Datasheet

LT3692
25
3692fa
For more information www.linear.com/3692
applicaTions inForMaTion
Shorted and Reverse Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3692 step-down regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
L
T3692
is absent. This may occur in battery charging
applications or in battery back-up systems where a battery
or some other supply is diode OR-ed with the LT3692’
s
output. If the V
IN1/2
pin is allowed to float and the SHDN
pin is held high (either by a logic signal or because it is
tied to V
IN
), then the LT3692’s internal circuitry will pull its
quiescent current through its SW pin. This is fine if your
system can tolerate a few mA in this state. If you ground
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the V
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT3692 can
pull large currents from the output through the SW pin
and the V
IN1/2
pin. Figure 19 shows a circuit that will run
only when the input voltage is present and that protects
against a shorted or reversed input.
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board (PCB) layout. Figure 20
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 circuit
board and their connections should be made on that layer.
Place a local, unbroken ground plane below these com
-
ponents, and tie this ground plane to system ground at
one location, ideally at the ground terminal of the output
capacitor C2. 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
LT3692 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
LT3692. 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. With these steps, the thermal resis
-
tance from die (or junction) to ambient can be reduced to
θ
JA
= 35°C/W.
Figure 19. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output
Figure 20. Subtracting the Current when the Switch is On (20a) from the Current when the Switch is Off (20b) Reveals the Path of the
High Frequency Switching Current (20c). Keep this Loop Small. The Voltage on the SW and BST Traces will Also Be Switched; Keep
These Traces as Short as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane
V
IN
GND
(20a)
LT3692
SW
V
IN
GND
(20c)
LT3692
SW
3692 F20
V
IN
GND
(20b)
LT3692
SW
V
IN
V
IN1/2
V
OUT1/2
SW
LT3692
D4
PARASITIC DIODE
3692 F19