New Technology Features and Applications of LED Lighting Brightness Control

2.2 Application of new dimming technology-PWM modulation technology

The wavelength of light emitted by an LED is closely related to the forward current driven into the device. To prevent color hue changes, the dimming method must be carefully selected. In the past, the most commonly used dimming method was to change the forward current or voltage on the device. Unfortunately, changes in current or voltage will change the wavelength of the light. This effect is proportional to the wavelength. Longer wavelengths experience the strongest change in color hue to current. In many applications, this result is unacceptable. If PWM modulation technology is used, the LED can be properly dimmed without causing wavelength changes. The on and off operation of the LED is achieved by changing the duty cycle. At this time, the forward current (1F) is a constant current.

2.2.1 Low-frequency and high-frequency dimming applications

⑴ Low frequency dimming. Since LEDs have a stable instantaneous drive current, they are suitable for low frequency dimming. The color temperature of the LED remains unchanged at all brightness levels. Another advantage of low frequency dimming is that the brightness can be reduced to 1%. Therefore, the dimming range is 100:1. The frequency is selected to avoid visible flicker. The PWM signal must be greater than 100Hz. If the selected frequency is too high, the built-in low-pass filter will start to merge the PWM signal and produce a nonlinear reaction. At the same time, the soft start function of the adjustment pin will cause a delay in the rise or fall of the PWM signal. This will make the LED current nonlinear, and the effect will be more significant when the frequency increases.

The common pulse width modulation diagram of low-frequency and high-frequency signals is shown in Figure 2. This figure is a pulse width modulation diagram taking the ZXLD1350 driver of ZETEX as an example.

Figure 2 Schematic diagram of pulse width modulation using the ZXLD1350 driver as an example

The upper limit of the low frequency is recommended to be 1kHz. The effect of possible audible noise from the inductor also needs to be considered. This may occur with some inductors with loose windings, and will be more noticeable at a PWM frequency of 1kHz than at 100Hz.

⑵High frequency dimming

If the system requires low radiation and input/output harmonics, high-frequency dimming is suitable. However, the dimming range will be reduced to 5:1. ZXLDl350 has a built-in low-pass filter that integrates high-frequency PWM signals and can perform DC dimming control. If the PWM frequency is higher than about 10kHz and the duty cycle is greater than the specified minimum value, the device will remain in operation and the output will remain unchanged.

⑶ Input buffer transistor

When PWM dimming is performed, the input bipolar transistor Q should use an open collector output (as shown in Figure 2). This ensures that the 200mV input shutdown threshold is reached. Direct PWM control can also be performed without a buffer transistor, but it must be done with extreme caution. This operation will overload the built-in 1.25V voltage reference. If 100% PWM (DC) uses a 2.5V input voltage, the output current into the LED will reach twice the normal current. And it may damage the ZXLDl350. Overdriving with a 5V logic signal will most likely damage the device when the rated voltage of the adjustment pin is exceeded.

⑷Soft start and decoupling capacitors

Any additional capacitor on the adjust pin will affect the PWM signal rise and fall. This needs to be taken into account as the rise time will increase by approximately 0.5ms/nF. Compare this to a 100Hz PWM, where the on time Ton at 50% duty cycle and the off time Toff are 5ms, and the on time Ton at 1% duty cycle is 0.1ms. A lnF on the adjust pin will result in a rise time of 0.5ms, which will cause errors and limitations when dimming at low duty cycles.

2.2.2 Accurately control brightness using linear and PWM input signals

This solution uses highly integrated technology. It is characterized by high efficiency and low power consumption, minimal external components, and accurate temperature/brightness control. The new LM3402/02HV switching regulator can meet the needs of this technology. Figure 3 shows the application diagram of the LM3402/02HV switching regulator.

The LM3402/02HV switching regulator in Figure 3 has a dedicated brightness control (DIM) pin that can accurately control brightness using linear and PWM input signals. Lighting systems that use LEDs to emit light generally use PWM dimming control to control light brightness, and this brightness control method has become a widely used standard in the industry. As long as the current of the forward LED is adjusted, the light output of the LED will increase or decrease in a linear manner, but most of the light bands will shift. Some applications do not have very strict requirements on color, so linear brightness control will still be used, but automotive lights such as brake lights, LCD backlights, and direct display RGB LEDs have extremely strict requirements on brightness and color, so these applications generally use PWM to control brightness.

Figure 3 Application diagram of LM3402/02HV switching regulator

The features of the LM3402/02HV switching regulator are as follows: input voltage range 6V ~ 75V, using the circuit layout of the buck regulator; can provide a constant drive current for the light-emitting diode, the feedback voltage is 200mV; when the RON pin is at a low potential, the shutdown current will be further reduced; accurate PWM brightness control; switching frequency up to 1MHz; with hysteresis function, and fixed conduction time. Therefore, the switching frequency (FSW) can be controlled within the entire input voltage range.