Datasheet

LT3690
24
3690fa
applicaTions inForMaTion
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 13 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT3690’s V
IN
, SW and GND pins and the input
capacitor (C
IN
). 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 circuit board, and their connec-
tions should be made on that layer. Place a local, unbroken
ground plane below these components. The SW and BST
nodes should be small as possible. If synchronizing the
part externally using the SYNC pin, avoid routing this signal
near sensitive nodes, especially V
C
and FB. Finally, keep
the FB and V
C
nodes small so that the ground traces will
shield them from the SW and BST nodes. The exposed
GND pad on the bottom of the package must be soldered
to ground so that the pad acts as a heat sink. To keep ther-
mal resistance low, extend the ground plane as much as
possible, and add thermal vias under
and near the LT3690
to
additional ground planes within the circuit board and
on the bottom side. In addition, the exposed SW pad on
the bottom of the package must be soldered to the PCB
to act as a heat sink for the low side switch. Add thermal
vias under the SW pad and to the bottom side.
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3690 cool.
The GND exposed pad on the bottom of the package must
be soldered to a ground plane and the SW exposed pad
must be soldered to a SW plane. Tie the ground plane and
SW plane to large copper layers below with thermal vias;
these layers will spread the heat dissipated by the LT3690.
Placing additional vias can reduce thermal resistance fur-
ther. With these steps, the thermal resistance from die (or
junction) to ambient can be reduced to θ
JA
= 40°C/W or less.
With 100 LFPM airflow, this resistance can fall by another
25%. Further increases in airflow will lead to lower thermal
resistance. Because of the large output current capability
of the LT3690, it is possible to dissipate enough heat to
raise the junction temperature beyond
the absolute maxi-
mum
of 125°C (150°C for H-grade or MP-grade). When
operating at high ambient temperatures, the maximum
load current should be derated as the ambient temperature
approaches the maximum junction temperature. Power
dissipation within the LT3690 can be estimated by calculat-
ing the total power loss from an efficiency measurement.
The die temperature is calculated by multiplying the LT3690
power dissipation by the thermal resistance from junction-
to-ambient. Thermal resistance depends on the layout of
the circuit board, but values from 20°C/W to 60°C/W are
typical. Die temperature rise was measured on a 4-layer,
6cm • 6cm circuit board in still air at a load current of 4A
(ƒ
SW
= 600kHz). For a 12V input to 3.3V output the die
temperature elevation above ambient was 43°C; for 24V
IN
to 3.3V
OUT
the rise was 52°C; for 12V
IN
to 5V
OUT
the rise
was 55°C and for 24V
IN
to 5V
OUT
the rise was 62°C.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain detailed descrip-
tions and design information for buck regulators and other
switching regulators. The LT1376 data sheet has a more
extensive discussion of output ripple, loop compensa-
tion and stability
testing. Design Note 318 shows how to
generate a bipolar output supply using a buck regulator.
Figure 13. Top Layer PCB Layout and Component
Placement in the LT3690 Demonstration Board
GND
V
OUT
V
IN
C
C
C
SS
R
C
R1
R2
C
F
R
T
C
BST
C
VCC
C
IN
C
OUT
L