How to drive white LEDs in portable devices

How to drive white LEDs in portable devices
How to drive white LEDs in
portable devices
by Jan Gripsborn, AnalogicTech, Sweden
In order to save power, many portable devices use white LEDs to provide display illumination. The author explains how a high-efficiency
driver IC simplifies design of the LED driver circuitry.
The explosive growth of colour displays in
portable appliances has been fuelled by the
consumer’s need to display more information and
the introduction of built-in (or ‘snap-on’) cameras.
In parallel with this growth, the development of
high-efficiency white LEDs has made them an
attractive alternative to illuminate the display.
For all colours to be displayed correctly, under
all ambient light conditions, a broad-spectrum
light source is needed (i.e. white light). Many
different backlighting technologies are available
for this purpose, e.g. cold cathode fluorescent
lamps (CCFL), electro-luminescent lamps (EL)
and more recently, white LEDs. However, while
the first two alternatives require a high alternating
voltage of several hundred volts, the white LED
can be driven from a low-voltage source below
5 V. It is this characteristic that makes white LEDs
very attractive, particularly for battery-driven
appliances.
Challenges
For a white LED, the brightness is (logarithmically)
proportional to the forward current through the
diode. For current to flow, the diode must be
biased with a voltage that is in excess of its forward
voltage drop. The diode forward voltage drop is
a characteristic of the LED colour and hence the
material used to manufacture the diode. Some
examples are:
Blue: 4,5 V, White: 3,6 V, Red: 2,0 V
The dominating power source for handheld
products is a Lithium-Ion or Lithium-Polymer
battery. This battery type has a cell voltage
that ranges from 4,2 V (depending on type)
down to about 2,7 V during the discharge
cycle. Therefore, to be able to drive the
white LED during the whole discharge cycle,
the battery voltage must be boosted from
2,7 V at its lowest, to a minimum voltage of
3,6 V. Either a DC/DC boost converter with an
external inductor can be used to achieve this
or alternatively the voltage can be boosted and
regulated with a capacitive DC/DC converter
or so-called ‘charge pump’.
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In many applications, it is
important to be able to control
backlight brightness. This can
be accomplished in two ways,
either via constant regulation of
the current through the diode
or via PWM control to set and
control the current. Usually
a PWM frequency between
100 Hz and 10 k Hz is used to
control the current for maximum
brightness between 15 and
30 mA.
In a typical application, several
LEDs can be used to provide
Fig. 1: The Lithium-Ion battery discharge curve.
a complete solution. In this
case, one problem to consider
is that due to manufacturing process variations Using a resistor in every branch can decrease
the forward voltage drop of a particular LED the influence of a varying forward voltage drop.
can vary from diode to diode. This variation If the LEDs forward voltage drop is less than the
can lead to deviations in brightness levels nominal value, the current will increase. This
between the LEDs, depending upon how they leads to an increased voltage drop over the
are connected. Connecting all the LEDs in series resistor and then again a decreased current. The
ensures that the same current will flow through resistor therefore provides negative feedback.
all of the LEDs, therefore they all emit the same On the other hand, there is now yet another
brightness. However, the disadvantage of this component dissipating power and therefore
is that it requires a higher compliance voltage reducing the efficiency.
for forward conduction (e.g. six diodes in series
would require a minimum of 22 V (6 x 3,6 V) for Methods for power conversion
correct operation). In addition, a white LED can
One of the most important parameters to
fail open circuit, cutting off the supply to the other
consider in selecting a backlighting solution is the
diodes. Therefore, not only will the backlight be
efficiency. Maximising battery life is a key design
completely off, additional damage may occur due
parameter and, if the efficiency of the chosen
to the inductive DC/DC converter increasing the
solution is low, then equipment operation time
output voltage to values in excess of component
and standby time are severely reduced. There are
operating limits, while trying to maintain a constant
several different methods for voltage conversion
current in an open loop!
and white LED driving. As mentioned earlier,
Variations of this arrangement use parallel both inductive and capacitive DC/DC converters
branches of serially connected diodes, e.g. two are used. They can be either constant current or
branches with three LEDs in each branch. There constant voltage topologies. However, it is not so
will be a difference in forward voltage drop easy to compare the efficiency between different
between each of the diodes, but as there are topologies. Since the brightness is proportional
several diodes in each branch, then generally the to the current, the best way to compare the
difference between the branches will be averaged efficiency is for a given diode current to look at the
total input power to the DC/DC converter.
out to become more or less equal.
