High contrast applications (day to night) require high dimming ratios, and ADI’s LT3950 provides up to 20,000:1 dimming ratio, providing high dynamic range dimming for day/night vision. The flexibility to combine with analog input dimming provides accurate LED brightness control and eliminates the need for a microcontroller.
The LT3950 is a multi-topology DC/DC converter designed specifically to drive high current LEDs. It contains a 1.5 A, 60 V DMOS switch and supports an external PWM dimming PMOS. The LT3950 has an internal PWM generator for dimming that maps an analog control signal to an exponential scale used to define PWM dimming duty ratio. Using an exponential scale to define duty ratio preserves dimming resolution across a wide range of LED current. In addition to operating as a constant current source, the LT3950 also provides output voltage regulation. This can be used to prevent damage to the part in case of open LED events. A programmable switching frequency offers flexibility in design for higher efficiency or reduced component size. Enabling spread spectrum frequency modulation reduces EMI. The switching frequency can also easily be synchronized by driving the SYNC/SPRD pin with an external clock. LED current is programmed with a single external sense resistor and can be adjusted from zero to full-scale by an analog signal at the CTRL pin.
DC2788A is a boost LED driver featuring the LT3950. This demonstration circuit powers a string of LEDs at 330 mA. The step-up topology can be used to drive a string of up to 28 V of LEDs as assembled. The maximum output voltage capability of the LT3950 is 60 V. DC2788A runs from an input voltage range of 6 V to 24 V as built. It also runs at 2 MHz and has the capability to turn on spread spectrum frequency modulation (SSFM) for a frequency range of 2.0 MHz to 2.5 MHz. Dimming control can be achieved with analog dimming or PWM dimming either from an external or an internally generated clock source. DC2788A features undervoltage lockout (UVLO) set at 6.6 V with a 1.0 V hysteresis for turn-on.
The UVLO voltage, LED current, output voltage range, switching frequency, brightness control, SSFM, and the topology can all be adjusted with simple modifications to the demonstration circuit.
- 20,000:1 external PWM (pulse-width modulation) dimming allows high dynamic range dimming of the LED load
- Accurate PWM control and short-circuit protection so it can maintain constant brightness of LEDs in side-by-side strings and protect LEDs from damage
- SSFM ensures low EMI performance; passes CISPR25 Class 5; synchronized to PWM frequency to eliminate visible LED flicker