LED brightness analog dimming and PWM dimming

2017-5-4 Vister: 1621

Adjust the LED brightness LED end products with the sustained and rapid development of LED application market, it appears very necessary. Is also such as LED lamps and displays must focus on the link.
Contrast generally are defined for the system can produce the brightest color (white) with the darkest color (black) luminosity ratio. For
LED brightness adjustment, mainly divided into analog dimming and PWM dimming, the following will be two dimming introduced:
Analog dimming:
For the current is proportional to the LED brightness within a certain range, LED analog dimming of each cycle of the LED current to be adjusted. More simply put, it is to constantly adjust the LED current. Analog dimming by adjusting the current detection resistor RSNS, or a dimming function of the analog voltage driver IC pin.
Analog dimming by adjusting the RSNS
When using the fixed CS voltage reference, RSNS value changes will correspond to the change of the LED current. If you can handle the high LED current can be found, at the same time can also provide sub-ohm value of the potentiometer, it will be a viable method of LED dimming.
2, the DC voltage drive CS pin analog dimming
More complex technology is through the direct control of the CS pin voltage drive of each cycle of the LED current. Usually the voltage source into sampling the LED current feedback loop buffer amplifier. The LED current can be controlled by the gain of the amplifier. With the feedback circuit and foldback current and heat functionality of the class in order to further protect the LED.
However, the disadvantage is that analog dimming of light emitted color temperature will change as a function of the LED current, the LED color is essential, or a specific LED's color temperature changed significantly in the LED current changes by changing the LED current, the output of the LED dimming will be banned.
PWM dimming:
PWM dimming, the LED forward current to reduce the duty cycle of 0-100% conversion for the brightness control. However, the frequency of the PWM dimming signal must be greater than 100Hz, to avoid flicker or jitter. In order to minimize can hear the noise and radiation, the dimming frequency range of the high-end lighting systems typically require tens of thousands of Hz. However, higher dimming frequency will be significantly reduced-driven dimming range, but reduces the maximum brightness of the system. The following will be explored in a fixed frequency, time delay hysteresis control and constant on-time buck LED driver, the high-frequency PWM dimming technology performance and test data to measure performance under different configurations.
Dimming range
PWM dimming, the LED forward current in a controlled duty cycle (DDim) on / off (ON / OFF) in order to achieve the desired brightness level. DDim the dynamic range is defined PWM dimming configuration can achieve the maximum brightness level. As mentioned above, the brightness of the LEDs and the LED forward current is proportional to, therefore, the average maximum and minimum LED current configuration of the PWM dimming by type 1 and type 2
ILED_Max = DDim_Max × ILED (1)
ILED_Min = DDim_Min × ILED (2)
The ILED for the LED current, the highest average value of the LED current ILED_Max, ILED_Min the average minimum value of the LED current, DDim_Max dimming duty cycle, DDim_Min minimum tone light duty cycle. Therefore, the ratio of the highest and lowest LED bright as a PWM dimming range using type 3.
Dimming range = DDim_Max / DDim_Min (3)
 Equation 3 represents the relationship between the PWM dimming range with maximum and minimum dimming duty cycle. For a given dimming frequency FDim,, DDim_Max said maximum duty cycle, the LED current in the next dimming before the start of the cycle, from the forward current required to reduce to zero; DDim_Min said that the minimum duty cycle, ie, the LED current rose from zero to the time of the forward current (IF).
        DDim_Max and DDim_Min expressed as follows:
        DDim_Max = (T-tSD) / T (4)
        DDim_Min = (tD + tSU) / T (5)
Among them, T-dimming cycle (T = 1/FDim), tD, for DIM rising edge to the delay between the first pulse of the power FET tSU rose from zero to the desired current for the LED current rise time, tSD for from DIM pulse falling edge to the LED current is zero between the fall time. Expressed in type 4 and type 5 DDim_Max, DDim_Min and LED drive transmission (power-train) the relationship between the characteristics and PWM dimming scheme.
PWM dimming due to its fast response, and to more accurately dimming. The most widely applied in actual use.

On one:Wide viewing angle, high brightness LCD module         Next: The next one is not recorded