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LT3754

16-Channel × 50mA LED Driver

Linear Technology 

LT3754

Applications Information

Analog Dimming

The LT3754 allows for LED dimming (brightness reduction)

by analog dimming or by PWM dimming. Analog dimming

uses the CTRL pin voltage below 1V to reduce LED

brightness by reducing LED current. For CTRL pin voltage

below 1V, the current in each LED pin is given by:

I(LEDX

)

≈

CTRL

•

295

RISET

(0.04

<

CTRL

<

1V

)

For CTRL pin voltages below 40mV (greater than 25:1

dimming) the LED current will approach zero current. The

CTRL pin voltage can be derived from a resistor divider

from VREF pin to ground or generated from an external

source. If analog dimming is not required, the pin can be

directly connected to the VREF pin. The only drawback of

analog dimming is that reducing LED current to reduce

the brightness of the LED also changes the perceived

color of the LED.

PWM Dimming

Many applications require an accurate control of the bright-

ness of the LED(s). In addition, being able to maintain a

constant color over the entire dimming range can be just

as critical. For constant color LED dimming the LT3754

provides a PWM pin and special internal circuitry to achieve

up to a 3000:1 wide PWM dimming range. This is achieved

by operating the LED at it’s programmed current and then

controlling the on-time of that LED current. The duty cycle

of the PWM pin controls the on-time of each LED pin

current source (Figure 5). For maximum PWM dimming

ratios (low PWM duty cycles) it is important to be able to

turn LED currents on/off as quickly as possible. For PWM

low, the LT3754 turns off the boost converter, turns off

all LED channel currents and disconnects the VC pin and

internal VOUT resistor divider connected to the OVP error

amplifier. This allows the part to quickly return to the last

state of operation when the PWM pin is returned high.

Some general guidelines for LED current dimming using

the PWM pin (see Figure 5):

1. PWM Dimming Ratio (PDR) = 1/(PWM Duty Cycle) =

1/TON(PWM) • fPWM

PWM

TPWM

TON(PWM)

(= 1/fPWM)

INDUCTOR

CURRENT

LED

CURRENT

MAX ILED

3754 F05

Figure 5. PWM Dimming Waveforms

2. Lower PWM frequency (fPWM) allows higher PWM

dimming ratios (typically choose 100Hz to maximize

PDR and to avoid visible flicker which can occur for

display systems with refresh rates at frequencies below

80Hz)

3. Higher fOSC value improves PDR (allows lower TON(PWM))

but will reduce efficiency and increase internal heating.

In general, minimum operational TON(PWM) = 3 • (1/fOSC)

4. Lower inductor value improves PDR

5. Higher output capacitor value improves PDR

6. Choose the Schottky diode for the LT3754 boost conver-

ter for minimum reverse leakage current.

7. Start-Up

LT3754 VOUT start-up requires SHDN/UVLO and PWM

pins to be asserted from off to on and the PWM on-time

to be above a minimum value. The lowest PWM on-time

allowed for fault detection is ≈3.2µs. The lowest PWM

on-time allowed for reaching VOUT regulation is typically

3.2µs but might be greater depending on external circuit

parameters. Once LED current is in regulation, PWM

on-time can be further reduced depending on external

component selection.

8. VOUT Collapse

If during normal operation VOUT collapses due to a

fault or because PWM on-time is too low, a restart is

required (see ‘Start-Up’ in item 7).

3754fc

17

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