Datasheet

LT3640
17
3640f
In the event of V
IN
undervoltage lockout, V
IN
overvoltage
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
the FB1 pin reaches 1.165V, 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 V
FB1
out of regulation, the V
IN2
pin falling
below 2.2V, or the EN pin going low, 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 4 shows the soft-start for a 3.3V and 1.8V
application.
Shorted-Output Protection
If an inductor is chosen that will not saturate excessively,
the LT3640 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 5 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 in this cycle (similar to
pulse-skipping operation). The inductor valley current is
kept below the NMOS current limit to ensure robustness
in shorted output condition (Figure 6).
Figure 4. Soft-Start of LT3640
Figure 5. The High Voltage Channel Reduces Frequency
to Protect Against Shorted Output With 30V Input
Figure 6. The Low Voltage Channel Operates in
Pulse-Skipping Mode to Protect Against Shorted Output
applicaTions inForMaTion
500µs/DIV
V
OUT2
1V/DIV
PGOOD
2V/DIV
V
OUT1
2V/DIV
EN
2V/DIV
3640 F04
V
IN
= 12V
R
T
SET = 2MHz
1µs/DIV
SW1
10V/DIV
I
L1
0.5A/DIV
3640 F05
V
IN
= 30V
V
OUT1
= SHORT
1µs/DIV
SW2
2V/DIV
I
L2
1A/DIV
3640 F06
V
IN2
= 5V
V
OUT2
= SHORT