LT3592

16

3592fc

APPLICATIONS INFORMATION

Shorted and Open LED Protection

In case of a shorted LED string or the OUT pin being

shorted to ground by any means, the current loop will

help to limit the output current for many conditions, but

the switch current may still reach the switch current limit

on some cycles despite the actions of the current loop.

For some conditions (especially cold), the output current

for shorted OUT will only be limited by the switch current

limit (which can be as high as 1.5A) and the switching

frequency foldback that occurs when OUT is close to

ground, and the current control loop will have little to

no effect. The total power dissipation will be quite low

in either case due to the frequency foldback and the fact

that the small current sense resistor will effectively be the

output load for shorted OUT. Peak switch and inductor

currents will be high, but the peaks will be brief and well

separated due to the lowered operating frequency. The

main concern in this condition is that the output inductor

not saturate and force the switch into an unsafe operating

condition of simultaneous high current and high voltage

drop. If the current sense resistor between CAP and OUT

becomes shorted or the CAP pin is shorted to ground, the

peak output current will be limited by the internal switch

current limit, which could be as high as 1.5A.

If an LED goes open circuit, the voltage control loop

through the R1-R2 resistor divider to FB will take control

and prevent the output voltages from flying up close to

below the absolute maximum for the CAP and OUT pins

but well above the maximum possible forward drop of the

LED at the programmed BRIGHT current.

Reversed Input Protection

In some systems, the output will be held high when the

input to the LT3592 is absent. This may occur in battery

charging applications or in battery backup systems where

a battery or some other supply is diode ORed with the

SHDN pin is held high (either by a logic signal or because

draw its quiescent current through its SW pin. This is fine

if the system can tolerate a few mA in this state. If you

RUN

10V/DIV

5V/DIV

50μs/DIV

500mA/DIV

10V/DIV

5V/DIV

50μs/DIV

500mA/DIV

SHDN

GND

LT3592

LT3592

3592 F07a

RUN

15k

0.1μF

SHDN

GND

3592 F07b

Figure 7. To Soft-Start the LT3592, Add a Resistor and Capacitor to the SHDN Pin