Author: ADI Power Products Division, Applications Engineering Team Leader Keith Szolusha, deputy Application Engineer Kyle Lawrence
The range of applications for LEDs has expanded from general lighting to automotive, industrial and test equipment, signage and safety equipment. Therefore, there are more and more design requirements for LED drivers. The latest LED solutions require drivers with compact size, high efficiency and low noise, and offer high dimming ratios and advanced fault protection. The LT3922 is very easy to meet these requirements.
LT3922 with integrated switch and internal PWM dimming capability
The LT3922 is a 36V, synchronous LED driver with an integrated 2A switch that can be configured as a boost, buck or boost-buck LED driver. The device integrates a high efficiency, synchronous power switch in a tiny 4mm x 5mm QFN package. The device uses ADI's state-of-the-art switching technology, which, while offering a small package size, provides significant power while controlling edge speed and reducing unwanted field emission. The integrated sync switch edges are controlled, do not ring, providing the right balance between high efficiency and low noise. These switches can also be switched at frequencies up to 2.5MHz, making them a compact solution. .
Topology selection: boost, buck, boost-buck mode
The LED string is driven by a controlled current that does not have to go directly back to ground. LED+ and LED− or either of these terminals can be connected to ground potential. This creates an opportunity to choose a floating output DC/DC LED driver topology that includes buck and boost-buck topologies. The LT3922's high-side PWMTG driver and sync switch can be configured as a boost, buck, or boost-buck mode LED driver while keeping all of the IC's features available, that is, internal PWM dimming, SSFM (spread spectrum) Frequency Modulation), Low EMI, ISMON Output Current Monitor, and Output Fault Protection are also available from standard boost topology to buck and boost-blow topologies.
Boost topology
When operating as a boost converter, the LT3922 can supply up to 34V LED strings with some space below 40V to prevent LED open overshoot. As shown in Figure 1, the 2MHz, 4V to 28V step-up LED driver delivers 330mA to a string of up to 34V. The device can be PWM dimmed externally at 120Hz and 2000:1 dimming ratio, or internally dimmed with an analog input voltage on the PWM pin with a 128:1 dimming ratio.
Figure 1: 2MHz conventional boost topology schematic, 120Hz, 2000:1 PWM dimming ratio
The device can withstand LED open and LED+ to ground shorts without damage and report such faults by determining the FAULT pin. The output current can be monitored via the ISMON pin, even during PWM dimming. At 2MHz switching frequency, its fundamental EMI harmonics reside at frequencies above the AM band, but their EMI is still low. It is also possible to increase spread spectrum frequency modulation to extend the switching frequency from 2MHz to 2.5MHz and to reduce EMI at the fundamental frequency and many harmonic frequencies. Thanks to the integrated synchronous switch, the efficiency of this 2MHz boost converter remains as high as 91% at 12VIN. At lower VIN, when the peak inductor current reaches its limit, the output current is smoothly reduced without flicker and the LED remains on.
Buck topology
As shown in Figure 2, when the LT3922 is used in a buck mode topology, the input voltage can be as high as 36V and can drive an LED string with up to 1.5A. The high side ISP and ISN current sense inputs and the PWMTG PMOS driver are easily transferred to the high side of the LED. In buck mode, the high side of the LED is connected to the input. LED− is connected directly to the inductor instead of ground. When driving two 1A LEDs at 6.5V, the synchronous buck mode is 94% efficient at VIN 12V and up to 89% at VIN 36V. The buck mode converter has a large bandwidth and is therefore capable of operating at 100Hz with a 1000:1 PWM dimming ratio.
Figure 2: 400kHz buck mode LED driver with brightness control at 1000:1, 100Hz PWM dimming
Boost-buck topology
The LT3922 boost-buck topology shown in Figure 3 supports an input voltage range above and below the LED string voltage. The sum of the LED string voltages and the input voltage must be kept below 35V to keep the ISP and ISN voltages below the 40V absolute maximum.
Figure 3: The input and output ripple of a 2MHz boost-buck LED driver is low. This solution passed the CISPR 25 Class 5 test.
This patented low EMI topology uses a boosted low ripple input inductor and a buck low ripple output inductor. The boost-buck converter can drive LED strings from 3V to 16V with a variety of battery inputs (5V, 12V, and 19V) with 4V to 18V automotive inputs or different chemical compositions.
