Figure 1 shows the LT3796 configured as a boost converter to drive a 34W LED string from a wide input range. The LED current is derated at low input voltages to prevent external power components from overheating. The front-end current sense amplifier monitors the input current by converting the input current to a voltage signal at the CSOUT pin with
The resistor network at the FB1 pin provides OPENLED protection, which limits the output voltage and prevents the ISP pin, ISN pin and sev-eral external components from exceeding their maximum rating. If an LED fails open or if the LED string is removed from the high power driver, the FB constant voltage loop takes over and regulates the output to 92.5V. The VMODE flag is also asserted to indicate an OPENLED event.
The LT3796 includes short-circuit protection independent of the LED current sense. The short-circuit protection feature prevents the development of excessive switching currents and protects the power components. The protection threshold (375mV, typ) is designed to be 50 percent higher than the default LED current sense threshold.
Figure 2: Short LED protection: hiccup mode (without R11 in Figure 1)
Once the LED overcurrent is detected, the GATE pin drives to GND to stop switching, the TG pin is pulled high to disconnect the LED array from the power path and the FAULT pin is asserted. The Schottky diode D2 is added to protect the drain of PMOS M2 from swinging well be-low ground when shorting to ground through a long cable. The PNP helper Q1 is included to further limit the transient short-circuit current.
If there is no resistor between the SS pin and VREF
pin, the converter enters hiccup mode and periodically retries as shown in the Figure 2
. If a resistor is placed between VREF
and SS pin to hold SS pin higher than 0.2V during LED short, then the LT3796 enters latchoff mode with GATE pin low and TG pin high, as shown in Figure 3
. To exit latchoff mode, the EN/UVLO pin must be toggled low to high.
Figure 3: Short LED protection: latchoff mode (with R11 in Figure 1)