DESCRIPCIÓN

The LT3756, LT3756-1 and LT3756-2 are DC/DC controllers designed to operate as a constant-current source for driving high current LEDs. They drive a low side external N-channel power MOSFET from an internal regulated 7.15V supply. The fixed frequency, current-mode architecture results in stable operation over a wide range of supply and output voltages. A ground referenced voltage FB pin serves as the input for several LED protection features, and also makes it possible for the converter to operate as a constant-voltage source. A frequency adjust pin allows the user to program the frequency from 100kHz to 1MHz to optimize efficiency, performance or external component size.

The LT3756/LT3756-1/LT3756-2 sense output current at the high side of the LED string. High side current sensing is the most flexible scheme for driving LEDs, allowing boost, buck mode or buck-boost mode configuration. The PWM input provides LED dimming ratios of up to 3000:1, and the CTRL input provides additional analog dimming capability.

 

CARACTERÍSTICAS

3000:1 True Color PWM™ Dimming

Wide Input Voltage Range: 6V to 100V

Output Voltage Up to 100V

Constant-Current and Constant-Voltage Regulation

100mV High Side Current Sense

Drives LEDs in Boost, Buck Mode, Buck-Boost Mode, SEPIC or Flyback Topology

Adjustable Frequency: 100kHz to 1MHz

Open LED Protection

Programmable Undervoltage Lockout with Hysteresis

Improved Open LED Status Pin (LT3756-2)

Frequency Synchronization (LT3756-1)

PWM Disconnect Switch Driver

CTRL Pin Provides Analog Dimming

Low Shutdown Current: <1µA

Arranque progresivo programable

Thermally Enhanced 16-Lead QFN (3mm × 3mm) and MSOP Packages

Cualificación AEC-Q100 para aplicaciones de automoción

 

APLICACIONES

High Power LED Applications

Current Limited Constant Voltage Applications

Battery Charging

 

OPERACIÓN

The LT3756 is a constant-frequency, current mode controller with a low side NMOS gate driver. The GATE pin and PWMOUT pin drivers, and other chip loads, are powered from INTVCC, which is an internally regulated supply. In the discussion that follows, it will be helpful to refer to the Block Diagram of the IC. In normal operation, with the PWM pin low, the GATE and PWMOUT pins are driven to GND, the VC pin is high impedance to store the previous switching state on the external compensation capacitor, and the ISP and ISN pin bias currents are reduced to leakage levels. When the PWM pin transitions high, the PWMOUT pin transitions high after a short delay. At the same time, the internal oscillator wakes up and generates a pulse to set the PWM latch, turning on the external power MOSFET switch (GATE goes high). A voltage input proportional to the switch current, sensed by an external current sense resistor between the SENSE and GND input pins, is added to a stabilizing slope compensation ramp and the resulting “switch current sense” signal is fed into the positive terminal of the PWM comparator. The current in the external inductor increases steadily during the time the switch is on. When the switch current sense voltage exceeds the output of the error amplifier, labeled “VC”, the latch is reset and the switch is turned off. During the switch off phase, the inductor current decreases. At the completion of each oscillator cycle, internal signals such as slope compensation return to their starting points and a new cycle begins with the set pulse from the oscillator.

 

INFORMACIÓN SOBRE APLICACIONES

Dimming Control

There are two methods to control the current source for dimming using the LT3756. One method uses the CTRL pin to adjust the current regulated in the LEDs. A second method uses the PWM pin to modulate the current source between zero and full current to achieve a precisely programmed average current. To make PWM dimming more accurate, the switch demand current is stored on the VC node during the quiescent phase when PWM is low. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect switch may be used in the LED current path to prevent the ISP node from discharging during the PWM signal low phase. The minimum PWM on or off time will depend on the choice of operating frequency and external component selection. With operation in discontinuous conduction mode (DCM), regulated current pulses as short as 1µs are achievable. But, the best overall combination of PWM and analog dimming (with CTRL) is available if the minimum PWM pulse is at least six switching cycles.