As LED applications continue to expand, ranging from general lighting to a wide range of automotive, industrial and test equipment, signage and safety equipment, the design requirements for LED drivers are also growing. The latest LED solutions require drivers With its compact size, high efficiency and low noise, high dimming ratio and advanced fault protection, the LT3922 easily meets these requirements.
The LT3922 features integrated switching 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. This device incorporates ADI's state-of-the-art switching technology that provides very high power while being small in package size while controlling edge speed and reducing unwanted field emission. The integrated synchronous switch edge Are controlled and ring-free to provide 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 strings are driven by a controlled current that does not have to be returned directly. The LED + and LED- or either of these terminals can be connected to a non-ground potential. This is where the choice of both buck and boost-buck topologies The floating-output DC / DC LED driver topology opens the door to opportunities The LT3922's high-side PWMTG driver and synchronous switch can be configured as a boost, buck or boost-buck LED driver while keeping all the features of the IC available, That is, internal PWM dimming, SSFM (spread spectrum frequency modulation), low-EMI, ISMON output current monitor, and output fault protection range from standard boost topologies to buck and buck-boost topologies Provided.
Boost topology
Operating as a step-up converter, the LT3922 can power up to 34V LED strings, leaving some room below 40V to prevent LED open overshoot. As shown in Figure 1, the 2MHz, 4V to 28V step-up LED driver 330mA current for LED strings up to 34V The device can be externally PWM dimming at 120Hz frequency and 2000: 1 dimming ratio or internally with a 128: 1 dimming ratio using the analog input on the PWM pin Voltage dimming.
The device can withstand LED open and LED + short to ground without damage, and by determining the FAULT pin to report such faults. The output current can be monitored by the ISMON pin, even when PWM dimming. At 2MHz switching frequency , Its fundamental EMI harmonics reside at frequencies above the AM band but its EMI is still low.Extended spectrum frequency modulation can also be added to extend the switching frequency from 2MHz to 2.5MHz and reduce the fundamental frequency and much harmonics EMI at Wave Frequencies The efficiency of this 2MHz step-up converter remains as high as 91% at 12VIN due to the integrated synchronous switch. At lower VIN, when the peak inductor current reaches its limit, the output current Smoothly without flickering, while 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 1.5 A. 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- Connects directly to the inductor instead of ground. When both 1A LEDs are driven at 6.5V, the synchronous buck The mode efficiency is as high as 94%, and the efficiency remains as high as 89% at VIN of 36 V. The buck mode converter has a large bandwidth and therefore operates at 100Hz with a PWM dimming ratio of 1000: 1.
Boost - buck topology
The LT3922 buck-boost topology shown in Figure 3 supports input voltage ranges 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 40V abs Maximum
This patented low-EMI topology features a step-down, low-ripple input inductor and a step-down, low-ripple output inductor with a variety of 4V to 18V automotive inputs or a wide variety of battery chemistries (5V, 12V and 19V), the buck-boost converter can drive LED strings from 3V to 16V.
As with other topologies, the PWMTG driver simplifies the connection of MOSFETs for PWM dimming. In floating LED topologies, open and short circuit protection is unaffected. An optional diode on the LED- terminal is protected from LED- to GND Short-circuit effects.
The 2-MHz converter in Figure 3 provides 85% efficiency (87% with no EMI filter) at 12V VIN, 15V VLED, 330mA ILED and a 2000: 1 PWM dimming ratio at 120Hz. The solution is small in size, With versatility and low EMI, to meet the daytime driving lights, signal lights or taillights LED driver requirements.
Car lighting
LED's many features make it ideal for automotive lighting. LED taillights and daytime running lights are visually appealing. High-efficiency LED headlights are solid and reliable, compared with the previous relatively easy to burn and compared to the filament-based headlights, LED headlamps can grow several orders of magnitude longer. LED drivers are small, highly efficient, have a wide input and output voltage range, and have low EMI.
The tiny EMI of the tiny LT3922 LED driver, coupled with the high efficiency and fault protection required in the automotive environment, operates over a 9V to 16V automotive input range and starts at 36V transient and cold-crank down to 3V The low EMI Silent Switcher® architecture, SSFM and controlled switching waveform edges make this device ideal for powering LEDs requiring low EMI. The versatility of this device allows it to be used for step-up, step-down and Step-up / step-down applications such as exterior daytime driving lights, signal lights, taillights and headlights, internal dashboards and head-up displays with high dimming ratio. To protect against short and open LED strings, protection is required Circuit, and the device's inherent flexibility and built-in fail-safe features help reduce the number of components needed 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 for the LT3922 as well as the Class 5 EMI limits. Term low EMI characteristics including, but not limited to, controlled switching waveform fringes and SSFM. Of course, the proper layout and a small amount of ferrite bead filtering (FB1 and FB2) are also used to obtain the best EMI result.
Inherent features help achieve low EMI
The LT3922 has features that make it easy for designers to achieve low-EMI solutions. First, the LT3922 uses ADI's patented Silent Switcher architecture, an internal synchronous switch that minimizes the size of the hot-swap loop , And the controlled switch waveform edge does not ring.
Figure 6 shows that the LT3922's pinout allows for placement of small, high frequency capacitors near both VOUT pins to minimize thermal loop size and EMI.
