lm27342sdx National Semiconductor Corporation, lm27342sdx Datasheet - Page 18

• Manufacturer Part Number: lm27342sdx
• Description: 2 Mhz 1.5a/2a Wide Input Range Step-down Dc-dc Regulator With Frequency Synchronization
• Manufacturer: National Semiconductor Corporation

www.national.com
R
temperature protection circuitry. When the silicon tempera-
ture 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 de-
vice will start to switch again. Knowing this, the R
PCB can be characterized during the early stages of the de-
sign by raising the ambient temperature in the given applica-
tion 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 meth-
ods. All that is needed for calculation is the estimate of
R
R
ohms. With this, the junction temperature, and the ambient
temperature R
Once this is determined, the maximum ambient temperature
allowed for a desired junction temperature can be found.
METHOD 3 EXAMPLE
The operating conditions are the same as the previous Effi-
ciency Calculation:
Internal Power Losses are:
Using a National Semiconductor eMSOP evaluation board to
determine the R
V
f
P
P
P
P
P
SW
θJA
DSON
DSON
IN
COND
SW
Q
BOOST
INTERNAL
= 12V
= 2 MHz
of the application. The LM27341/LM27342 has over-
at 165 °C. This can be extracted from the graph of
vs. Temperature. The value is approximately 0.267
= I
= 2
= (V
= (12V x 2A x 2 MHz x 10nS)
= I
= 1.5 mA x 12V
= I
= 7 mA x 4.5V
= P
OUT
Q
BOOST
2
θJA
COND
IN
x V
x 0.267Ω x .314
θJA
2
x I
can be determined.
x R
IN
of the board. The four layer PCB is con-
OUT
+ P
x V
V
V
OUT
D1
DSON
BOOST
SW
x f
= 0.5V
= 3.3V
SW
+ P
x D
x t
Q
FALL
+ P
BOOST
)
I
R
OUT
FIGURE 9. Buck Converter Current Loops
DCR
= 2A
= 20 mΩ
= 335 mW
= 480 mW
= 29 mW
= 37 mW
________
= 881 mW
θJA
for any
18
structed 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 ac-
cessed by fourteen 10 mil vias. The board measures 2in x 2in
(50.8mm x 50.8mm). It was placed in an oven with no forced
airflow.
The ambient temperature was raised to 132 °C, and at that
temperature, the device went into thermal shutdown. R
now be calculated.
To keep the Junction temperature below 125 °C for this lay-
out, the ambient temperature must stay below 92 °C.
This calculation of the maximum ambient temperature is only
2.3 °C different from the calculation using method 2. The
methods described above to find the junction temperature in
the eMSOP package can also be used to calculate the junc-
tion temperature in the LLP package. The 10 pin LLP package
has a R
layout. R
scribed in method 3.
PCB Layout Considerations
COMPACT LAYOUT
The performance of any switching converter depends as
much upon the layout of the PCB as the component selection.
The following guidelines will help the user design a circuit with
maximum rejection of outside EMI and minimum generation
of unwanted EMI.
Parasitic inductance can be reduced by keeping the power
path components close together and keeping the area of the
loops small, on which high currents travel. Short, thick traces
or copper pours (shapes) are best. In particular, the switch
node (where L1, D1, and the SW pin connect) should be just
large enough to connect all three components without exces-
sive heating from the current it carries. The LM27341/
LM27342 operates in two distinct cycles (see Figure 2) whose
high current paths are shown below in Figure 9:
θJC
T
θJA
A_MAX
= 9.1°C/W, while R
T
can be calculated in the same manner as de-
A_MAX
= 125 °C - (37.46 °C/W x 0.881 W)
= T
T
J_MAX
A_MAX
- (R
30005660
= 92 °C
θJA
θJA
can vary depending on the
x P
INTERNAL
)
θJA
can