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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
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
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,
Automotive
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Figure 2. LED dimming engine
38 l New-Tech Magazine Europe