LT3755 LINER [Linear Technology], LT3755 Datasheet - Page 12

• Manufacturer Part Number: LT3755
• Description: 40VIN, 75VOUT LED Controller
• Manufacturer: LINER [Linear Technology]

LT3755/LT3755-1
APPLICATIONS INFORMATION
When V
regulated to:
The LED current programming feature can increase total
dimming range by a factor of 10. The CTRL pin should
not be left open (tie to V
can also be used in conjunction with a thermistor to
provide overtemperature protection for the LED load, or
with a resistor divider to V
switching current when V
varying differential voltage signal (ripple) across ISP and
ISN at the switching frequency is expected. The amplitude
of this signal is increased by high LED load current, low
switching frequency and/or a smaller value output fi lter
capacitor. Some level of ripple signal is acceptable: the
compensation capacitor on the VC pin fi lters the signal so
the average difference between ISP and ISN is regulated
to the user-programmed value. Ripple voltage amplitude
(peak-to-peak) in excess of 20mV should not cause mis-
operation, but may lead to noticeable offset between the
average value and the user-programmed value.
Programming Output Voltage (Constant Voltage
Regulation) or Open LED/Overvoltage Threshold
For a boost application, the output voltage can be set by
selecting the values of R1 and R2 (see Figure 2) according
to the following equation:
12
I
V
LED
OUT
=
CTRL
Figure 2. Feedback Resistor Connection for
Boost or SEPIC LED Driver
=
100
1 25
R
.
LED
mV
is higher than 1.1V, the LED current is
•
R
1
R
LT3755
+
2
R
2
REF
IN
FB
IN
is low. The presence of a time
to reduce output power and
if not used). The CTRL pin
V
OUT
R1
R2
37551 F02
For a boost type LED driver, set the resistor from the output
to the FB pin such that the expected V
operation will not exceed 1.1V. For an LED driver of buck or
a buck-boost confi guration, the output voltage is typically
level-shifted to a signal with respect to GND as illustrated
in Figure 3. The output can be expressed as:
ISP/ISN Short-Circuit Protection Feature
The ISP and ISN pins have a protection feature indepen-
dent of the LED current sense feature that operates at
ISN below 3V. The purpose of this feature is to provide
continuous current sensing when ISN is below the LED
current sense common mode range (during start-up or
an output short-circuit fault) to prevent the development
of excessive switching currents that could damage the
power components. The action threshold (150mV, typ) is
above the default LED current sense threshold, so that no
interference will occur over the ISN voltage range where
these two functions overlap. This feature acts in the same
manner as SENSE current limit — it prevents GATE from
going high (switch turn-on) until the ISP/ISN difference
falls below the threshold.
Dimming Control
There are two methods to control the current source for
dimming using the LT3755. One method uses the CTRL
pin to adjust the current regulated in the LEDs. A second
method uses the PWM pin to modulate the current source
V
OUT
Figure 3. Feedback Resistor Connection for
Buck Mode or Buck-Boost Mode LED Driver
=
LT3755
V
BE
+ 1 25
FB
.
•
R
R
R1
R2
2
1
+
–
V
OUT
100k
R
FB
SEN(EXT)
37551 F03
during normal
LED
ARRAY
37551fa