Datasheet

LT3480
16
3480fe
For more information www.linear.com/LT3480
APPLICATIONS INFORMATION
Figure 7. To Soft-Start the LT3480, Add a Resisitor
and Capacitor to the RUN/SS Pin
3480 F07
I
L
1A/DIV
V
RUN/SS
2V/DIV
V
OUT
2V/DIV
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
Soft-Start
The RUN/SS pin can be used to soft-start the LT3480,
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
of the resistor so that it can supply 20µA when the RUN/
SS pin reaches 2.5V.
Synchronization
To select Low-Ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
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 flag. 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 section. 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 significantly 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 float 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 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
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.