lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 20

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3695
Soft-Start
The RUN/SS pin can be used to soft-start the LT3695,
reducing the maximum input current during start-up. The
RUN/SS pin is driven through an external RC network to
create a voltage ramp at this pin. Figure 6 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 7.5μA when the
RUN/SS pin reaches 2.5V. For fault tolerant applications,
see the discussion of the RUN/SS resistor in the Fault
Tolerance section.
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 oscillator of the LT3695 to an external
frequency 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 LT3695 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 maximum load current that the part can supply is
reduced when a clock signal is applied to SYNC.
20
0.22μF
RUN
15k
Figure 6. To Soft-Start the LT3695, Add a
Resistor and Capacitor to the RUN/SS Pin
RUN/SS
GND
V
5V/DIV
RUN/SS
5V/DIV
5V/DIV
1A/DIV
V
V
RUN
OUT
I
L
5ms/DIV
3695 F05
The LT3695 may be synchronized over a 300kHz to 2.2MHz
range. The R
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal is 360kHz,
the R
safe operation the LT3695 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
section for more information. It is also important to note
that slope compensation is set by the R
subharmonic oscillations, calculate the minimum inductor
value using the frequency determined by R
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, the LT3695 will tolerate a shorted output. When
operating in short-circuit condition, the LT3695 will reduce
its frequency until the valley current is at a typical value of
1.6A (see Figure 7). There is another situation to consider
in systems where the output will be held high when the
input to the LT3695 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
T
500mA/DIV
should be chosen for 300kHz. To assure reliable and
Figure 7. The LT3695 Reduces its Frequency to
Protect Against Shorted Output with 36V Input
20V/DIV
V
SW
0A
I
T
L
resistor should be chosen to set the LT3695
T
resistor. See the Inductor Selection
2μs/DIV
3695 F07
T
value; to avoid
T
0V
.
3695f