LT3760EFE 查看數據表（PDF） - Linear Technology

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LT3760EFE

8-Channel × 100mA LED Driver

Linear Technology 

LT3760

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 LT3760 constant switching frequency is programmable

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 switching 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 1V

reference. VOUT is regulated to ensure the lowest LED pin

voltage of any connected LED string is maintained at 1V.

If any of the LED strings are open, the LT3760 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 LT3760 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 LT3760 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 LT3760 monitors system

conditions and performs soft start for startup after any of

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

or MOSFET current too high. The LT3760, when entering

these faults, discharges an internal soft start node and

prevents switching at the GATE pin. When exiting these

faults the LT3760 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 LT3760 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 LT3760 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 LT3760 also allows it’s own junction temperature

to be monitored and regulated by derating LED currents

when a junction temperature programmed by the TSET

pin is exceeded.

3760fc

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