The board measures 2in x 2in (50.8mm x 50.8mm). It was placed in a
container with no airflow. The case temperature measured on this LM27342MY Demo Board was 48.7°C.
Therefore,
T
J
= (9.5 °C/W x 733 mW) + 48.7 °C (58)
T
J
= 55.66 °C (59)
To keep the Junction temperature below 125 °C for this layout, the ambient temperature must stay below 94.33
°C.
T
A_MAX
= T
J_MAX
- T
J
+T
A
(60)
T
A_MAX
= 125 °C - 55.66 °C + 25 °C (61)
T
A_MAX
= 94.33 °C (62)
Method 3:
The third method can also give a very accurate estimate of silicon junction temperature. The first step is to
determine R
θJA
of the application. The LM27341/LM27342 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. We can
calculate the internal power dissipation from the above methods. All that is needed for calculation is the estimate
of R
DSON
at 165 °C. This can be extracted from the graph of R
DSON
vs. Temperature. The value is approximately
0.267 ohms. With this, the junction temperature, and the ambient temperature R
θJA
can be determined.
(63)
Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be
found.
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