LT3684
APPLICATIONS INFORMATION
Soft-Start
D4
MBRS140
The RUN/SS pin can be used to soft-start the LT3684,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
create a voltage ramp at this pin. Figure 7 shows the start-
up and shut-down waveforms with the soft-start circuit.
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
oftheresistorsothatitcansupply20µAwhentheRUN/SS
pin reaches 2.3V.
V
V
BOOST
SW
IN
IN
LT3684
V
RUN/SS
OUT
V
C
GND FB
BACKUP
3684 F07
Figure 7. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output. It Also
Protects the Circuit from a Reversed Input. The LT3684
Runs Only When the Input is Present
I
L
LT3684 can pull large currents from the output through
1A/DIV
RUN
15k
the SW pin and the V pin. Figure 7 shows a circuit that
IN
will run only when the input voltage is present and that
protects against a shorted or reversed input.
V
RUN/SS
GND
RUN/SS
2V/DIV
0.22µF
V
OUT
2V/DIV
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 8 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
3481 F06
2ms/DIV
Figure 6. To Soft-Start the LT3684, Add a Resisitor
and Capacitor to the RUN/SS Pin
currents flow in the LT3684’s V and SW pins, the catch
IN
Shorted and Reversed Input Protection
diode (D1) and the input capacitor (C1). The loop formed
bythesecomponentsshouldbeassmallaspossible.These
components,alongwiththeinductorandoutputcapacitor,
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 BOOST nodes should be as small as possible.
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3684 buck 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
LT3684 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3684’s
Finally, keep the FB and V nodes small so that the ground
C
traces will shield them from the SW and BOOST nodes.
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 LT3684 to additional ground planes within the circuit
board and on the bottom side.
output. If the V pin is allowed to float and the RUN/SS
IN
pin is held high (either by a logic signal or because it is
tied to V ), then the LT3684’s internal circuitry will pull
IN
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 RUN/SS pin, the SW pin current will drop to
essentially zero. However, if the V pin is grounded while
the output is held high, then parasitic diodes inside the
IN
3684f
16
Linear [ Linear ]
