With the continuous rapid development of the LED application market, the adjustment of LED brightness in LED terminal products is very necessary. It is also a link that must be paid attention to such as LED lamps and displays.
Contrast is generally defined as the ratio of the illuminance of the brightest color (white) to the darkest color (black) that the system can produce. For the adjustment of LED brightness, it is mainly divided into analog dimming and PWM dimming. The following two dimming methods will be introduced:
Analog dimming:
Since the brightness of the LED is proportional to the current in a certain range, the analog dimming of the LED is to adjust each cycle of the LED current. More simply, it is constantly adjusting the current of the LED. Analog dimming can be done by adjusting the current sense resistor RSNS or by using a dimming function pin of the analog voltage driver IC.
1. Simulate dimming by adjusting RSNS
When a fixed CS reference voltage is used, the change in RSNS value will correspond to the change in LED current. If you can find a potentiometer that can handle high LED current while also providing a sub-1 ohm value, this will be a viable method for LED dimming.
2. Simulate dimming by driving the CS pin with DC voltage
A more sophisticated technique is to directly control the current of each LED cycle by driving the CS pin with a voltage. The voltage source is typically inserted into a feedback loop that samples the LED current and is buffered by the amplifier. The LED current can be controlled by the gain of the amplifier. Using this feedback circuit, functions such as current and heat return can be implemented to further protect the LED.
However, the disadvantage of analog dimming is that the color temperature of the emitted light changes with a certain function of the LED current. When the color of the LED is critical, or the color temperature of a particular LED changes greatly when the LED current changes, Dimming the output of the LED by changing the LED current will be disabled.
PWM dimming:
In PWM dimming, the LED forward current is switched between 0% and 100% with a reduced duty cycle for brightness control. However, the frequency of the PWM dimming signal must be greater than 100 Hz to avoid flicker or jitter. To minimize audible noise and radiation, the dimming frequency range of high-end lighting systems typically requires tens of thousands of Hertz. However, a higher dimming frequency will greatly reduce the dimming range of the drive, which in turn will reduce the maximum brightness of the system. The performance of high-frequency PWM dimming techniques in fixed-frequency, time-delay hysteresis control, and fixed-on-time buck LED drivers is discussed below, and test data is used to measure performance in different configurations.
Dimming range
In PWM dimming, the LED forward current is turned ON/OFF with a controlled duty cycle (DDim) to achieve the desired brightness level. DDim's dynamic range defines the maximum brightness level that can be achieved with a PWM dimming configuration. As mentioned above, the LED brightness is proportional to the LED forward current, so the highest and lowest LED current averages obtained using the PWM dimming configuration are represented by Equations 1 and 2, respectively.
ILED_Max=DDim_Max×ILED (1)
ILED_Min=DDim_Min×ILED (2)
Among them, ILED is the LED current, ILED_Max is the average maximum value of the LED current, ILED_Min is the average minimum value of the LED current, DDim_Max is the maximum dimming duty ratio, and DDim_Min is the minimum dimming duty. Therefore, the ratio of the highest and lowest LED brightness is also seen as the PWM dimming range, expressed in Equation 3.
Dimming range = DDim_Max / DDim_Min (3)
Equation 3 represents the relationship between the PWM dimming range and the maximum and minimum dimming duty ratios. For a given dimming frequency FDim, DDim_Max represents the maximum duty cycle, which is the time from the required forward current to zero before the start of the next dimming cycle; DDim_Min represents the minimum duty cycle, ie the LED current The time from zero to the required forward current (IF).
DDim_Max and DDim_Min are expressed as follows:
DDim_Max=(T-tSD)/T (4)
DDim_Min=(tD+tSU)/T (5)
Where T is the dimming period (T=1/FDim), tD is the delay from the rising edge of the DIM pulse to the first pulse of the power FET, and tSU is the rise time of the LED current from zero to the required current, tSD The falling time from the falling edge of the DIM pulse to the LED current equal to zero. Equations 4 and 5 express the relationship between DDim_Max, DDim_Min and the power-train characteristics of the LED driver and the PWM dimming scheme.
PWM dimming is fast due to its fast response and can be dimmed more accurately. It is the most widely used in practical use.
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