Like other topologies, the PWMTG driver simplifies the connection of MOSFETs for PWM dimming. In floating LED topologies, open and short circuit protection is not affected. An optional diode on the LED− terminal protects against LED– to GND shorts.
The 2MHz converter in Figure 3 has an efficiency of 85% at 12V VIN, 15V VLED, 330mA ILED (87% without EMI filter) and a 2000:1 PWM dimming ratio at 120Hz. This solution is small in size, versatile and low EMI, meeting the requirements of automotive daytime running lights, signal indicators or taillight LED drivers.
Car lighting
The many features of LED make it ideal for automotive lighting. LED taillights and daytime running lights are visually appealing. High-efficiency LED headlamps are rugged and reliable, and the life of LED headlamps is orders of magnitude longer than previous filament-based headlamps. LED drivers are small, highly efficient, have a wide input and output voltage range, and have low EMI.
The tiny LT3922 LED driver has low EMI and high efficiency and fault protection for automotive environments. The device operates from a 9V to 16V automotive input range and can operate with 36V transients and cold startdowns as low as 3V. Its low EMI Silent Switcher® architecture, SSFM and controlled switching waveform edges make this device ideal for powering LEDs that require low EMI. The versatility of the device enables it to be used in boost, buck, and boost-buck applications such as external daytime running lights, signal indicators, taillights and headlights, as well as internal dashboards and high dimming ratios Head-up display. To protect against LED string shorts and opens, protection circuitry is required, and the inherent flexibility and built-in fault protection of the device helps reduce the number of components required to protect the circuit.
The 400kHz automotive step-up LED driver shown in Figure 4 passed the CISPR 25 Class 5 EMI test, as shown in Figure 5, which shows the conducted and radiated EMI test results of the LT3922 and the Class 5 EMI limits. This is the result of a combination of the low EMI characteristics of the LT3922, including but not limited to controlled switching waveform edges and SSFM. Of course, proper layout and a small amount of ferrite bead filtering (FB1 and FB2) should also be used for best EMI results.
Figure 4: The 400kHz automotive step-up LED driver uses a filter for low EMI with an internally generated 100%, 10% or 1% PWM dimming. The EMI test (Figure 5) shows that this solution passed the CISPR 25 Class 5 test.
Figure 5: The EMI curve for the 400kHz LED driver shown in Figure 4. The driver uses the smallest EMI filter and passes the CISPR 25 Class 5 test. If you need to further reduce EMI in order to meet the specific EMI requirements proposed by the manufacturer, you can also add a larger LC filter to the input.
Inherent features help achieve low EMI
Some of the features of the LT3922 make it easy for designers to implement low EMI solutions. First, the LT3922 uses ADI's patented Silent Switcher architecture, in which the internal synchronous switch minimizes the size of the hot-switching loop and the controlled switching waveform edges do not ring.
Figure 6 shows that the LT3922's pinout allows small, high-frequency capacitors to be placed close to the two VOUT pins to minimize thermal loop size and EMI.
Figure 6: Dual-loop layout and high-frequency 0402 split capacitors form a small, opposite-direction thermal loop to help reduce high-frequency EMI
The LT3922 controls the switching edge rate, eliminating high frequency ringing, and high frequency ringing is common in switching converters without such control. Since the switching edge of the LT3922 is controlled, the high frequency EMI generated by the power switch can be reduced without reducing efficiency and power supply capability.
The LT3922's SSFM extends the switching frequency range of the resistor-set, 400kHz converter from 100% up and down to 125% at 1.6kHz. This reduces the peak EMI and average EMI of the converter at low and high frequencies. This feature can be turned on or off very easily by connecting the SYNC/SPRD pins to INTVcc or GND, respectively.
Internally generated PWM dimming
Analog dimming with an adjustable voltage on the CTRL pin is always easier than performing more accurate PWM dimming. To date, PWM dimming has always required an external clock or microsignal that controls the brightness at its duty cycle through the PWM input pin. However, the LT3922 has an internally generated PWM dimming signal that requires only an external voltage on the PWM pin to set the duty cycle to achieve a 128:1 PWM dimming ratio. The PWM period (for example, 122 Hz) is set by a single resistor on the RP pin.
