LT3640IUFD View Datasheet(PDF) - Linear Technology
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LT3640IUFD Datasheet PDF : 24 Pages
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Applications Information
In the event of VIN undervoltage lockout, VIN 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 VIN2 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 VFB1 out of regulation, the VIN2 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 VIN2 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).
LT3640
EN
2V/DIV
VOUT1
2V/DIV
VOUT2
1V/DIV
PGOOD
2V/DIV
VIN = 12V
RT SET = 2MHz
500µs/DIV
3640 F04
Figure 4. Soft-Start of LT3640
SW1
10V/DIV
IL1
0.5A/DIV
VIN = 30V
VOUT1 = SHORT
1µs/DIV
3640 F05
Figure 5. The High Voltage Channel Reduces Frequency
to Protect Against Shorted Output With 30V Input
SW2
2V/DIV
IL2
1A/DIV
VIN2 = 5V
VOUT2 = SHORT
1µs/DIV
3640 F06
Figure 6. The Low Voltage Channel Operates in
Pulse-Skipping Mode to Protect Against Shorted Output
3640f
17
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