Datasheet

LT3745-1
17
37451f
LED bus voltage and should be programmed high enough
to supply the worst-case LED string across temperature,
current, and manufacturing variation.
Adaptive-Tracking-Plus-Precharging
Higher system efficiency and faster transient re-
sponse are two highly anticipated specifications in an
individually-modulated multi-channel LED driver chip.
The LT3745-1 uses a patent pending adaptive-tracking-
plus-precharging technique to achieve both of them
simultaneously.
Besides 16 internal
grayscale PWM dimming signals,
the part also generates another internal precharging
signal PRECHG. As shown in Figure 4, the PRECHG
signal divides any grayscale PWM dimming cycle
into two phases: tracking phase when PRECHG = 0
and precharging phase when PRECHG = 1. During
each grayscale PWM dimming cycle – 4096 PWMCK
clock cycles, the PRECHG signal stays low for the first
3584 clock cycles (7/8 of the grayscale PWM dimming
period) and goes high
for the rest 512 clock cycles (1/8
of the grayscale PWM dimming period). In the event of
all the LED channels being not
active (i.e., either fault
or off) before the 3585th PWMCK clock, the PRECHG
signal will go high immediately.
To better explain the operation of the adaptive-tracking-
plus-precharging technique, a simplified application
system with 3-channel LED array is presented in Fig-
ure 5. Each channel
consists of a single LED with the
forward voltage drop equal to 3.1V, 3.5V, and 3.9V,
respectively. Three internal grayscale PWM dimming
signals PWM1, PWM2, and PWM3 are used to modulate
each LED channel.
At the beginning of each grayscale PWM dimming
cycle, all three LED channels are turned on and the
tracking phase starts with PRECHG = 0. The amplifier
G
M2
is enabled and takes the control from the amplifier
G
M1
, regulating the minimum active LED pin voltage to
0.7V. With the V
LED3
equal to 0.7V, the output LED bus
voltage is tracked to 4.6V. Subsequently, at a certain
time instant t
1
when the third channel is turned off, the
minimum active LED pin voltage goes to V
LED2
, 1.1V.
Then the amplifier G
M2
tracks the output LED bus voltage
OPERATION
Figure 4. Grayscale PWM Dimming and Precharging Signal Timing Chart
PWMCK 40964095 1 235841 2 3
C1/EN
I(LED00)
REG = 0x002
REG = 0xFFF
TRACKING PHASE PRECHARGING PHASE
REG = 0x000
I(LED01)
t
WH-PWM
t
WL-PWM
t
PD-PWM
I(LED15)
PRECHG
3585
37451 F04