For vehicles with redundant clusters, it is necessary to ensure the accuracy of the LED current. Obviously, the brightness of the lights on both sides must match. LEDs fabricated in the same way may produce different brightness when using the same drive current. The LT3922's internal dimming function can be used to fine tune the brightness near or near 100% duty cycle and then set an accurate 10:1 or 100:1 dimming ratio. This allows the lamp manufacturer to avoid increasing costs due to special graded LEDs.
When a higher dimming ratio is required, the LT3922 can be dimmed from the outside in the usual way. The large bandwidth 400kHz buck mode LED driver in Figure 2 produces a 1000:1 PWM dimming ratio at 100Hz. The 2MHz boost LED driver in Figure 1 achieves a 2000:1 dimming ratio at 120Hz, as shown in Figure 7a. By placing a resistor set at 122 Hz on the RP pin and setting the PWM pin voltage between 1.0V and 2.0V, the same circuit can be set to internally generate PWM dimming. The dimming ratio is 128:1, as shown in Figure 7b. In some applications, the LT3922 can be set to operate with an external PWM dimming ratio of 5000:1, and PWM dimming can also be combined with the analog dimming of the LT3922 to provide brightness control over 50,000:1.
Figure 7: (a) Figure 1 circuit externally generates 2000:1 or 4000:1 PWM dimming; (b) Figure 1 circuit internally generates 128:1 PWM dimming.
Machine vision
In industrial assembly applications, machine vision (Figure 8) uses high-speed digital photography and digital imaging to quickly provide visual feedback on the device. This helps to quickly identify and isolate defective products with little or no manual inspection. The illumination of the machine vision system must be synchronized with the speed of the pipeline while producing consistent light pulses for indefinite turn-off times.
For conventional LED drivers, the driver cannot maintain the output voltage as long as the PWM input signal remains low, regardless of the hold time. This is because the output capacitor is gradually discharging, so the general purpose LED driver is not suitable for applications such as machine vision. However, the LT3922 digitally samples the output state of the converter on the falling edge of the PWM signal. Then, by performing a "hold switching" when the PWM is turned off and the LED is turned off by the high side PMOS, the device can maintain its output voltage for a long time off. When standard PWM dimming is performed at frequencies above 100 Hz, the maximum off time is 10 ms or less, at which time too much leakage current is pulled from the output. Machine vision and strobe applications can have turn-off times of 100ms and 5s (or longer), so leakage currents can increase by tens to hundreds of times.
“Keep switching†ensures that the output capacitor remains at the voltage recorded during the previous sampling period of the LT3922. Assuming that the IC gets an uninterrupted power input, the digital sample values ​​of the converter state can be stored indefinitely. This allows the LT3922 to provide a consistent output current waveform for any given off-time, as shown in Figure 9.
Figure 8: Overview of the pipeline system with machine vision applications
CAMERA: camera
IMAGE LIBRARY & PROCESSING: Image Library and Image Processing
ROBOTICS: Robot
LED FLASH: LED flash
CONVEYOR WITH OBJECTS: Conveyor belt carrying objects
1ms – 1s PERIOD: 1ms to 1s period
LT3922 PRODUCES CONSISTENT LED OUTPUT REGARDLESS OF PERIOD BETWEEN FLASHES:
The LT3922 produces consistent LED output regardless of the interval between flashes
Figure 9: The waveform of the camera flash is the same regardless of idle or downtime. The waveforms in the figure show the pulses after 10ms and one hour later. After one hour of idle and 10ms of idle, the waveform of the flash is the same. These waveforms are derived from the circuit shown in Figure 1.
10ms SINCE LAST PULSE: 10ms after the last pulse
10ms BEFORE NEXT PULSE: 10ms before the next pulse
1 hour SINCE LAST PULSE: 1 hour after the last pulse
100ms, 1s, 1 hour, 1 day BEFORE NEXT PULSE: 100ms, 1s, 1 hour, 1 day before the next pulse
in conclusion
The LT3922 36V LED driver with internal sync, 2A switch is a compact, versatile LED driver. The device is very easy to use in boost, buck, and boost-buck topologies. Regardless of the topology used, all features work, including high PWM dimming ratio capability and internally generated PWM dimming. Low EMI is very easy to achieve with its Silent Switcher layout and SSFM. Its compact synchronous switch maintains high efficiency, even at frequencies up to 2MHz. With rugged and reliable fault protection, this IC is easy to meet the needs of automotive and other demanding applications.
Table 1: LED driver with a wide input range
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