Research on dimming technology of switchable LED driver

As the number of LED lighting devices grows, new integrated circuits are being developed to provide controlled power to LEDs . Energy conservation is a global issue, and power-hungry linear current sources no longer meet this requirement, so switching LED drivers have become the industry standard. Applications ranging from flashlights to stadium scoreboards require accurate control of current. In many cases, we also find that a mechanism is needed to change the intensity of the LED output in real time. This function is called "dimming control". This article aims to explain basic LED theory and several dimming control techniques for switching LED drivers.

LED Brightness and Color Temperature

LED brightness

The unit of measurement for the brightness of LED visible light is the luminous flux "intensity", candela (cd). The unit of measurement for the total power output of LED is lumen (lm). In addition, we must also understand that the average forward LED current determines the brightness of the LED.

LED color temperature

Color temperature is a measurement that describes the color of light emitted by an LED, and this information can be quantified on an LED data sheet. The color temperature of a particular LED will be specified within a certain range, and will vary depending on forward current, junction temperature, and age. Light with low color temperatures tends to be reddish-yellow (warm color temperatures), while higher values ​​tend to be bluish-green (cool color temperatures). Many colored LEDs specify a dominant wavelength instead of a color temperature, and provide a wavelength shift.

LED dimming method

Switching driver circuit for adjusting LED brightness There are two most common methods: pulse width modulation dimming (PWM Dimming) and analog dimming (Analog Dimming). Both methods control the average current through the LED or LED string, but when carefully comparing the advantages and disadvantages of these two dimming technologies, we can find obvious differences.

Vin must be higher than the total voltage of LED + RSNS . The inductor current is the LED current. The current can be regulated by monitoring the voltage at the CS pin. When CS starts to drop below the set voltage, the duty cycle of the current pulses through L1 (causing the LED and RSNS to increase) increases the average LED current. Selecting the correct RSNS for the CS voltage of the controller sets the LED current.

Analog dimming

LED analog dimming means adjusting the LED current per cycle. Simply put, this is a way to adjust the "constant" LED current level. Analog dimming can be achieved by adjusting the current sensing resistor RSNS or driving an analog voltage on the IC's partial DIM function pin. Figure 1 illustrates two possible analog dimming methods.

Adjust RSNS for analog dimming

From Figure 1, we can see that a fixed CS reference voltage will produce a change in LED current when the value of RSNS is changed. If you can find a potentiometer that can handle high LED currents and has a sub-1 ohm value, you can use this method to adjust the LED light.

Drive DC voltage on CS pin for analog dimming

Another more complex dimming technique is to directly control the LED current per cycle by driving a voltage into the CS pin. The voltage source is usually inserted into the feedback loop that samples the LED current and is buffered by an amplifier. The gain of the amplifier controls the LED current. With this feedback circuit, functions such as current and thermal protection mechanisms can be implemented to protect the LED.

The disadvantage of analog dimming is that the color temperature of the emitted light may change due to the LED current. In the case where the LED color temperature reaches a critical level, or when a particular LED shows a large change in color temperature due to changes in LED current, it is impossible to change the LED current to adjust the LED output.

PWM dimming

PWM dimming is the actual start-up and restart action of the LED current in a short period of time. The frequency of the start-restart cycle must be faster than the human eye can see to avoid light flickering. Generally speaking, a frequency of 200Hz or faster is an acceptable frequency.

This LED dimming is proportional to the duty cycle of the dimming wavelength and is determined by the following equation:

IDIM-LED = DDIM x ILED

Where IDIM-LED is the average LED current, DDIM is the duty cycle of the dimming wavelength, and ILED is the rated LED current setting with RSNS selection.

Adjusting LED Drivers

Many LED drivers now have a special PWM DIM pin that accepts a wider range of PWM frequencies and amplitudes, making it easy to interface with external logic. The DIM function only turns off the output driver while allowing the internal circuitry to continue operating, thus avoiding the delay of restarting the IC. We can also use the output enable pin with other logic shutdown functions.

[page] Two-wire PWM dimming

Two-wire PWM dimming is a common dimming method for automotive interior lighting. As VIN is adjusted to less than 70% of VIN-NOMINAL, the voltage change can be detected and the PWM wavelength can be converted to the corresponding PWM of the output drive. The disadvantage of this method is that the converter power supply must include circuitry that can overlap the PWM wavelength to its own DC output.

