Linear’s LT3791 is a synchronous four-switch boost-buck LED driver controller that can make VIN higher, lower, or equal to VOUT with up to 98.5% efficiency, input Voltage 4.7V~60V, output voltage 0V~60V, LED Current accuracy ±6%, true color PWM and analog dimming, input power of 100W or more per IC. Mainly used in automotive headlights/driving lights and general lighting.
Its continuous constant frequency and forced constant current mode architecture makes it possible to adjust, or synchronize frequencies from 200kHz to 700kHz. No top MOSFET switching cycles are required for buck or boost operation. The device is 60V input, 60V output. The seamless transition between the LT3791 and the working area makes the LT3791 an ideal choice for LED drivers in automotive, industrial, and battery-powered systems.
The LT3791 features an input current monitor, LED current monitor, and LED open or short circuit protection during restart or latch-up.
Figure 1 LT3791 block diagram
LT3791 main features
• 4-switch single-Inductor architecture allows VIN to be above, below or equal to VOUT
• Synchronous switching: up to 98.5% efficiency
• Wide VIN range of 4.7V~60V:
• Wide VOUT range: 0V~60V (52V LED)
• ±6% LED current accuracy: 0V≤VLED<52V
• True Color PWM and analog dimming
• LED current monitor output and input current regulation
• No Top MOSFET refresh during buck or boost
• During shutdown, VOUT is disconnected from VIN
• Open or short LED fault protection
• 100W or higher per IC
• 38-pin TSSOP with exposed pad
• Car headlights/driving lights
• General lighting
Figure 2 LT3791 efficiency 98% 50W (25V2A) buck-boost LED driver circuit diagram
Figure 3 LT3791 efficiency 98.5% 100W (33.3V3A) buck-boost LED driver circuit diagram
Figure 4 DC1666A demo board circuit diagram
Demonstration board DC1666A
The DC1666A demonstration circuit is a synchronous four-switch buck-boost LED driver controller. Its input voltage is 4.7V~60V, and it can drive up to 2A, 25V LED. The DC1666A features PWM and analog dimming of LED strings. When the LED string is moved, its OPENLED flag is displayed; when the output is shorted to GND, its SHORTLED flag is displayed. In both cases, the IC remains in control and is well protected.
The DC1666A has very high efficiency, its switching frequency is 300kHz, and it has continuous conduction mode (CCM). Its synchronous four-switch topology delivers up to 98% efficiency when regulating 50W of constant LED output current (boost and buck). The circuit can be varied for LED applications over 100W. An optional SYNC terminal is used to synchronize an external clock to the CLKOUT terminal and provides a synchronization resource for another converter and the internal clock (LT3791). Three sense resistors provide constant output current control and monitoring, peak switch current control, and DC input current limit and monitoring. Its 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 to save space and cost. The LED open circuit overvoltage protection uses an IC constant voltage regulation loop to limit the output (about 28.3V if the LED string is turned on). For low input voltage operation, when the input voltage is lower than 6.5V, the CTRL pin voltage is reduced and reduces the brightness of the LED and limits the peak switch current to limit the size of the inductor and switch. When VIN drops below 4.7V, UVLO turns the LED off. The overvoltage lockout feature turns the switch off as protection when the input rises above 57.6V, and turns it back on when VIN drops to 56.2V.
The DC1666A PWM dimming is simple, the PWM dimming MOSFET uses the input of the PWM dimming terminal to turn the LED string on and off. For the highest PWM dimming ratio, it is recommended to use 100Hz as the PWM dimming frequency. For information on PWM dimming ratios and performance, see the LT3791 data sheet in the Applications Information section. Analog dimming is also simple, using a simple CTRL-terminated voltage source.
The DC1666A can be modified to convert the board to higher or lower power, or to a LED driver to a constant voltage regulator or battery charger.
The LT3791 data sheet gives a complete description of the operation and application information. The datasheet must be read along with the user manual to understand the DC1666A circuit. The LT3791 is packaged in a 38-pin plastic TSSOP package with thermally enhanced ground pads. Proper board layout is essential for thermal performance.
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