LT3640EFE LINER [Linear Technology], LT3640EFE Datasheet - Page 17

• Manufacturer Part Number: LT3640EFE
• Description: Dual Monolithic Buck Regulator with Power-On Reset and Watchdog Timer
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
In the event of V
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
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
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 1.35A
when the FB1 voltage is above 0.2V, and is 0.8A when the
FB1 voltage is below 0.2V. Figure 5 shows the high voltage
channel operation under shorted output.
Because of the low V
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
1.1A 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 1.1A to ensure
robustness in shorted output condition (Figure 6).
IN
FB1
undervoltage lockout, V
out of regulation, the V
IN2
voltage, the low voltage channel
IN2
voltage is above 2.3V,
IN
IN2
overvoltage
pin falling
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
0.5A/DIV
10V/DIV
PGOOD
2V/DIV
2V/DIV
1V/DIV
2V/DIV
2V/DIV
1A/DIV
V
V
OUT1
OUT2
SW1
SW2
EN
I
I
L1
L2
V
R
V
V
V
V
IN
IN
OUT1
IN2
OUT2
T
Figure 4. Soft-Start of LT3640
SET = 2MHz
= 12V
= 30V
= 5V
= SHORT
= SHORT
500μs/DIV
1μs/DIV
1μs/DIV
3640 F04
3640 F05
3640 F06
LT3640
17
3640p