LT3592
APPLICATIONS INFORMATION
LT3592
I
L
500mA/DIV
RUN
SHDN
GND
V
SW
10V/DIV
3592 F07a
V
OUT
5V/DIV
50μs/DIV
RUN
15k
LT3592
SHDN
GND
I
L
500mA/DIV
0.1μF
V
SW
10V/DIV
3592 F07b
V
OUT
5V/DIV
50μs/DIV
Figure 7. To Soft-Start the LT3592, Add a Resistor and Capacitor to the SHDN Pin
Shorted and Open LED Protection
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.
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
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
V . Program the desired open circuit voltage to a value
IN
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
LT3592’s output. If the V pin is allowed to float and the
IN
SHDN pin is held high (either by a logic signal or because
it is tied to V ), then the LT3592’s internal circuitry will
IN
draw its quiescent current through its SW pin. This is fine
if the system can tolerate a few mA in this state. If you
3592fc
16
Linear Systems [ Linear Systems ]
