to achieve dimming, the PWM3 is used as a modulator of the CCP output while driving the MOSFET Q2 to rapidly cycle the LED on and off. The modulation is made possible through the DSM module and the modulated output signal is fed to the COG. PWM3 provides pulse with variable duty cycle which controls the average current of the driver and in effect controls the brightness of the LED.
The LED dimming engine can not only accomplish what the typical LED driver controller does but it also has features that solve the typical problems that an LED driver poses. We’ll now walk through these problems and how a LED dimming engine can be used to avoid them.
Flickering is one of the challenges that typical dimmable LED drivers may have. While flickering can be a fun effect when it’s intentional, when LEDs inadvertently flicker it can ruin the user’s desired lighting design. In order to avoid flickering and provide a smooth dimming experience, the driver should perform the dimming step from 100% light output all the way down to its low-end light level with a continuously fluid effect. Since the LED responds instantaneously to current changes and doesn’t have a dampening effect, the driver must have enough dimming steps so the eye does not perceive the changes. To meet this requirement, the LED dimming engine employs PWM3 for controlling the dimming of the LED. The PWM3 is a 16-bit resolution PWM that has 65536 steps from
100% to 0% duty cycle, ensuring a smooth lighting-level transition.
LED Color Temperature Shifting
The LED driver can also shift the LED’s color temperature. Such color change can be noticeable to the consumer and undermine claims made about the high-quality lighting experience of LEDs. Figure 3 shows a typical PWM LED dimming waveform. When the LED is off, the LED current gradually declining due the slow discharge of the output capacitor. The color is changing with the current change so the gradual discharge of the output capacitor can lead to color temperature shifting and higher power dissipation of the LED.
Fig. 3: LED dimming waveform.
The ideal current should be rectangular just like the one shown in Figure 3. To achieve this, the slow discharging of the output should be eliminated. This can be done by using a load switch. For example, in Figure 2, the circuit used Q2 as a load switch and the LED dimming engine synchronously turns off the COG PWM output and Q2 in order to cut the path of the decaying current and allows the LED to turn off quickly.
When using a power converter for driving the LED, a feedback circuit is usually employed to regulate the LED current. However, during dimming, the feedback circuit can create current peaking (see Figure 3) when the operation is not handled properly. Looking back at Figure 2, when the LED is on, a current is delivered to the LED and the voltage across RSENSE2 is fed to the EA. When the LED turns off, no current is