Analog Devices LT3922 | Demoboard DC2247A

Description

Demonstration circuit DC2247A is a 36V 2A synchronous2MHz boost LED driver featuring the LT®3922. It drivesa single string of LEDs at 330mA up to 34V when VIN isbetween 7V and 28V. It runs down to 4VIN with reducedILED and can withstand VIN as high as 36V, but has overvoltagelockout (OVLO) set for 28V for this application.DC2247A runs at 2MHz switching frequency. Spreadspectrum frequency modulation (SSFM) can be turnedon with a simple jumper, reducing EMI. DC2247A comeswith low EMI features including optimized layout, SSFM,and input and output filters. It passes CISPR 25 class 5conducted and radiated EMI. It is protected against bothopen and short LED conditions and reports the faults.The LT3922 has an input voltage range from 2.8V to 36V.Its internal synchronous 2A 40V switches allow up to 34Vof LEDs on the output with room for overvoltage protectionand overshoot during an open LED event. It has adjustableswitching frequency between 200kHz and 2.5MHz. It canbe synchronized to an external source or programmed withSSFM for low EMI. The PWMTG high side PWM MOSFETdriver assists with short-circuit protection and versatility.LT3922 can be configured as boost, boost-buck, or buckmode LED driver and maintain all of its low-EMI, PWMdimming and fault diagnostic features.The LT3922 can be PWM dimmed with an external PWMsignal or an internally-generated PWM signal. DC2247Ahas a jumper that can be set to switch between internallygeneratedPWM signal, externally-generated PWM signal,and no PWM signal (100% on). It can be analog dimmedwith a control voltage on its CTRL pin.When run with both PWM dimming and SSFM, the spreadspectrum aligns itself with the PWM signal for flicker-freeoperation.L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of LinearTechnology Corporation. All other trademarks are the property of their respective owners.Small ceramic input and output capacitors are used tosave space and cost. The board is designed with tiny, highfrequency capacitors on both sides of the VOUT pins fora reduction in radiated EMI. The open LED overvoltageprotection uses the IC’s constant voltage regulation loopto regulate the output to approximately 37.5V if the LEDstring is opened although it may reach almost 40V peakduring transient from running LEDs to open. There is aprotection diode from LED+ to GND to prevent negativeringing during a short circuit. The output current can bemonitored through the ISMON output pin.Undervoltage and overvoltage lockout can be adjusted onthe circuit with a few simple resistor choices.There is an EMI filter on the input of DC2247A. There is alsoa small ferrite bead output filter. These filters, combinedwith proper board layout and SSFM are effective to helpthe PCB pass CISPR 25 class 5 conducted and radiatedEMI. Please follow the recommended layout and four-layerthickness of DC2247A for low EMI applications.The LT3922 data sheet gives a complete description ofthe part, operation and applications information. Thedata sheet must be read in conjunction with this DemoManual for DC2247A. The LT3922EUFD is assembled in a28-lead plastic 4mm x 5mm QFN package with a thermallyenhanced ground pad. Proper board layout is essentialfor maximum thermal performance. See the data sheetsection Layout Considerations.

More information

• Datasheet