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LT3745
OPERATION
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
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
4096*T PWMCK
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
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 and maintains a shorter
minimum LED on-time.
PWM 1
PWM 2
PWM 3
V OUT = 4.6V
3.1V
(1)
+
3.5V
(2)
+
3.9V
(3)
+
PRECHG
IDEAL V OUT
4.6V
4.2V
3.8V
4.6V
4.2V
3.8V
–
+
1.5V
–
+
1.1V
–
+
0.7V
LT3745 V OUT
4.6V
–
–
–
CONSTANT V OUT
3745 F05
t 1
t 2
t 3 t 4
Figure 5. Adaptive-Tracking-Plus-Precharging LED Bus Voltage Technique
3745f
18
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