For more information www.linear.com/LT3697
LT3697
25
3697f
(either by a logic signal or because it is tied to V
IN
), then
the LT3697’s internal circuitry will pull its quiescent cur-
rent through
its SW pin. This is fine if your system can
tolerate
a 1mA in this state. If you ground the EN pin, the
SW pin current will drop to zero. However, if the V
IN
pin
is grounded while the output is held high, regardless of
EN, parasitic diodes inside the LT3697 can pull current
from the output through the SW pin and out of V
IN
pin,
possibly causing high power dissipation in and damage
to the LT3697 depending on the magnitude of the current.
Figure 9 shows a circuit that is robust to output shorts
high and reversed input.
The exposed pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3697 to additional ground planes within the circuit
board and on the bottom side.
High Temperature Considerations and Thermal
Shutdown
For higher ambient temperatures, care should be taken in
the
layout of the PCB to ensure good heat sinking of the
LT3697.
The exposed pad on the bottom of the package
must be soldered to a ground plane. This ground should be
tied to large copper layers below with thermal vias; these
layers will dissipate the heat generated by the LT3697.
Placing additional vias can reduce the thermal resistance
further. When operating at high ambient temperatures, the
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
Power dissipation within the LT3697 can be estimated by
calculating the total power loss from an efficiency measure
-
ment and
subtracting the catch diode loss and inductor
loss.
The die temperature is calculated by multiplying the
LT3697 power dissipation by the thermal resistance from
junction to ambient.
The LT3697 has thermal shutdown to protect the part
during periods of high power dissipation, particularly in
high ambient temperature environments. The thermal
shutdown feature detects when the LT3697 is too hot
and shuts the part down, preventing switching. When the
thermal event passes and the LT3697 cools, the part will
restart and resume switching.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design
information for buck regulators
and
other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing.
applicaTions inForMaTion
Figure 9. Diodes D
IN
and D
OUT
Prevent High Current
Flow in the LT3697 if the Input Is Grounded or
Floating and the Output Is Pulled High.
INPUT OUTPUT
3697 F09
LT3697
GND
C
OUT
C
IN
V
IN
SW
D
IN
D
OUT
C
BULK
L
+
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 10 shows
a good PCB layout example with component, trace, ground
plane and via locations. Note that large currents with high
dI/dt flow
in the LT3697’s V
IN
and SW pins, the catch diode
(D
CATCH
), and the input capacitor (C
IN
). The loop formed by
these components should be as small and low inductance
as possible. 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. The SW and BST nodes should be as
small as possible to minimize the capacitive coupling on
these nodes to any fixed voltage like GND or V
IN
. Finally,
keep the VC, RT, RLIM and RCBL nodes small so that the
ground traces will shield them from the SW and BST nodes.
