lt3692a Linear Technology Corporation, lt3692a Datasheet - Page 26

• Manufacturer Part Number: lt3692a
• Description: Monolithic Dual Tracking 3.5a Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

LT3692A
APPLICATIONS INFORMATION
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board (PCB) layout. Figure 18
shows the high di/dt paths in the buck regulator circuit.
Note that large switched currents flow in the power switch,
the catch diode and the input capacitor. The loop formed
by these components should be as small as possible.
These components, along with the inductor and output
capacitor, should be placed on the same side of the cir-
cuit board and their connections should be made on that
layer. Place a local, unbroken ground plane below these
components, and tie this ground plane to system ground
at one location, ideally at the ground terminal of the out-
put capacitor C2. Route all small signal analog returns
to the ground connection at the bottom of the package.
Additionally, the SW and BST traces should be kept as
short as possible.
Thermal Considerations
The PCB must also provide heat sinking to keep the LT3692A
cool. The exposed metal on the bottom of the package
must be soldered to a ground plane. This ground should
be tied to other copper layers below with thermal vias;
these layers will spread the heat dissipated by the LT3692A.
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Figure 19. PCB Top Layer and Component Placement for TSSOP and QFN Packages
DC1403A TSSOP LAYOUT
Place additional vias near the catch diodes. Adding more
copper to the top and bottom layers and tying this copper
to the internal planes with vias can further reduce thermal
resistance. The topside metal and component outlines
in Figure 19 illustrate proper component placement and
trace routing.
The LT3692A’s powerful 3.8A switches allow the converter
to source large output currents. Depending on the con-
verter’s operating conditions, the resulting internal power
dissipation can raise the junction temperature beyond
its maximum rating. Operating conditions include input
voltages, output voltages, switching frequencies, output
currents, and the ambient environmental temperature,
etc. An estimation of the junction temperature rise above
ambient temperature helps determine whether a given
design may exceed the maximum junction ratings for
specific operating conditions. However, temperature rise
depends on PCB design and the proximity to other heat
sources. The final converter design must be evaluated
on the bench.
An estimation of the junction temperature rise begins by
determining which circuit components dissipate power.
In order to simplify the power loss estimation, only the
inductors, catch diodes, and the LT3692A will be considered
QFN LAYOUT
3692a F19
3692af