LT3693
15
3693f
Synchronization
Synchronizing the LT3693 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 LT3693 may be synchronized over a 250kHz to 2MHz
range. The R
T
resistor should be chosen to set the LT3693
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 LT3693 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 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 signifi cantly reduced which may
require a larger inductor value to prevent subharmonic
oscillation.
The minimum operating voltage of an LT3693 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3693 is turned on with its RUN/SS pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 5 shows a plot
of minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
start. The plots show the worst-case situation where V
IN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V
IN
; however, this restricts the
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above V
OUT
. At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3693, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3693,
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 6 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.
APPLICATIONS INFORMATION
Figure 6. To Soft-Start the LT3693, Add a Resisitor
and Capacitor to the RUN/SS Pin
3693 F07
I
L
1A/DIV
V
RUN/SS
2V/DIV
V
OUT
2V/DIV
RUN/SS
GND
RUN
15k
2ms/DIV
