Datasheet

LT3745-1
18
37451f
down to 4.2V to maintain the minimum active LED pin
voltage equal to 0.7V again. Similarly, at the next time
instant t
2
, the output LED bus voltage is tracked down
to 3.8V. In this manner, the adaptive-tracking technique
eliminates unnecessary power dissipation across the
current sinks and yields superior system efficiency when
compared to a constant 4.6V output voltage.
At a later time instant t
3
when the PRECHG signal goes
high, the amplifier G
M2
is disabled and gives the control
back to the amplifier G
M1
. The amplifier GM1 regulates
the output LED bus voltage towards the FB programmed
maximum value 4.6V to guarantee shorter minimum
LED on-time for the next grayscale PWM dimming
OPERATION
cycle. Without the precharging phase, the output LED
bus voltage will stay at 3.8V before the next grayscale
PWM dimming cycle, when all the 3 LED channels will be
turned on again. At that time the 3.8V LED bus voltage
is too low to keep all the LED channels in regulation,
and the minimum LED on-time is greatly increased to
accommodate the slow transient response of the switch-
ing buck converter charging the output capacitor from
3.8V to 4.6V. This adaptive-tracking-plus-precharging
LED bus voltage technique simultaneously lowers the
power dissipation in the LT3745-1 and maintains a
shorter minimum LED on-time.
Figure 5. Adaptive-Tracking-Plus-Precharging LED Bus Voltage Technique
PWM
1
PWM
2
PWM
3
PRECHG
IDEAL V
OUT
LT3745-1 V
OUT
CONSTANT V
OUT
4096*T
PWMCK
4.6V
4.6V
4.2V
3.8V
t
1
t
2
t
3
t
4
4.6V
4.2V
3.8V
3.1V
(1)V
OUT
= 4.6V
37451 F05
(2) (3)
1.5V 1.1V 0.7V
–
– –
+
–
+
–
+
+
–
+ +
3.5V 3.9V