DC2247A: 330mA @ 34V Max, 7 ~ 28V in

Demonstration circuit DC2247A is a 36V 2A synchronous 2MHz boost LED driver featuring the LT®3922. It drives a single string of LEDs at 330mA up to 34V when VIN is between 7V and 28V. It runs down to 4VIN with reduced ILED and can withstand VIN as high as 36V, but has overvoltage lockout (OVLO) set for 28V for this application. DC2247A runs at 2MHz switching frequency. Spread spectrum frequency modulation (SSFM) can be turned on with a simple jumper, reducing EMI. DC2247A comes with low EMI features including optimized layout, SSFM, and input and output filters. It passes CISPR 25 class 5 conducted and radiated EMI. It is protected against both open and short LED conditions and reports the faults.

The LT3922 can be PWM dimmed with an external PWM signal or an internally-generated PWM signal. DC2247A has a jumper that can be set to switch between internally generated PWM signal, externally-generated PWM signal, and no PWM signal (100% on). It can be analog dimmed with a control voltage on its CTRL pin. When run with both PWM dimming and SSFM, the spread spectrum aligns itself with the PWM signal for flicker-free operation.

There is an EMI filter on the input of DC2247A. There is also a small ferrite bead output filter. These filters, combined with proper board layout and SSFM are effective to help the PCB pass CISPR 25 class 5 conducted and radiated EMI. Please follow the recommended layout and four-layer thickness of DC2247A for low EMI applications.

The LT3922 data sheet gives a complete description of the part, operation and applications information. The data sheet must be read in conjunction with this Demo Manual for DC2247A. The LT3922EUFD is assembled in a 28-lead plastic 4mm x 5mm QFN package with a thermally enhanced ground pad. Proper board layout is essential for maximum thermal performance. See the data sheet section Layout Considerations.