New-Tech Europe | March 2017 | Digital Edition

Automotive Special Edition

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 delivered to the LED and RSENSE2 voltage becomes zero. During this dimming off-time, EA output increases to its maximum and overcharges the EA compensation network. When the modulated PWM turns on again, it takes several cycles before it recovers while high-peak current is driven to the LED. This current peaking scenario shortens the lifetime of the LED. To avoid this problem, the LED dimming engine allows the PWM3 to be used as an override source of the OPA. When the PWM3 is low, the output of the EA is tristate which completely disconnects the compensation network from the feedback loop and holds the last point of the stable feedback as a charge stored in the compensation capacitor. When the PWM3 is high and the LED turns on again, the compensator network reconnects and the EA output voltage immediately jumps to its previously stable state (before PWM3 is low) and restores the LED current set value almost instantly. Complete Solution As mentioned earlier, a LED dimming engine can operate with minimal to no CPU intervention. Therefore, while offloading all of the work for controlling the LED driver to the CIPs, the CPU has

Figure 2. LED dimming engine

output capacitor. This event can lead to color temperature shifting and higher power dissipation of the LED. The slow discharging of the output capacitor can be eliminated 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 allow the LED to turn off quickly. Current Peaking When using a switched-mode power converter for driving the LED, the feedback circuit is employed to regulate the LED current. However,

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 weaken 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 diminishes due the slow discharge of the

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