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LM21215AMHE-1 Datasheet(PDF) 29 Page - Texas Instruments

Part No. LM21215AMHE-1
Description  15-A Ultra High-Efficiency Synchronous Buck Converter
Download  38 Pages
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM21215AMHE-1 Datasheet(HTML) 29 Page - Texas Instruments

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OUT
JA
A
J
OUT
V
1
1
R
T
T
I
˜
K
K
˜
T
SW
PVIN
VOUT
GND
LM21215A-1
Low-side
NMOS
gate driver
High-side
PMOS
gate driver
CIN
COUT
Q1
Q2
LF
#1
#2
PGND
High
di/dt
loop
VIN
LM21215A
www.ti.com
SNOSB87C – MARCH 2011 – REVISED JANUARY 2016
Layout Guidelines (continued)
10.1.1 Compact PCB Layout for EMI Reduction
Radiated EMI generated by high di/dt components relates to pulsing currents in switching converters. The larger
area covered by the path of a pulsing current, the more electromagnetic emission is generated. The key to
reducing radiated EMI is to identify the pulsing current path and minimize the area of that path. The main
switching loop of the LM21215A power stage is denoted by #1 in Figure 41. The topological architecture of a
buck converter means that particularly high di/dt current flows in loop #1, and it becomes mandatory to reduce
the parasitic inductance of this loop by minimizing its effective loop area. For loop #2 however, the di/dt through
inductor LF and capacitor COUT is naturally limited by the inductor. Keeping the area of loop #2 small is not nearly
as important as that of loop #1. Also important are the gate drive loops of the low-side and high-side MOSFETs,
which are inherently tight by virtue of the integrated power MOSFETs and gate drivers of the LM21215A
Figure 41. LM21215A Power Stage Circuit Switching Loops
High-frequency ceramic bypass capacitors at the input side provide the primary path for the high di/dt
components of the pulsing current. Placing ceramic bypass capacitors as close as possible to the PVIN and
PGND pins is the key to EMI reduction. Keep the SW trace connecting to the inductor as short as possible, and
just wide enough to carry the load current without excessive heating. Use short, thick traces or copper pours
(shapes) for current conduction path to minimize parasitic resistance. Place the output capacitors close to the
VOUT side of the inductor and route the return using GND plane copper back to the PGND pins and the exposed
pad of the LM21215A.
10.1.2 Thermal Design
As with any power conversion device, the LM21215A dissipates internal power while operating. The effect of this
power dissipation is to raise the internal junction temperature of the LM21215A above ambient. The junction
temperature (TJ) is a function of the ambient temperature (TA), the power dissipation and the effective thermal
resistance of the device and PCB combination (RθJA). The maximum operating junction temperature for the
LM21215A is 125°C, thus establishing a limit on the maximum device power dissipation and therefore the load
current at high ambient temperatures. Equation 20 shows the relationships between these parameters.
(20)
Copyright © 2011–2016, Texas Instruments Incorporated
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