Pulse Width Modulation (PWM) dimming offers a dimming ratio of 3000:1 or higher without compromising accuracy or LED color. In this article, we'll explore how the LT3761 enables efficient LED dimming with an internal PWM signal generator.
LED dimming is typically done in two ways: analog and PWM. Analog dimming adjusts the DC current of the LED string to control brightness, while PWM dimming changes the duty cycle of the constant current to adjust the average current. Although analog dimming is simple, it suffers from significant accuracy loss at low brightness levels—over 25% at just 10:1—and can cause color shifts. On the other hand, PWM dimming maintains high accuracy and consistent color even at extreme dimming ratios, making it ideal for applications where precision matters.
The LT3761 combines the simplicity of analog dimming with the precision of PWM. It features an internal PWM signal generator that allows for high dimming ratios without requiring external components like microcontrollers or oscillators. The internal PWM provides up to 25:1 dimming, while an external PWM input can achieve up to 3000:1. This flexibility makes the LT3761 suitable for a wide range of lighting applications.
As a high-power LED driver, the LT3761 supports various topologies such as boost, SEPIC, buck-boost, and buck configurations. With an input voltage range of 4.5V to 60V and an output voltage range of 0V to 80V, it’s ideal for demanding applications. The device also includes features like LED open-circuit protection, short-circuit protection, and support for constant current or constant voltage operation.
Figure 1 shows a 60W automotive headlamp application using the LT3761 in boost mode with 94% efficiency and 25:1 internal PWM dimming. Unlike other drivers, the LT3761 generates its own PWM signal internally, eliminating the need for extra pins or components.
The internal PWM signal is programmable, allowing users to set both frequency and duty cycle. The frequency is determined by a capacitor connected between the PWM pin and ground, while the duty cycle is controlled via a small current on the DIM/SS pin. This design ensures precise control and compatibility with digital signals from microcontrollers.
When using the internal PWM, the minimum duty cycle is around 4%, and the PWM pin can be tied to INTVCC for full brightness. Additionally, the LT3761 can accept external PWM signals for even higher dimming ratios.
Figures 2 through 4 illustrate the internal PWM signal, LED current waveform, and the μA-level control on the DIM/SS pin. These visuals highlight the effectiveness of the LT3761's dimming capabilities.
In conclusion, the LT3761 is a powerful and versatile LED driver that simplifies high-performance dimming with its built-in PWM signal generator. Whether you're designing automotive lighting, industrial systems, or smart lighting solutions, the LT3761 delivers accurate, reliable, and efficient performance.
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