LT3992EFE View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT3992EFE' PDF : 36 Pages View PDF
LT3992
Applications Information
Shorted and Reverse Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3992 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
LT3992 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 LT3992’s
output. If the VIN1/2 pin is allowed to float and the SHDN
pin is held high (either by a logic signal or because it is
tied to VIN), then the LT3992’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 VIN pin is grounded while the output
is held high, then parasitic diodes inside the LT3992 can
pull large currents from the output through the SW pin
and the VIN1/2 pin. Figure 17 shows a circuit that will run
only when the input voltage is present and that protects
against a shorted or reversed input.
PARASITIC DIODE
D4
VIN1/2
VIN
SW
VOUT1/2
LT3992
3992 F17
Figure 17. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output
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.
VIN LT3992 SW
GND
(18a)
VIN LT3992 SW
GND
(18b)
VIN LT3992 SW
GND
(18c)
3992 F18
Figure 18. Subtracting the Current When the Switch Is On (18a)
from the Current When the Switch Is Off (18b) Reveals the Path of
the High Frequency Switching Current (18c). 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
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
Thermal Considerations
The PCB must also provide heat sinking to keep the LT3992
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 LT3992.
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.
3992fa
For more information www.linear.com/LT3992
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