Say goodbye to sources of efficiency loss, this device powers high-current LEDs at any time

The number of applications of high-power LEDs in modern lighting systems continues to proliferate, covering applications such as automotive headlamps, industrial/commercial signage, architectural lighting, and various consumer electronics. The industry turned to LED technology because solid-state lighting offers distinct advantages over traditional light sources: the conversion of electrical energy into light output is not only efficient, but also has a long service life.

The number of applications of high-power LEDs in modern lighting systems continues to proliferate, covering applications such as automotive headlamps, industrial/commercial signage, architectural lighting, and various consumer electronics. The industry turned to LED technology because solid-state lighting offers distinct advantages over traditional light sources: the conversion of electrical energy into light output is not only efficient, but also has a long service life.

As more and more applications adopt LED lighting, there is a growing need for higher LED currents in order to increase light output. One of the biggest challenges in driving high-current LED strings is maintaining high efficiency at the power converter stage to provide a regulated LED current. The inefficiency of the power converter is manifested in the heating phenomenon caused by the switching elements of the current regulator circuit.

The LT3762 is a synchronous boost LED controller designed to reduce the sources of efficiency losses commonly found in high power boost LED driver systems. The synchronous operation of the device minimizes losses typically associated with the forward voltage drop of clamping diodes in asynchronous DC-DC converters. This efficiency improvement enables the LT3762 to deliver higher output current than comparable asynchronous boost LED drivers, especially at low input voltages.

To improve operation at low input voltages, an on-board DC-DC regulator is configured to supply 7.5 V to the gate drive circuit even if the input voltage drops below 7.5 V. Providing a strong gate drive voltage source at low input voltages allows the MOSFETs to generate less heat as the input voltage decreases, allowing an operating voltage input range down to 3 V.

Say goodbye to sources of efficiency loss, this device powers high-current LEDs at any time

Figure 1. The LT3762 demonstration circuit (DC2342A) drives an LED at 2 A (up to 32 V) over a wide input voltage range. This demo circuit can be easily modified with additional MOSFETs and capacitors to increase output power.

This step-up LED controller can be configured to operate from a fixed switching frequency of 100 kHz to 1 MHz and offers a −30% × fSW spread spectrum modulation option to reduce switching-related EMI energy peaks. The LT3762 can drive LEDs in boost, buck or boost/buck topologies. A high-side PMOS disconnect switch facilitates PWM dimming and protects the device from potential damage when the LEDs are open/shorted.

The LT3762 employs an internal PWM generator that utilizes a single capacitor and a DC voltage to set the frequency and pulse width to achieve PWM dimming ratios up to 250:1, or up to 3000:1 dimming ratios using an external PWM signal.

The schematic in Figure 2 shows a demonstration circuit application (DC2342A) using the LT3762 configured to drive an LED at 2 A (up to 32 V) from an input voltage range of 4 V to 28 V. The LT3762 synchronous boost LED controller is available in 4 mm × 5 mm QFN and 28-pin TSSOP packages.

Say goodbye to sources of efficiency loss, this device powers high-current LEDs at any time

Figure 2. 32 V, 2 A LT3762 boost LED driver.

Sync switch

In asynchronous DC-DC converter topologies, Schottky clamp diodes are used as passive switches to simplify the converter’s control scheme for pulse width modulation of a single MOSFET. While this does simplify control, it limits the amount of output current. Schottky diodes, like PN junction devices, have a forward voltage drop before any current flows through the device. Since the power dissipation of a Schottky diode is the product of its forward voltage drop and current, too high an output current level will generate several watts of on-state dissipation, which will heat up the Schottky diode and ultimately reduce converter efficiency.

Unlike asynchronous converters, the LT3762 synchronous switching converter has no output current limitation because the synchronous converter uses a second MOSFET instead of a Schottky diode. Unlike Schottky diodes, MOSFETs have no forward voltage drop. Conversely, when the MOSFET is fully enhanced, its drain-to-source resistance is very small. At high currents, MOSFETs generate much lower conduction losses than Schottky diodes because power dissipation is proportional to the product of the square of the drain-source resistance and the current through the device. Even at full power input voltages as low as 7 V, the MOSFETs will only experience a temperature rise of about 30°C (as shown in Figure 3).

