LT3754 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT3754

16-Channel × 50mA LED Driver

Linear Technology 

LT3754

Operation

to define the turn on/off voltages for VIN. SHDN/UVLO pin

current switches from 2.4µA to 0µA when SHDN/UVLO

pin voltage exceeds 1.476V.

The LT3754 constant switching frequency is program-

mable from 100kHz up to 1MHz using a single resistor

at the RT pin to ground. A SYNC pin is also provided to

allow an external clock to define the converter switch-

ing frequency. The GATE output provides a ±0.8A peak

gate drive for an external N-channel power MOSFET to

generate a boosted output voltage VOUT using a single

inductor, Schottky diode and output capacitor. With LED

strings connected from VOUT to every LED pin, the lowest

voltage on each LED pin is monitored and compared to

an internal 1.1V reference. VOUT is regulated to ensure

the lowest LED pin voltage of any connected LED string

is maintained at 1.1V. If any of the LED strings are open,

the LT3754 regulation loop will ignore the open LED pin.

If all of the LED strings are open VOUT charges up until

a user programmable OVP (overvoltage protection) level

is reached. This programmable OVP level allows the user

to protect against LED damage when the LED strings are

opened and then reconnected.

Since the LT3754 boost controller uses a current mode

topology, the VC pin voltage determines the peak current

in the inductor of the converter and hence the duty cycle

of the GATE switching waveform. The basic loop uses a

pulse from an internal oscillator to set an RS flip-flop and

turn on the external power MOSFET. Current increases

in the MOSFET and inductor until the VC commanded

peak switch current is exceeded and the MOSFET is then

turned off. Inductor current is sensed during the GATE on

period by a sense resistor RS in the source of the external

N-channel power MOSFET. As with all current mode

converters, slope compensation is added to the control

path to ensure stability for duty cycles above 50%. Any

over current fault condition in the MOSFET turns off the

MOSFET and triggers soft-start internally. In this fault mode

the LT3754 only allows MOSFET turn-on approximately

every 2ms. This hiccup mode significantly reduces the

power rating required for the MOSFET.

LED current programming and dimming can be achieved

using the ISET, CTRL and PWM pins. A single resistor at

the ISET pin programs LED current. Analog dimming of

LED brightness is achieved using the CTRL pin below 1V.

PWM dimming of LED brightness is achieved by control-

ling the duty cycle of the PWM pin.

For robust operation the LT3754 monitors system

conditions and performs soft-start for start-up after any of

the following faults: VIN, SHDN or INTVCC voltages too low or

MOSFET current too high. The LT3754, when entering these

faults, discharges an internal soft-start node and prevents

switching at the GATE pin. When exiting these faults the

LT3754 ramps up an internal soft-start node to control VC

pin voltage rise and hence control MOSFET peak switch

current rise. In addition the soft-start period gradually

ramps up switching frequency from approximately 33%

to 100% of full scale.

The LT3754 monitors each LED pin voltage. If the LED

string has an open fault (V(LEDX)<0.5V) the FAULT flag

is pulled low.

For LED protection, the LT3754 CTRL pin allows an LED

current derating curve to be programmed versus the

ambient temperature of the LED strings. An NTC resistor

placed close to the LEDs decreases CTRL pin voltage and

hence decreases LED current as LED ambient temperature

increases.

The LT3754 also allows its own junction temperature to

be monitored and regulated by derating LED currents

when a junction temperature programmed by the TSET

pin is exceeded.

3754fc

10

Share Link: