The LT®3791 is a synchronous 4-switch buck-boost LED driver controller. The controller can regulate LED current up to 52V of LED string with input voltages above, below, or equal to the output voltage. The constant-frequency, forced-continuous current mode architecture allows its frequency to be adjusted or synchronized from 200kHz to 700kHz. No top MOSFET refresh switching cycle is needed in buck or boost operation. With 60V input, 60V output capability and seamless transitions between operating regions, the LT3791 is ideal for LED driver applications in automotive, industrial, and even battery-powered systems.
The LT3791 provides input current monitor, LED current monitor, and open or shorted LED fault condition, during which the LT3791 either restarts or latches off.
4-Switch Single Inductor Architecture Allows VIN Above, Below or Equal to VOUT
Synchronous Switching: Up to 98.5% Efficiency
Wide VIN Range: 4.7V to 60V
Wide VOUT Range: 0V to 60V (52V LED)
±6% LED Current Accuracy: 0V ≤ VLED < 52V
True Color PWM™ and Analog Dimming
LED and Input Current Regulation with Current Monitor Outputs
No Top MOSFET Refresh in Buck or Boost
VOUT Disconnected From VIN During Shutdown
Open or Shorted LED Fault Protection
Capable of 100W or Greater per IC
38-Lead TSSOP with Exposed Pad
Automotive Head Lamps/Running Lamps
General Purpose Lighting
Demonstration circuit DC1666A is a synchronous fourswitch buck-boost LED driver controller. It accepts an input voltage from 4.7V to 60V, and drives up to 25V of LEDs at 2A. DC1666A features both PWM and analog dimming of the LED string. It has an OPENLED flag that indicates when the LED string has been removed and it has a SHORTLED flag that indicates that the output has been shorted to GND. In both cases, the IC remains in control and well protected.
DC1666A features very high efficiency at 300kHz switching frequency and continuous conduction mode (CCM). The synchronous four-switch topology both steps up and steps down voltage while regulating up to 50W of constant LED output current at efficiencies up to 98%. The circuit can be altered for applications requiring over 100W of LEDs.
An optional SYNC terminal is provided for synchronizing to an external clock and CLKOUT terminal provides asource to sync another converter to the internal clock of the LT®3791.
Three sense resistors provide constant output current control and monitoring, peak switch current control, and DC input current limit and monitoring. The ISMON and IVINMON outputs tell the user how much current is flowing through the output and input sense resistors.
Small ceramic input and output capacitors are used to save space and cost. The open LED overvoltage protection uses the ICs constant voltage regulation loop to limit the output to approximately 28.3V if the LED string is opened.
For low input voltage operation, the CTRL pin voltage is reduced as the input voltage drops below 6.5V, reducing LED brightness and restraining the peak switch currents in order to limit inductor and switch size. UVLO turns the LEDs off when VIN drops below 4.7V. When input rises above 57.6V, overvoltage lockout turns the switches off to protect them and they turn back on when VIN drops below 56.2V.
DC1666A PWM dimming is simple. The PWM dimming MOSFET turns the LED string on and off with an input to the PWM dimming terminal. For the highest PWM dimming ratio, it is recommended to use 100Hz as a PWM dimming frequency. Information regarding PWM dimming ratios and performance can be found in the Applications Information section of the LT3791 data sheet. Analog dimming is also simple with a simple voltage source on the CTRL terminal.
Modifications can be made to DC1666A in order to convert the board to higher or lower power or from an LED driver to a constant voltage regulator or battery charger. Please consult the factory or the LT3791 data sheet for details.
The LT3791 data sheet gives a complete description of the part, operation and applications information. The data sheet must be read in conjunction with this demo manual for demonstration circuit DC1666A. The LT3791 is assembled in a 38-lead plastic TSSOP package with a thermally enhanced ground pad. Proper board layout is essential for maximum thermal performance. See the Layout Considerations section in the data sheet.