LT3745EUJ Просмотр технического описания (PDF) - Linear Technology

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LT3745EUJ

16-Channel 50mA LED Driver with Buck Controller

Linear Technology 

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

GM2 is enabled and takes the control from the amplifier

GM1, regulating the minimum active LED pin voltage to

0.7V. With the VLED3 equal to 0.7V, the output LED bus

voltage is tracked to 4.6V. Subsequently, at a certain

time instant t1 when the third channel is turned off, the

minimum active LED pin voltage goes to VLED2, 1.1V.

Then the amplifier GM2 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 t2, 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 t3 when the PRECHG signal goes

high, the amplifier GM2 is disabled and gives the control

back to the amplifier GM1. 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.

PWM1

PWM2

PWM3

PRECHG

IDEAL VOUT

LT3745 VOUT

CONSTANT VOUT

4096*TPWMCK

4.6V 4.2V

4.6V 4.2V

3.8V

3.8V

4.6V

t1 t2

t3 t4

VOUT = 4.6V

(1)

+

3.1V

(2)

+

3.5V

(3)

+

3.9V

–

–

–

+

+

+

1.5V

1.1V

0.7V

–

–

–

3745 F05

Figure 5. Adaptive-Tracking-Plus-Precharging LED Bus Voltage Technique

3745f

18

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