Fast PWM dimming using shunt devices

The PWM dimming frequency and duty cycle range are limited due to the delay in the turn-off and turn-on of the converter output. To solve the delay problem, an external shunt device such as a FET is placed in parallel with the LED or LED string to quickly bypass the converter output current around the LED . During the "LED off time", the current continues to flow into the inductor to avoid the long delay of the current going back and forth to the inductor current base. The delay time now translates to the limitation of the rise and fall time of the shunt device.

Care must be taken when shunting LED current using a current mode converter because of the output current overshoot when turning on the FET. The LM340x family of LED drivers are real-time controlled converters that do not have overshoot issues. We should keep the output capacitance on the LED low to maximize the on/off/on transition speed.

The disadvantage of using a fast dimming circuit with the output turned off is a loss in efficiency. When the shunt device is on, the power consumed by the VShunt device X ILED is lost as heat. Using a low RDS-ON FET can reduce this efficiency loss.

Another disadvantage of using shunted LEDs for faster switching times is that this action does not work in a boost converter topology. In a boost topology, shorting the load is equivalent to shorting Vin to ground through the inductor.

LM3409 Multiple Dimming Functions

The LM3409 from National Semiconductor is a unique LED driver that provides full analog and PWM dimming capabilities. There are 4 possible methods of LED dimming on this device:

1. Directly use 0V~1.24V voltage source to drive IADJ pin for analog dimming

2. Place a potentiometer between the IADJ pin and ground for analog dimming

3. Use the start pin for PWM dimming

4. Use external shunt FET for PWM dimming

We must use a potentiometer to connect the LM3409 to perform analog dimming. The internal 5uA current source will establish a voltage on RADJ, thereby changing the internal current sensing setting. We can also use a DC voltage to directly drive the IADJ pin to achieve the same effect.

LED Efficiency

LED luminous efficacy can be roughly defined as the input power wattage divided by the visible output light power lumens; expressed as lumens per watt. All power that enters the LED but does not become visible light is converted into heat or invisible electromagnetic radiation. The nature of LEDs is that they can achieve higher efficiency at low output levels.

At low dimming levels, using analog dimming of LEDs is more efficient than PWM dimming. Assuming the PWM dimmed LED has a low duty cycle, the LED peak current will be higher than using the average time LED current.

Adjusting offline LED lamps

The AC voltage that offline LED lighting can operate on is 100 VAC in Japan and over 300 VAC in some industrial applications. One of the advantages of replacing fluorescent bulbs with LEDs is improved efficiency. LED lamps can be used for about 80,000 hours before they need to be repaired or replaced, while fluorescent lamps may only last 6,000 hours. In the long run, the cost savings of switching to LED lamps usually far exceeds the cost of the LED lamps themselves.

Residential lighting applications are more likely to require the use of dimmable LED lamps. The two most common types of dimming are TRIAC and 10 VDC, with TRIAC dimmers being more common. TRIAC controllers are available at any home improvement store and are more common in most homes. TRIACs "cut out" some of the AC wavelength to increase duty cycle efficiency and provide current to the lamp. The disadvantage of TRIAC dimming is that it does not completely solve the problem of flickering light output when dimming is low. Advances in LED driver technology have improved TRIAC dimming technology, and today's drivers are more capable of "decoding" low dimming settings to output light without flicker.

The so-called 10 VDC dimming is to apply a DC voltage of 0V to 10V to the dimming controller on a 10V lamp, where 0V corresponds to no light and 10V produces full-scale light output. 10 VDC lighting requires the installation of additional wiring from the dimmer to the lamp, and is more common in higher-grade lamps because such lamps require more linear dimming control and better dimming ratios than TRIAC dimmers.

Conclusion

There are many ways to dim the light of an LED powered by a switching regulator. The two main dimming methods, PWM and analog, each have their own advantages and disadvantages. PWM dimming greatly reduces the color variation of the LED, has different brightness levels, and provides additional logic to establish the PWM wavelength. Analog dimming is a simpler circuit, but is not suitable for applications that require a constant color temperature.