LT8253/LT8253A

11

Rev. 0

For more information www.analog.com

OPERATION

Start-Up and Fault Protection

Figure 6 shows the start-up and fault sequence for the

LT8253/LT8253A. 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 LT8253/LT8253A wait 10µs so

that the SS pin can be fully discharged to ground. After

the 10µs, the LT8253/LT8253A enter the UP/PRE state

when the VOUTEN signal goes high.

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 LT8253/

LT8253A enter the UP/TRY state. After 10µs in the UP/

TRY state, the LT8253/LT8253A enter the UP/RUN state.

POR

• SS hard pull down

• Switching disabled

• No short detection

INIT

• SS hard pull down

• Switching disabled

• No short detection

POR = 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 and

VOUTEN = HI

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

VOUTEN = HI

SHORT

SS < 1.7V

Figure 6. Start-Up and Fault Sequence

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

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

than 1V, the LT8253/LT8253A regulate 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 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 above, the output voltage

Once the SS pin is charged above 1.75V, the LT8253/

LT8253A enter the OK/RUN state, where the output

< 0.25V. When the output short happens, the LT8253/

LT8253A enter 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 LT8253/LT8253A enter 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 VOUTEN signal is still high, the LT8253/LT8253A go

back to the UP/RUN state.

In an output short condition, the LT8253/LT8253A can

be set to hiccup, latch-off, or keep-running fault pro-

pins. Without any resistor, the LT8253/LT8253A 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 resis-

tor, the LT8253/LT8253A will latch off until the EN/UVLO

is toggled. With a 100kΩ resistor, the LT8253/LT8253A

will keep running regardless of the fault.The front page

shows a typical LT8253/LT8253A application circuit. This

Applications Information section serves as a guideline of

selecting external components for typical applications. The

examples and equations in this section assume continuous

conduction mode unless otherwise specified.

Switching Frequency Selection

The LT8253/LT8253A use a constant frequency control

scheme, 150kHz-650kHz for LT8253 and 600kHz-2MHz