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LT8390

8390fa

For more information www.linear.com/LT8390

OPERATION

Start-Up and Fault Protection

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

LT8390. 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 LT8390 wait 10µs so that the SS pin can

be fully discharged to ground. After the 10µs, the LT8390

enters the UP/PRE state when the LOADON signal goes

high. The LOADON high signal happens when CTRL pin is

above its rising latch-off thresholds (0.325V typical) and

the LOADEN is high.

Figure 8. Start-Up and Fault Sequence

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

12.5µA pull-up current while the switching is disabled

and the LOADTG is turned off. Once the SS pin is charged

above 0.25V, the LT8390 enters the UP/TRY state, where

the LOADTG is turned on first while the switching is still

disabled. If an excessive current flowing through the

current sense resistor triggers the ISP/ISN over current

(ISOC) signal, it will reset the LT8390 back into the POR

state. After 10µs in the UP/TRY state without triggering

the ISOC signal, the LT8390 enters the UP/RUN state.

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 LT8390 regulates the FB pin voltage to the SS

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

SS pin to be used to program soft-start by connecting an

external capacitor 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 LT8390 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 LT8390 enters the FAULT/RUN

state,wherea1.25µApull-downcurrentslowlydischarges

the SS pin with the other conditions the same as the OK/

RUN state. Once the SS pin is discharged below 1.7V, the

LT8390enterstheDOWN/STOPstate,wheretheswitching

is disabled and the short detection is deactivated with the

previousfaultlatched.OncetheSSpinisdischargedbelow

0.2V and the LOADON signal is still high, the LT8390 goes

back to the UP/RUN state.

In an output short condition, the LT8390 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 LT8390 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 LT8390 will latch off until the

EN/UVLO is toggled. With a 100kΩ resistor, the LT8390

will keep running regardless of the fault.

POR

• SS hard pull down

• Switching disabled

• LOADTG turned off

• No short detection

INIT

• SS hard pull down

• Switching disabled

• LOADTG turned off

• No short detection

POR = HI or

ISOC = HI

SS < 0.2V

UP/TRY

• SS 12.5µA pull up

• Switching disabled

• LOADTG turned on

• No short detection

UP/PRE

• SS 12.5µA pull up

• Switching disabled

• LOADTG turned off

• No short detection

Wait 10µs and

LOADON = HI

SS > 0.25V

UP/RUN

• SS 12.5µA pull up

• Switching enabled

• LOADTG turned on

• No short detection

OK/RUN

• SS 12.5µA pull up

• Switching enabled

• LOADTG turned on

• Short detection

Wait 10µs

SS > 1.75V

DOWN/STOP

• SS 1.25µA pull down

• Switching disabled

• LOADTG turned on

• No short detection

FAULT/RUN

• SS 1.25µA pull down

• Switching enabled

• LOADTG turned on

• Short detection

SS < 0.2V and

LOADON = HI

SHORT

SS < 1.7V

8390 F08

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