LT3480
16
3480fb
APPLICATIONS INFORMATION
Synchronizing the LT3480 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.3V
and peaks that are above 0.8V (up to 6V).
The LT3480 will not enter Burst Mode at low output loads
while synchronized to an external clock, but instead will
skip pulses to maintain regulation.
The LT3480 may be synchronized over a 250kHz to 2MHz
range. The R
T
resistor should be chosen to set the LT3480
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be 250kHz
and higher, the R
T
should be chosen for 200kHz. To assure
reliable and safe operation the LT3480 will only synchronize
when the output voltage is near regulation as indicated by the
PG fl ag. It is therefore necessary to choose a large enough
inductor value to supply the required output current at the
frequency set by the R
T
resistor. See Inductor Selection sec-
tion. It is also important to note that slope compensation
is set by the R
T
value: When the sync frequency is much
higher than the one set by R
T
, the slope compensation will
be signifi cantly reduced which may require a larger inductor
value to prevent subharmonic oscillation.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively,
an LT3480 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 LT3480
is absent. This may occur in battery charging applications
or in battery backup systems where a battery or some
other supply is diode OR-ed with the LT3480’s output. If
the V
IN
pin is allowed to fl oat and the RUN/SS
pin is held
high (either by a logic signal or because it is tied to V
IN
),
then the LT3480’s internal circuitry will pull 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
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT3480 can
pull large currents from the output through the SW pin
and the V
IN
pin. Figure 8 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 layout. Figure 9 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents fl ow in the LT3480’s V
IN
and SW pins, the catch
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.
Figure 7. To Soft-Start the LT3480, Add a Resisitor
and Capacitor to the RUN/SS Pin
3480 F07
I
L
1A/DI
V
RUN/
2V/DI
V
OUT
2V/DI
RUN/SS
GND
RUN
15k
2ms/DIV
0.22μF
Figure 8. 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 LT3480
Runs Only When the Input is Present
V
IN
BOOST
GND FB
RUN/SS
V
C
SW
D4
MBRS140
V
IN
LT3480
3480 F08
V
OUT
BACKUP
