LT3754
21
3754fc
applicaTions inForMaTion
LED Open Circuit and PWM Dimming Ratios
The LT3754 monitors each LED pin voltage to determine if
the LED string has an open fault (LED pin voltage < 0.5V).
If an open LED fault occurs, the FAULT flag is pulled low.
To avoid false detection of faults during the initial converter
start-up when V
OUT
is low, the LT3754 ignores low LED
pin voltages until V
OUT
reaches 90% of its maximum al-
lowed OVP level. Once this condition is met, the LT3754
monitors all LED pins for open LED faults. To avoid false
detection of faults during PWM dimming edges (where
LED pins can possibly ring and trip fault detection levels)
the LT3754 only monitors/updates fault conditions during
PWM high (and only after a blank duration of 2µs following
each PWM rising edge).
LED Short Circuit
A short circuit fault between the positive terminal of an LED
string (V
OUT
) and the negative terminal of the LED string
(LEDx pin) causes the channel to be disabled in order to
protect the internal current source. A resistive short is
allowed as long as (V
OUT
-V
LEDx
) < 6V. During the short,
however, cable inductance can cause the LED pin voltage
to
overshoot past V
OUT
voltage. To avoid LED pin voltage
exceeding its absolute maximum rated voltage, a diode
may be required to clamp the LED pin. 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.
Loop Compensation
Be sure to check the stability of the loop with the LEDs
connected (LED regulation loop) and disconnected
(Overvoltage Protection (OVP) regulation loop). Various
application circuits are shown in the data sheet which
cover a range of V
IN
, V
OUT
, f
OSC
, output power and inductor
current ripple values. For application requirements which
deviate from the circuits shown in the data sheet be sure
to check the stability of the final application over the full
V
IN
range, LED current range (if analog dimming) and
temperature range. Be aware that if the V
C
pin components
represent the dominant pole for the converter loop and
they have been adjusted to achieve stability, the V
C
pin
might move more slowly during load transient conditions
such
as an all-LEDs-open fault. A slower moving V
C
pin
will add to V
OUT
overshoot during an all-LEDs-open fault.
An alternative compensation approach is to place the
dominant pole of the converter loop at the output. This
requires an increased output capacitor value but will allow
a much reduced Vc capacitor. The combination will allow
V
C
to move more quickly and V
OUT
to move more slowly
resulting in less overshoot during an all-LEDs-open fault.
Thermal Considerations
The internal power dissipation of the LT3754 comes from 3
main sources: V
IN
quiescent current (I
Q
total), V
IN
current
for GATE switching (I
GATE
) and the LT3754 LED current
sources. Since the maximum operational V
IN
voltage is
40V, care should be taken when selecting the switching
frequency and type of external power MOSFET since the
current required from V
IN
for GATE switching is given by,
I
GATE
= f
OSC
• Qg
where Q
g
is the gate charge (at V
GS
= INTV
CC
) specified
for the MOSFET and f
OSC
is the programmed switching
frequency for the LT3754. A low Q
g
MOSFET should al-
ways be used when operating the LT3754 from high V
IN
voltages. The internal junction temperature
of the LT3754
can be estimated as:
T
J
= T
A
+ [V
IN
• (I
QTOTAL
+ (f
OSC
• Q
g
)) + (16 • I(LED
X
) • 1.1V)]
• θ
JA
where, T
A
is the ambient temperature for the LT3754
I
QTOTAL
represents the V
IN
quiescent current for the LT3754
(not switching, PWM = 1.5V and CTRL = 0.1V) - illustrated
in the Typical Characteristics Graphs – plus the base cur-
rents of active channels (typically 16 • I(LED)/75). θ
JA
is
the thermal resistance of the package (34°C/W for the
5mm × 5mm QFN package).
