Datasheet

LT3685
15
3685fb
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
Synchronizing the LT3685 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 LT3685 may be synchronized over a 250kHz to 2MHz
range. The R
T
resistor should be chosen to set the LT3685
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 LT3685 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.
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 LT3685, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3685,
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.
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3685 F05
LOAD CURRENT (A)
1
INPUT VOLTAGE (V)
4.0
4.5
5.0
10000
3.5
3.0
2.0
10 100 1000
1 1000010 100 1000
2.5
6.0
5.5
TO START
(WORST CASE)
TO RUN
LOAD CURRENT (A)
INPUT VOLTAGE (V)
5.0
6.0
7.0
4.0
2.0
3.0
8.0
TO RUN
V
OUT
= 3.3V
T
A
= 25°C
L = 8.2μH
f = 700kHz
V
OUT
= 5V
T
A
= 25°C
L = 8.2μH
f = 700kHz
TO START
(WORST CASE)
APPLICATIONS INFORMATION
Figure 6. To Soft-Start the LT3685, Add a Resisitor
and Capacitor to the RUN/SS Pin
3685 F06
I
L
1A/DIV
V
RUN/SS
2V/DIV
V
OUT
2V/DIV
RUN/SS
GND
RUN
15k
0.22μF
2ms/DIV