LT3758 datasheet(15/28 Pages) AD

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LT8392

Rev. 0

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OPERATION

Start-Up and Fault Protection

Figure 8 shows the start-up and fault sequence for the

LT8392. During the POR state, the SS pin is hard pulled

down with a 100Ω to ground. In a pre-biased condition,

the SS pin has to be pulled below 0.2V to enter the INIT

state, where the LT8392 wait 10µs so that the SS pin can

be fully discharged to ground. After the 10µs, the LT8392

enters the UP/PRE state.

During the UP/PRE state, the SS pin is charged up by a

12.5µA pull-up current while the switching is disabled.

Once the SS pin is charged above 0.25V, the LT8392

enters the UP/TRY state. After 10µs in the UP/TRY state,

the LT8392 enters the UP/RUN state.

Figure 8. Start-Up and Fault Sequence

During the UP/RUN state, the switching is enabled and

the start-up of the output voltage VOUT is controlled by

the voltage on the SS pin. When the SS pin voltage is less

than 1V, the LT8392 regulates the FB pin voltage to the SS

pin voltage instead of the 1V reference. This allows the SS

pin to program soft-start by connecting an external capac-

itor from the SS pin to GND. The internal 12.5µA pull-up

current charges up the capacitor, creating a voltage ramp

on the SS pin. As the SS pin voltage rises linearly from

0.25V to 1V (and beyond), the output voltage VOUT rises

smoothly to its final regulation voltage.

Once the SS pin is charged above 1.75V, the LT8392

enters the OK/RUN state, where the output short detection

is activated. The output short means VFB < 0.25V. When

the output short happens, the LT8392 enters the FAULT/

RUN state, where a 1.25µA pull-down current slowly dis-

charges the SS pin with the other conditions the same as

the OK/RUN state. Once the SS pin is discharged below

1.7V, the LT8392 enters the DOWN/STOP state, where

the switching is disabled and the short detection is deac-

tivated with the previous fault latched. Once the SS pin

is discharged below 0.2V, the LT8392 goes back to the

UP/RUN state.

In an output short condition, the LT8392 can be set to

hiccup, latch-off, or keep-running fault protection mode

with a resistor between the SS and VREF pins. Without any

resistor, the LT8392 will hiccup between 0.2V and 1.75V

and go around the UP/RUN, OK/RUN, FAULT/RUN, and

DOWN/STOP states until the fault condition is cleared.

With a 499kΩ resistor, the LT8392 will latch off until the

EN/UVLO is toggled. With a 100kΩ resistor, the LT8392

will keep running regardless of the fault.

POR

• SS hard pull down

• Switching disabled

• No short detection

INIT

• SS hard pull down

• Switching disabled

• No short detection

POR = HI or

ISOC = HI

SS < 0.2V

UP/TRY

• SS 12.5µA pull up

• Switching disabled

• No short detection

UP/PRE

• SS 12.5µA pull up

• Switching disabled

• No short detection

Wait 10µs

SS > 0.25V

UP/RUN

• SS 12.5µA pull up

• Switching enabled

• No short detection

OK/RUN

• SS 12.5µA pull up

• Switching enabled

• Short detection

Wait 10µs

SS > 1.75V

DOWN/STOP

• SS 1.25µA pull down

• Switching disabled

• No short detection

FAULT/RUN

• SS 1.25µA pull down

• Switching enabled

• Short detection

SS < 0.2V and

LOADON = HI

SHORT

SS < 1.7V

8392 F08

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