Say goodbye to sources of efficiency loss, this device powers high-current LEDs at any time

Figure 3. The synchronous LT3762 (left) drives a 2 A, 32 V LED string with a much lower temperature rise than the asynchronous LT3755-2 circuit (right) under the same test conditions using similar components. This thermal performance improvement is due to the replacement of Schottky clamp diodes with synchronous MOSFETs, which eliminates losses due to diode forward voltage drop.

Low input voltage operation

Another challenge for high power boost LED controllers occurs during low input voltage operation. Most boost DC-DC regulator ICs use an internal LDO regulator powered by the input of the device to provide a lower voltage supply for the analog and digital control circuits in the IC. Of the circuits that draw power from the internal LDO regulator, the gate driver dissipates the most power, and its performance is affected by fluctuations in the LDO regulator output. When the input voltage drops below the LDO’s output voltage, the LDO output begins to dip, which limits the gate driver’s ability to properly boost the MOSFET. When MOSFETs are not fully boosted, they operate in a higher resistance state, so power is dissipated as heat as current flows through the device.

The low input voltage operating characteristics in boost converter topologies will result in higher input currents, exacerbating conduction losses when this current must flow through the more resistive MOSFET devices. Depending on the gate drive voltage of the regulator IC, this severely limits the low input voltage range that the device can achieve without overheating.

The LT3762 uses an integrated buck-boost DC-DC regulator instead of an LDO regulator to provide 7.5 V to the internal circuitry even when the input voltage is low. This buck-boost regulator occupies only three pins of the LT3762 IC and requires only two additional components. Compared to internal LDO controller devices with 4.5 V and 6 V minimum input voltages, the LT3762 extends the lower input voltage operating range to 3 V. The 7.5 V output of the buck-boost converter powers the gate drivers and allows the use of 6 V/7 V gate drive MOSFETs. MOSFETs with higher gate drive voltages tend to have lower drain-to-source resistance and operate more efficiently (except for switching losses) than similar devices with lower gate drive voltages.

Say goodbye to sources of efficiency loss, this device powers high-current LEDs at any time

Figure 4. The 32 V, 2 A LT3762 LED driver maintains high efficiency over a wide input range. Low VIN foldback helps avoid excessive switch/Inductor currents. Asynchronous switching starts with 24 V input voltage.

flexible topology

Like most other boost LED drivers from Analog Devices, the LT3762 drives LEDs in a reconfigurable mode that can be used in either a boost configuration or in buck, boost-buck, and buck-boost modes. Among these topological variants of boost converters, the use of ADI’s patented boost-buck mode configuration enables operation as a boost/buck converter with the benefit of low EMI operation. This topology utilizes two inductors, one facing the input and the other facing the output, to help filter out the noise created by the switching. These two inductors help suppress EMI coupled to the input power supply, other devices that may be connected, and the LED load.

Additional circuitry can also be added to the boost-buck mode topology to provide short-circuit protection from the LEDC node to GND. The schematic in Figure 5 shows the LT3762 in a boost-buck mode configuration with the addition of this protection circuit. When LEDC is shorted to GND, M4 is forcibly turned off to block the conduction path through the inductor to the input and prevent excessive current consumption. When M4 is forced off, D3 pulls the EN/UVLO pin low, preventing the converter from switching until the short is removed. Combining this additional protection circuit with the LT3762’s built-in open/short detection provides a robust solution capable of handling various fault conditions in harsh environments.

Say goodbye to sources of efficiency loss, this device powers high-current LEDs at any time

Figure 5. The LT3762 is available in a 25 V, 1.5 A boost-buck configuration with additional short-circuit protection from LEDC to GND.

in conclusion

In normal operation of an asynchronous boost converter, it is often difficult to avoid supplying high output currents without incurring significant power losses and heating the clamping diodes. In addition to the losses created by Schottky diodes, these converters have difficulty maintaining maximum power output capability as the input voltage decreases, which limits the power output over the input range. Asynchronous DC-DC converters simply cannot handle higher power levels, so a synchronous switching scheme must be employed to meet application specifications. The LT3762 step-up LED controller solves the problem of delivering high current output with its synchronous switching, it is able to operate at lower input voltages thanks to the on-board DC-DC converter, and is flexible with various circuit topologies .

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