LT3756-1 LINER [Linear Technology], LT3756-1 Datasheet - Page 13

• Manufacturer Part Number: LT3756-1
• Description: 60VIN LED Controller with Internal PWM Generator
• Manufacturer: LINER [Linear Technology]

applicaTions inFormaTion
LED current to zero, although relative accuracy decreases
with the decreasing voltage sense threshold. When the
CTRL pin voltage is less than 1V, the LED current is:
When the CTRL pin voltage is between 1V and 1.2V the LED
current varies with CTRL, but departs from the previous
equation by an increasing amount as the CTRL voltage
increases. Ultimately, the LED current no longer varies for
CTRL ≥ 1.2V. At CTRL = 1.1V, the value of I
the equation’s estimate. Some values are listed in Table 1.
Table 1. (ISP-ISN) Threshold vs CTRL
When CTRL is higher than 1.2V, the LED current is regu-
lated to:
The CTRL pin should not be left open (tie to V
used). The CTRL pin can also be used in conjunction with
a thermistor to provide overtemperature protection for
the LED load, or with a resistor divider to V
output power and switching current when V
The presence of a time 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 filter capacitor. Some level of ripple
signal is acceptable: the compensation capacitor on the
V
ISP and ISN is regulated to the user-programmed value.
Ripple voltage amplitude (peak-to-peak) in excess of
C
I
I
pin filters the signal so the average difference between
LED
LED
=
=
250mV
V
V
R
CTRL
CRTL
1.05
1.15
LED
1.0
1.1
1.2
R
LED
(V)
− 100mV
• 4
(ISP-ISN) Threshold (mV)
244.5
248.5
225
236
250
LED
IN
is ~98% of
IN
to reduce
REF
is low.
if not
50mV should not cause mis-operation, but may lead to
noticeable offset between the current regulation and the
user-programmed value.
Programming Output Voltage (Constant Voltage
Regulation) or Open LED/Overvoltage Threshold
For a boost or SEPIC application, the output voltage can
be set by selecting the values of R3 and R4 (see Figure 2)
according to the following equation:
For a boost type LED driver, set the resistor from the output
to the FB pin such that the expected voltage level during
normal operation will not exceed 1.17V. For an LED driver
of buck mode or a buck-boost mode configuration, 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:
V
V
OUT
OUT
Figure 2. Feedback Resistor Connection for
Boost or SEPIC LED Driver
LT3761
Figure 3. Feedback Resistor Connection for
Buck Mode or Buck-Boost Mode LED Driver
= 1.25 •
= V
BE
FB
+ 1.25 •
R3 + R4
LT3761
R4
R3
R4
R3
R4
FB
+
–
V
OUT
100k
V
OUT
R3
R4
3761 F02
R
SEN(EXT)
LED
ARRAY
3761 F03
LT3761
C
OUT
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