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(DSM), comparator, programmable
ramp generator (PRG), op amp
(OPA), and pulse-width modulator
3 (PWM3). Combining these CIPs
with other on-chip peripherals, such
as fixed-voltage regulators (FVR),
digital-to-analog converters (DAC)
and Capture/Compare/PWM (CCP),
completes the whole engine. The
COG provided the high frequency
switching pulse to MOSFET Q1 to
allow the transfer of energy and
supply current to the LED string. The
switching period of the COG output
is set by the CCP and the duty cycle,
which maintains the LED constant
current and is dictated by the
comparator output. The comparator
produces an output pulse whenever
the voltage across Rsense1 exceeds
the output of PRG module. The PRG,
whose input is derived from OPA
output in the feedback circuit, is
configured as a slope compensator
to counteract the effect of inherent
subharmonic oscillation when the
duty cycle is greater than 50%.
The OPA module is implemented as
an error amplifier (EA) with a Type II
compensator configuration. The FVR
is used as the DAC input to provide
voltage reference to the OPA non-
inverting input based on the LED
constant current specification.
In order 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
Flickering is one of the challenges
that
typical
switched-mode
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.
Automotive
Special Edition
Figure 1: Diagram of four LED strings being controlled by a
Microchip PIC16F1779 8-bit microcontroller
New-Tech Magazine Europe l 37