Datasheet

LT3599
16
3599fe
APPLICATIONS INFORMATION
a soft-start latch is set causing the SS pin to be discharged
(see the Soft-Start Pin Timing Diagram in Figure 11). All
faults are detected internally and do not require external
components. When all faults no longer exist and the SS
pin has been discharged to at least 0.25V, the soft-start
latch is reset and an internal 11μA supply charges the SS
pin. During start-up or recovery from a fault, the SS pin
ramp up controls the ramp up of switch current limit.
Soft-start ramp rate is given by:
ΔV
SS
ΔT
=
I
SS
C
SS
I
SS
=11µA typ
(
)
A 10nF capacitor from the SS pin to ground will therefore
provide a 1V/ms ramp rate on the SS pin.
In addition, during soft-start, switching frequency is
reduced to protect the inductor from high currents.
A useful feature of the LT3599 is that it waits for the fi rst
PWM pin active high (minimum 200ns pulse width) before
it allows the soft-start of V
C
pin to begin. This feature
ensures that during start-up of the LT3599 the soft-start
ramp has not timed out before PWM is asserted high.
Without this ‘wait for PWM high’ feature, systems which
apply PWM after V
IN
and SHDN are valid, can potentially
turn on without soft-start and experience high inductor
currents during wake up of the converter’s output voltage.
It is important to note that when PWM subsequently goes
low, the soft-start ramp is not held at its present voltage
but continues to ramp upwards. If the soft-start ramp
voltage was held every time PWM goes low, this would
Figure 11. Soft-Start Pin Timing Diagram
cause very slow start-up of LED displays for applications
using very high PWM dimming ratios.
OPENLED FLAG
The OPENLED pin is an open-collector output and needs
an external resistor tied to a supply (see Figure 12). If any
LED string is open during normal operation, the OPENLED
pin will be pulled down.
The open LED detection is enabled only when the PWM
signal is enabled. There is a delay for OPENLED fl ag
generation when the PWM signal is enabled to avoid
generating a spurious fl ag signal.
During start-up (see the Operation section), the open LED
detection is disabled.
SHORTLED FLAG
The SHORTLED pin is an open-collector output, and needs
an external resistor tied to a supply (see Figure 12). If any
LED pin is shorted to V
OUT
during normal operation, the
SHORTLED pin will be pulled down. In addition, the shorted
LED string (channel) is immediately disabled, thereby
protecting the LT3599. During the short circuit fault, cable
inductance can cause the LED pin voltage to overshoot
past V
OUT
voltage. To avoid LED pin voltage exceeding it’s
absolute maximum rated voltage, the anode of a 1N4148WS
diode should be connected to the LED pin and the cathode
of the diode connected to the V
OUT
pin.Keep the traces as
short as possible. A Schottky diode should not be used
due to high reverse bias leakage currents.
The short LED detection is enabled only when the PWM signal
is enabled. There is a delay for SHORTLED fl ag generation
when the PWM signal is enabled to avoid spurious signal
being generated.
During start-up, the SHORTLED fl ag is disabled (see the
Operation section).
SW
SS
3599 F12
V
IN
< 2.7V
OR
SHDN < 1.4V
SS < 0.25V
AND
V
IN
> 2.7V
AND
SHDN > 1.4V
AND
PWM > 1V (FOR >200ns)
FAULTS TRIGGERING
SOFT-START LATCH
WITH SW TURNED OFF
IMMEDIATELY:
SOFT-START
LATCH RESET:
SOFT-START
LATCH SET
0.3V (ACTIVE THRESHOLD)
0.25V (RESET THRESHOLD)
0.15V
Figure 12. OPENLED and SHORTLED Connection
3599 F13
LT3599
OPENLED
R1
SHORTLED
R2