LT3641
17
3641fa
Soft-Start
The LT3641 has a soft-start pin for each channel. The
feedback pin voltage is regulated to the lower of the
corresponding SS pin and the internal references, which
is 1.265V for the high voltage channel, and 600mV for the
low voltage channel. A capacitor from the SS pin to ground
is charged by an internal 2μA current source resulting in an
output ramping linearly from 0V to the regulated voltage.
The duration of the ramp is:
t
SS1
= C
SS1
•
1.265V
2μA
t
SS2
= C
SS2
•
600mV
2μA
where t
SS1
is the ramping time for the SS1 pin, t
SS2
is
the ramping time for the SS2 pin, C
SS1
is the capacitance
from the SS1 pin to ground, and C
SS2
is the capacitance
from the SS2 pin to ground.
At power-up, a latch is set to discharge the SS1 pin.
After the SS1 pin is discharged to below 100mV, the latch
is reset. The internal 2μA current source starts to charge
the SS1 pin when the (BST-SW) voltage is charged to
above 2V.
In the event of V
IN
undervoltage lockout, or the EN/UVLO
pin being driven below 1.26V, the soft-start latch is set,
triggering a start-up sequence.
A latch is set to discharge the SS2 pin at power-up. After
EN/UVLO is enabled, the V
IN2
voltage is above 2.3V, the
EN2 pin is enabled, and the SS2 pin is below 100mV, the
latch is reset. The internal 2μA current source starts to
charge the SS2 pin.
In the event of the V
IN2
pin falling below 2.2V, or the EN2
pin going below its threshold, the SS2 discharging latch
is set, triggering a start-up sequence.
The SS pins can also be pulled up by external current
sources or resistors for output tracking. The external pull-
up current should not exceed 100μA for either SS pin.
Figure 3 shows the soft-start for a 3.3V and 1.8V
application.
Figure 3. Soft-Start of LT3641
APPLICATIONS INFORMATION
500μs/DIV
V
OUT2
1V/DIV
PGOOD2
2V/DIV
V
OUT1
2V/DIV
EN/UVLO
2V/DIV
3641 F04
V
IN
= 12V
R
T
SET = 2MHz
EN2 TIED TO FB2
Shorted-Output Protection
If an inductor is chosen that will not saturate excessively,
the LT3641 will tolerate a shorted output. For the high
voltage channel, the DA current comparator extends the
internal oscillator period until the catch diode current is
below its limit. Both the top switch and the DA comparator
have current foldback to help limit load current when the
output is shorted to ground. The DA current limit is 1.7A
when the FB1 voltage is above 0.2V, and is 1A when the
FB1 voltage is below 0.2V. Figure 4 shows the high voltage
channel operation under shorted output.
Because of the low V
IN2
voltage, the low voltage channel
does not have current foldback. The low voltage channel
does not extend the internal oscillator in shorted output
condition allowing the high voltage channel to operate
in constant frequency. If the bottom MOSFET current
exceeds the NMOS current limit at the start of a clock
cycle, the top MOSFET is kept off until the overcurrent
situation clears. The inductor valley current is kept below
the NMOS current limit to ensure robustness in shorted
output condition (Figure 5).