The LT3922 controls the switching edge rate, eliminating high frequency ringing, which is common in switching converters without this control. Since the switching edges of the LT3922 are controlled, the LT3922 can operate at high efficiency without sacrificing efficiency and power supply Lower the high frequency EMI generated by the power switch.
The LT3922's SSFM expands the resistor-set, 400kHz converter's switching frequency range from 100% up and down to 125% at 1.6kHz, which reduces the peak EMI and the average of the converter at low and high frequencies EMI. This feature can be turned on or off very easily by connecting the SYNC / SPRD pins to INTVcc or GND, respectively.
Internal PWM dimming generated
Analog dimming with the adjustable voltage on the CTRL pin is always easier than with more accurate PWM dimming.To date, PWM dimming always requires an external clock or micro-sigma, which is fed through the PWM input pin However, the LT3922 has an internally generated PWM dimming signal that enables 128: 1 PWM dimming by simply adding an external voltage to the PWM pin to set the duty cycle The period (eg 122Hz) is set by a single resistor on the RP pin.
For vehicles with redundant lamp sets it is necessary to ensure the accuracy of the LED current.It is clear that the brightness of the lamps on both sides must be matched.When LEDs of the same type are manufactured using the same drive current the resulting brightness The internal dimming feature of the LT3922 can be used to fine tune the brightness at or near 100% duty cycle and then set the exact 10: 1 or 100: 1 dimming ratio. This will allow the lamp group manufacturer to avoid Special grade LED and increase costs.
The LT3922 externally dims in the usual way when higher dimming ratios are required. The large bandwidth 400kHz buck mode LED driver in Figure 2 produces a 1000: 1 PWM dimming ratio at 100Hz. The 2MHz boost in Figure 1 Type LED driver achieves a 2000: 1 dimming ratio at 120Hz as shown in Figure 7a by placing a resistor at the RP pin with a frequency setting of 122Hz and setting the PWM pin voltage between 1.0V and 2.0V , The same circuitry can be set to internally generate PWM dimming with a dimming ratio of 128: 1, as shown in Figure 7b. In some applications, the LT3922 can be configured to operate at an external PWM dimming ratio of 5000: 1 Operation, PWM dimming can also be combined with the analog dimming of the LT3922 to provide brightness control in excess of 50000: 1.
Machine vision
In industrial-line applications, machine vision (Figure 8) uses high-speed digital photography and digital imaging to quickly provide visual feedback about the device, helping to quickly detect and isolate with little or no need for human inspection Defective products. The lighting of a machine vision system must be synchronized with the speed of the pipeline while producing a consistent light pulse for an indefinite turn-off time.
For conventional LED drivers, the driver can not hold the output voltage for as long as the PWM input signal stays low, because the output capacitors are gradually discharging, so general purpose LED drivers are not suitable for applications such as machine vision. The LT3922 digitally samples the output state of the converter at the falling edge of the PWM signal, and then performs 'keep-alive' by shutting down the PWM while the PWM is off and the LED is off by the high-side PMOS, Maintains its output voltage. When performing standard PWM dimming at frequencies above 100Hz, the longest turn-off time is 10ms or less without too much drain current drawn from the output. Machine Vision and Strobe Applications May With 100ms and 5s (or longer) turn-off times, the leakage current can increase tens to hundreds of times.
'Keeping the switch' ensures that the output capacitor holds the voltage recorded during the previous sampling period of the LT3922. The digital samples of the converter state can be stored indefinitely, provided the IC receives uninterrupted power input. This allows the LT3922 to operate at any given During the turn-off time, a consistent output current waveform is provided, as shown in Figure 9.
CAMERA: Camera IMAGE LIBRARY & PROCESSING: Image Library and Image Processing ROBOTICS: ROBOT FLASH: LED FLASH CONVEYOR WITH OBJECTS: Conveyor Belt with Object 1ms - 1s PERIOD: 1ms to 1s Period
LT3922 PRODUCES CONSISTENT LED OUTPUT REGARDLESS OF PERIOD BETWEEN FLASHES: The LT3922 produces a consistent LED output no matter how long between flashes
10ms SINCE LAST PULSE: 10ms10ms after the last pulse BEFORE NEXT PULSE: 10ms1 hour before the next pulse SINCE LAST PULSE: 1 hour after the last pulse 100ms, 1s, 1 hour, 1 day BEFORE NEXT PULSE: 100ms before the next pulse, 1s , 1 hour, 1 day
in conclusion
36V LED Driver with Internal Synchronization, 2A Switch The LT3922 is a compact, versatile LED driver that is easy to use in boost, buck and boost-buck topologies. Regardless of the topology used, all features Both high PWM dimming capability and internally generated PWM dimming.It is very easy to achieve low EMI with its Silent Switcher layout and SSFM.The compact synchronous switch maintains high efficiency even at frequencies up to 2MHz The same is true with its rugged, reliable fail-safe features that make it easy to meet automotive and other demanding applications.
Table 1: Enter a wide range of LED driver
36V, Synchronous LED Driver with 2A Switches LT3922 Configurable as Boost, Buck or Boost - Step-Down LED Driver
