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LT3746
OPERATION
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
Figure 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 0.5V.
With the V LED3 equal to 0.5V, the output LED bus voltage
is tracked to 4.4V. 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 , 0.9V. Then the
amplifier G M2 tracks the output LED bus voltage down
to 4V to maintain the minimum active LED pin voltage
equal to 0.5V again. Similarly at the next time instant t 2 ,
the output LED bus voltage is tracked down to 3.6V. 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 4.4V 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 4.4V 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 3.6V before the next grayscale
PWM dimming cycle, when all the 3 LED channels will be
turned on again. At that time the 3.6V 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
3.6V to 4.4V. This adaptive-tracking-plus-precharging
LED bus voltage technique simultaneously lowers the
power dissipation in the LT3746 and maintains a shorter
minimum LED on-time.
18
For more information www.linear.com/3746
3746fa
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