lt3684emse-trpbf Linear Technology Corporation, lt3684emse-trpbf Datasheet - Page 16

• Manufacturer Part Number: lt3684emse-trpbf
• Description: 36v, 2a, 2.8mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3684
Soft-Start
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 fi lter 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
of the resistor so that it can supply 20µA when the RUN/SS
pin reaches 2.3V.
Shorted and Reversed Input Protection
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
output. If the V
pin is held high (either by a logic signal or because it is
tied to V
its quiescent current through its SW pin. This is fi ne 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
the output is held high, then parasitic diodes inside the
0.22µF
16
RUN
15k
Figure 6. To Soft-Start the LT3684, Add a Resisitor
and Capacitor to the RUN/SS Pin
IN
RUN/SS
), then the LT3684’s internal circuitry will pull
GND
IN
pin is allowed to fl oat and the RUN/SS
IN
2ms/DIV
pin is grounded while
3481 F06
I
1A/DIV
V
2V/DIV
V
2V/DIV
L
RUN/SS
OUT
LT3684 can pull large currents from the output through
the SW pin and the V
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 layout. Figure 8 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents fl ow in the LT3684’s V
diode (D1) and the input capacitor (C1). 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 components.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and V
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.
V
IN
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
MBRS140
D4
V
RUN/SS
V
IN
C
LT3684
GND FB
IN
BOOST
pin. Figure 7 shows a circuit that
C
SW
nodes small so that the ground
IN
and SW pins, the catch
3684 F07
V
BACKUP
OUT
3684f