Elektron January 2005
ensures that even during a GSM transmission burst
(when the battery voltage might decrease up to
400 mV) there will be enough voltage to keep
the programmed current and avoid flickering
of the backlight.
Fig. 2:. Capacitive converter with constant output voltage using the
AAT3112.
Fig. 3: Capacitive converter with constant current output using the
AAT3141.
Capacitive converters with constant output
voltage
Many of the commercially-available LCD
modules already include the LEDs and bias
resistors for the backlight. The LEDs are often
connected in parallel.
Those modules only require a single 5 V supply.
In this case, a constant output voltage, capacitive
DC/DC converter, could provide a solution.
Capacitive converters with constant output
current
The AAT3141 capacitive DC/DC converter has
four constant current outputs. This low noise
driver IC uses constant frequency tri-mode,
(1x), fractional (1,5x) and doubling (2x) charge
pump conversion to maximise efficiency while
controlling white LED brightness, without the
need for ballast resistors but with accurate
current matching.
Having three operation modes ensures that a
very high efficiency is achieved over the whole
Lithium-Ion battery voltage range. If a pure
voltage doubler were used, the output from
a fully charged battery would be in excess of
8 V. Regulation down to 3,6 V would then be
required, representing a significant power loss. If
the battery voltage is increased only 1,5 times, this
loss is decreased and the efficiency will be much
higher. When the input voltage is higher than
the forward voltage for the LED, the converter
enters 1x mode controlling the current in a linear
fashion without using the charge pump at all.
The switchover between the
1x, 1x and 2x mode is fully
automatic and transparent to
the user.
One should also note the
typical white LED VF is around
3,5 V. The Li-Ion battery in
most applications spends most
of its useable life (87%) at and
above 3,6 V. Thus, for close
to 87% of the discharge cycle,
the 3141 operates in 1x mode
consuming only 500 uA with
an excellent efficiency.
Fig. 4:. Inside the AAT3141.
Elektron January 2005
Having a 2x doubling mode
With four 30 mA outputs, the converter uses a
single wire to control the brightness level in 32
independent steps. Outputs can be paralleled for
driving higher current LEDs or the outputs can
be used individually. The output drivers are very
flexible and can be operated either as a group of
four, with all outputs programmed to the same
brightness level, or as a group of three and one,
where the output brightness can be controlled
independently. The latter is for applications where
there is a main and a sub-display. Again, this is
achieved with a single control wire.
Utilising a proprietary switching arrangement
between the internal power supplies and each
output, the converter monitors each diode and
selects the optimum power supply voltage
according to the VF of the diode being driven.
This ensures that no unnecessary power is
wasted. In less efficient, competitive solutions,
only one diode is monitored - which could result
in the remaining LED driver stages dissipating
excessive power. Compared to competitive
solutions, this converter uses up to 25% less
current.
The simple serial control (S2Cwire) digital input
is used to enable, disable and set the LED drive
current with a 32-level logarithmic scale LED
brightness control. In the majority of competing
converters, the diode brightness is controlled by
a low frequency PWM signal between 100 Hz
and 10 kHz. As this is within the audio frequency
band, additional filtering may be required.
Alternatively, the current could be programmed
with an external resistor. The simple single-wire
interface used in the converter results in lower
noise and better accuracy over temperature than
competing resistive solutions.
Brightness (light) is logarithmically proportional
to the LED current, and the output current from
the converter is programmed accordingly.
Therefore, every step change in current will
result in an even change in brightness. 32 steps
are sufficient to simulate a step-less increase/
decrease of brightness if required.
Conclusion
The choice of converter depends on many
design criteria. It is important not only to look
at the efficiency but also to carefully study how
the LEDs are connected in order to minimise
brightness differentials. Noise, number of
external components, control and, not least,
price are other important parameters to consider
when selecting your backlighting solution.
Contact Stephen Delport,
Hi-Q Electronics,
Tel (021) 595-1307,
[email protected] ‰
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