LT3965/LT3965-1
10
39651fa
For more information www.linear.com/LT3965
APPLICATIONS INFORMATION
OVERVIEW
The LT3965/LT3965-1 is an 8-channel LED bypass switch-
ing device
with I
2
C serial interface, designed for dimming
LED strings using a common current source. Each of
the eight channels can be independently programmed to
bypass the LED string in constant on or off, or dimming
with or without fade transition. Operation can be best
understood by referring to the block diagram in Figure 1.
The LT3965/LT3965-1 operates over the V
DD
input sup-
ply range of 2.7V to 5.5V. The eight channel switches are
powered
by the V
IN
input supply and can be connected in
parallel and/or in series. Each of the eight channel switches
can bypass one or more LEDs up to 17V in a string.
Each channel has an LED fault detector which can be pro
-
grammed to detect
an open LED fault at one of the four
threshold levels: 4.5V, 9V (default setting of LT3965-1),
13.5V and 18V (default setting of LT3965). If EN/UVLO is
high, when an open LED fault is detected in a channel, the
channel switch will be turned on to bypass the faulty LED to
maintain the continuity of the string and for
self protection
.
The PWM dimming for this channel is interrupted until reset
by the serial interface. With a proper LED reference voltage
(<4V) applied to the LEDREF pin, each channel LED fault
detector can be programmed to detect a single-shorted LED
fault (default setting), a 2-shorted LED fault, a 3-shorted LED
fault or a 4-shorted LED fault in a multi-LED segment. When
a shorted LED fault is detected in a channel, the channel
switch will continue with the programmed PWM dimming.
Besides LED faults, the LT3965/LT3965-1 also detects and
reports an overheat fault condition (≥170°C). The LT3965/
LT3965-1 asserts (pulls down) the ALERT pin to interrupt
the bus master when an LED fault and/or an overheat fault
is detected. The master can use the alert response address
(ARA) to determine which device is sending the alert.
The LT3965/LT3965-1 I
2
C serial interface contains nine
command registers for configuring channel switches and
LED fault detectors. It also contains two read-only fault
status registers for reporting the LED and overheat faults.
The I
2
C serial interface supports random addressing of
any register. The LT3965/LT3965-1 address select pins
ADDR4, ADDR3, ADDR
2 and ADDR1
allow up to 16 LT3965/
LT3965-1 devices to share the I
2
C bus.
If a resistor is connected between the RTCLK pin and the
ground, the internal oscillator is chosen and the LED dimming
frequency is set by the resistor. If the RTCLK pin is driven by
an external clock source, the external clock source is used to
override the internal oscillator and the dimming frequency
equals the external clock frequency divided by 2048.
DIFFERENCES BETWEEN LT3965 AND LT3965-1
The LT3965 and the LT3965-1 have different default
command register values, which result in different initial
switch states and different open LED threshold settings after
POR (Power On Reset). Otherwise they are the same (see
Table 1 and Table 2 for default register values).
Details of the LT3965/LT3965-1 operation are found in
the following sections.
EN/UVLO SHUTDOWN
The EN/UVLO pin resets the internal logic and controls
whether the LT3965/LT3965-1 is enabled or is in shutdown
state. The LT3965/LT3965-1 indicates that the part is in
shutdown state by setting all OLFREG and SLFREG register
bits high and deasserting the ALERT pin. In the shutdown
state, the serial interface is alive as long as
V
DD
is applied.
Any data written while EN/UVLO is low will be reset when it
transitions high. The eight channel switches are off and the
alert function is disabled in shutdown condition. Because
V
IN
must be at least 7.1V higher than the channel source
voltage for proper channel switch bypass operation, it is
recommended to enable the IC when V
IN
is at least 7.1V
higher than V
LED
+
. The PNP based level shifter shown in
Figure 1 can be used to generate EN/UVLO input signal. A
micropower 1.24V reference, a comparator and controllable
current source, I
S1
, allow the user to accurately program
the V
IN
– V
LED
+
voltage at which the IC turns on and off
(see Figure 1). When EN/UVLO is above 0.7V, and below
the 1.24V threshold, the small pull-down current source,
I
S1
, (typical 2.7μA) is active. The purpose of this current
is to allow the user to program the rising hysteresis.
The typical falling threshold voltage and rising threshold
voltage can be calculated by the following equations:
V
IN
– V
LED
+
)
(FALLING)
=1.24•
R2
+ V
BE
V
IN
– V
LED
+
)
(RISING)
= 2.7µA •R1+(V
IN
– V
LED
+
)
(FALLING)
