LT3755 datasheet(18/32 Pages) AD

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LT8393

Rev. A

For more information www.analog.com

OPERATION

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 LT8393 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 LED string voltage.

Once the SS pin is charged above 1.75V, the LT8393

enters the OK/RUN state, where the LED fault (both open

LED and short LED) detection is activated. The open LED

fault means that VFB > 0.95V and V(ISP-ISN) < 10mV, and

the short LED fault means that VFB < 0.25V. Both the open

LED and short LED faults are combined to the FAULT pin.

When either fault happens, the LT8393 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 LT8393 enters the DOWN/STOP state, where

the switching is disabled and the LED fault detection is

deactivated with the previous fault latched. Once the SS

pin is discharged below 0.2V and the PWMON signal is still

high, the LT8393 goes back to the UP/RUN state.

In an open or short LED condition, the LT8393 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 LT8393 will hiccup with SS pin 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 499k resistor, the LT8393 will latch off until

the EN/UVLO is toggled. With a 100k resistor, the LT8393

will keep running regardless of the fault.

APPLICATIONS INFORMATION

The front page shows a typical LT8393 application circuit.

ThisApplicationsInformationsectionservesasaguideline

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 LT8393 uses a constant frequency control scheme

between 350kHz and 2MHz. Selection of the switching

frequency is a tradeoff between efficiency and component

size. Low frequency operation improves efficiency by

reducing MOSFET switching losses, but requires larger

inductor and capacitor values. For high power applica-

tions, consider operating at lower frequencies to minimize

MOSFET heating from switching losses. For low power

applications, consider operating at higher frequencies to

minimize the total solution size.

Inaddition,thespecificapplicationalsoplaysanimportant

role in switching frequency selection. In a noise-sensitive

system,theswitchingfrequencyisusuallyselectedtokeep

the switching noise out of a sensitive frequency band.

Switching Frequency Setting

The switching frequency of the LT8393 can be set by the

internal oscillator. With the SYNC/SPRD pin pulled to

ground, the switching frequency is set by a resistor from

the RT pin to ground. Table 1 shows RT resistor values

for common switching frequencies.

Table 1. Switching Frequency vs RT Value (1% Resistor)

fOSC (MHz)

RT (k)

0.4

374

0.6

237

0.8

169

1.0

130

1.2

102

1.4

84.5

1.6

71.5

1.8

60.4

2.0

51.1

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