LT3790 datasheet(16/30 Pages) LINER

16 / 30 page

LT8390A

8390afa

For more information www.linear.com/LT8390A

OPERATION

Start-Up and Fault Protection

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

LT8390A. 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 LT8390A wait 10µs so that the SS pin can

be fully discharged to ground. After the 10µs, the LT8390A

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.

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

Figure 8. Start-Up and Fault Sequence

above 0.25V, the LT8390A 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 LT8390A back into the POR

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

the ISOC signal, the LT8390A 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 LT8390A 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 LT8390A

enters the OK/RUN state, where the output short detec-

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

When the output short happens, the LT8390A enters

the FAULT/RUN state, where a 1.25µA pull-down current

slowly discharges the SS pin with the other conditions the

same as the OK/RUN state. Once the SS pin is discharged

below 1.7V, the LT8390A enters the DOWN/STOP state,

where the switching is disabled and the short detection is

deactivated with the previous fault latched. Once the SS

pin is discharged below 0.2V and the LOADON signal is

still high, the LT8390A goes back to the UP/RUN state.

In an output short condition, the LT8390A 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 LT8390A 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 LT8390A will latch off until the

EN/UVLO is toggled. With a 100kΩ resistor, the LT8390A

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

8390a F08

LT3790 датащи(PDF) 16 Page - Linear Technology

LT3790 датащи(HTML) 16 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

датащи скачать

ссылки URL