Datasheet

LT3761
13
3761f
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:
I
LED
=
V
CTRL
−
100mV
R
LED
• 4
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
LED
is ~98% of
the equation’s estimate. Some values are listed in Table 1.
Table 1. (ISP-ISN) Threshold vs CTRL
V
CRTL
(V) (ISP-ISN) Threshold (mV)
1.0 225
1.05 236
1.1 244.5
1.15 248.5
1.2 250
When CTRL is higher than 1.2V, the LED current is regu-
lated to:
I
LED
=
250mV
R
LED
The CTRL pin should not be left open (tie to V
REF
if not
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
IN
to reduce
output power and switching current when V
IN
is low.
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
C
pin filters 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
FB
LT3761
V
OUT
R4
3761 F02
R3
Figure 2. Feedback Resistor Connection for
Boost or SEPIC LED Driver
FB
LT3761
V
OUT
R4
100k
3761 F03
R3
LED
ARRAY
R
SEN(EXT)
C
OUT
+
–
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:
V
OUT
= 1.25 •
R3 +R4
R4
Figure 3. Feedback Resistor Connection for
Buck Mode or Buck-Boost Mode LED Driver
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
OUT
= V
BE
+ 1.25 •
R3
R4