lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 23

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 17 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
fl ow in the LT3686A’s V
(D1) and the input capacitor (C2). The loop formed by
these components should be as small as possible and tied
to system ground in only one place. These components,
along with the inductor and output capacitor, should be
placed on the same side of the circuit 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 output capacitor C1. The SW
and BOOST nodes should be as small as possible. Finally,
keep the FB node small so that the ground pin and ground
traces will shield it from the SW and BOOST nodes. Include
vias near the exposed GND pad of the LT3686A to help
remove heat from the LT3686A to the ground plane.
IN
and SW pins, the catch diode
FB
BD
Figure 17. PCB Layout
SS
OUT
V
C2
IN
RT
High Temperature Considerations
The die temperature of the LT3686A must be lower than
the maximum rating of 125°C (150°C for LT3686AH). For
high ambient temperatures, care should be taken in the
layout of the circuit to ensure good heat sinking of the
LT3686A. The maximum load current should be derated
as the ambient temperature approaches the maximum
allowed junction temperature. The die temperature is
calculated by multiplying the LT3686A power dissipa-
tion by the thermal resistance from junction to ambient.
Power dissipation within the LT3686A can be estimated
by calculating the total power loss from an effi ciency
measurement and subtracting the catch diode loss. The
resulting temperature rise at full load is nearly independent
of input voltage. Thermal resistance depends on the layout
of the circuit board, but 43°C/W is typical for the (3mm
× 3mm) DFN package.
C1
UVLO
SW
DA
SYNC/
D1
MODE
BST
3686A F17
LT3686A
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3686af