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integrated current delivered therefore
follows the profile of the reference
signal. Various techniques can be
used to control the reference signal
and therefore manipulate the power
output. For smart-home use, power
converters can integrate additional
inputs that tune the reference signal
based on the preferred dimming
level.
The power converters for LEDs may
also be designed to handle different
supply types. For example, a simple
retrofit bulb that includes the LED
ballast, dimming control and RF
interface to a smart-home network,
will generally work with a 50 Hz/60
Hz mains supply. However, the
power converters for bulbs that are
designed to go into low-voltage strips
and buses will need to be able to
handle DC supply inputs instead.
Inrush current is an important
consideration in LED lighting design.
The power supply to the LED presents
a capacitive load to the mains supply,
voltage transmitted to its low-voltage
VCC pin from the high-voltage
supply as a means of detection. The
ripple on the VCC rail is used using
a sigma-delta modulation scheme
to provide a reference signal to the
PWM switching circuitry. When a DC
input is applied, the lack of ripple on
the VCC rail results in a reference
signal suitable for DC-mode power
conversion without PFC.
To support remote control, the
FL7701 has an analog input pin that
can support 0 V to 10 V dimming
control by using a resistive divider.
This signal is fed into the digital
controller and used to determine
the output of a digital-to-analog
converter that provides the reference
signal to the PWM control circuitry.
A microcontroller can be used to
generate the input signal to the
LED ballast, potentially receiving its
commands from an RF interface.
The FL7701 employs a soft-start
function to limit inrush current,
applying a delay to the internal
voltage reference signal used by
the PWM circuitry so that it rises
Figure 1: Block diagram of the FL7701
which leads to the potential for a
high current to be drawn for a short
period when the LED is switched on
as the capacitors around the supply
charge. Because this current can
be much higher than the steady-
state current, it can lead to overload
problems when a large number of
lamps attached to one mains circuit
are activated. Limiting inrush current
at startup provides a design that
has greater compatibility with home
electrical circuits.
Power-factor correction (PFC) is a
further consideration in terms of
mains compatibility. The switched-
mode techniques needed for efficient
LED dimming should not be allowed
to drive additional harmonics onto
the electricity supply.
The need for efficient power delivery
and support for PFC has led to the
introduction of parts such as the
Fairchild FL7701. The Fairchild FL7701
has been designed to work with both
AC and DC supplies, using the ripple
32 l New-Tech Magazine Europe