Datasheet
0 2 4 6 8 10 12
75
85
95
105
115
125
MAX. AMBIENT TEMPERATURE (°C)
I
OUT
(A)
2 3 4 5 6 7 8 9 10
10
12
14
16
18
20
22
24
26
28
30
THERMAL RESISTANCE (
JA
)
BOARD AREA (in
2
)
LM21212-2
SNVS715A –MARCH 2011–REVISED MARCH 2013
www.ti.com
Figure 34. Thermal Resistance vs PCB Area (4 Layer Board)
Figure 35 shows a plot of the maximum ambient temperature vs. output current for the typical application circuit
shown in Figure 37, assuming a θ
JA
value of 24 °C/W.
Figure 35. Maximum Ambient Temperature vs. Output Current (0 LFM)
PCB LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance
of a DC-DC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss
in the traces. These can send erroneous signals to the DC-DC converter resulting in poor regulation or instability.
Good layout can be implemented by following a few simple design rules.
1. Minimize area of switched current loops. In a buck regulator there are two loops where currents are switched
at high slew rates. The first loop starts from the input capacitor, to the regulator PVIN pin, to the regulator
SW pin, to the inductor then out to the output capacitor and load. The second loop starts from the output
capacitor ground, to the regulator GND pins, to the inductor and then out to the load (see Figure 36). To
minimize both loop areas, the input capacitor should be placed as close as possible to the VIN pin.
Grounding for both the input and output capacitor should be close. Ideally, a ground plane should be placed
on the top layer that connects the PGND pins, the exposed pad (EP) of the device, and the ground
connections of the input and output capacitors in a small area near pins 10 and 11 of the device. The
inductor should be placed as close as possible to the SW pin and output capacitor.
2. Minimize the copper area of the switch node. The six SW pins should be routed on a single top plane to the
pad of the inductor. The inductor should be placed as close as possible to the switch pins of the device with
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