lt3980 Linear Technology Corporation, lt3980 Datasheet - Page 16

• Manufacturer Part Number: lt3980
• Description: 58v, 2a, 2.4mhz Step-down Switching Regulator With 85?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3980
create a voltage ramp at this pin. Figure 7 shows the start-
up and shutdown 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 0.5V (this can be ground or a logic output).
Synchronizing the LT3980 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 LT3980 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will skip pulses to maintain regulation.
The LT3980 may be synchronized over a 150kHz to 2MHz
range. The R
switching frequency 25% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the R
To assure reliable and safe operation the LT3980 will only
synchronize when the output voltage is near regulation
16
T
resistor should be chosen to set the LT3980
T
should be chosen for 200kHz.
0.22μF
RUN
15k
RUN/SS
Figure 7. To Soft-Start the LT3980, Add a
Resistor and Capacitor to the RUN/SS Pin
GND
2ms/DIV
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
the Inductor Selection section. It is also important to note
that slope compensation is set by the R
sync frequency is much higher than the one set by R
slope compensation will be signifi cantly reduced which
may require a larger inductor value to prevent subharmonic
oscillation.
Shorted and Reversed Input Protection
If an inductor is chosen that will not saturate excessively,
an LT3980 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 LT3980
is absent. This may occur in battery charging applications
or in battery backup systems where a battery or some
other supply is diode ORed with the LT3980’s output. If
the V
high (either by a logic signal or because it is tied to V
then the LT3980’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
is held high, then parasitic diodes inside the LT3980 can
pull large currents from the output through the SW pin
IN
pin is allowed to fl oat and the RUN/SS pin is held
3680 F07
I
1A/DIV
V
2V/DIV
V
2V/DIV
L
RUN/SS
OUT
IN
pin is grounded while the output
T
value: When the
T
resistor. See
T
, the
IN
3980f
),