Download Auto-biasing white LED drivers reduce overall power

Auto-biasing white LED drivers reduce overall power
By Manfred Plankensteiner
Engineering Manager
Energy Management and Audio
Product Group
Dialog Semiconductor
E-mail: manfred.plankensteiner
@diasemi.com
The growth in use of mobile
phone handsets with color displays continues to place heavy
demands on batter y power.
Since cellphones and other portable devices, such as communicators and PDAs with wireless
capability, are constantly used
for multiple functions, there is
significant demand on the backlighting requirements of the
LCD module, which needs to be
illuminated all the time. While
there are many manufacturers
of standalone white LED drivers, one technique for currentcontrol boost converters automatically optimizes current efficiency, regardless of the number of LEDs it supports.
All color displays need some
form of backlight, which is usually enabled by white LED drivers. To ensure uniform brightness over the complete display,
the equipment normally uses
multiple LEDs. In most applications four LEDs are used, either
in parallel or in series. If connected in parallel, each LED can
be controlled and adjusted separately. The disadvantage of this
approach is that there are more
connections to the LCD module
and a tighter tolerance requirement with respect to the currents in each LED.
On the other hand, LEDs
connected in serial will guarantee that each sees exactly the
same current. Another advantage of the serial approach is
that it decreases system cost,
since only two connections to
the LCD module are needed.
However, the drawback is that
it is not possible to independently control the LEDs.
With parallel LEDs, there is
a chance that a mismatch in
currents will occur, depending
on the matching of the resistors
and for ward voltage of the
LED. With serial LEDs, the
currents are identical, producing more uniform backlighting.
Only the forward voltage variation has to be taken into account, which will influence the
overall brightness but is not
seen as a local effect.
In Li-ion battery systems,
white LEDs always need a boost
converter. Parallel LEDs require
a minimum of 5V. However, by
increasing the boost to a much
higher voltage system, the cost
goes down because fewer contacts are needed to connect the
LCD module to the LED driver.
Lowering complexity, cost
An important consideration is
how to optimize power consumption. Where standalone LED
drivers are used, this has implications for both the cost and
overall power consumption of
the entire system. Integrating
several functions with similar requirements onto the same piece
of silicon produces several design and system efficiencies.
There is an argument for integrating the white LED driver
within the power management
system itself, because of the
similarities of the functional
blocks and their demands. For
example, both functions require similar voltage levels and
therefore can be implemented
using a similar process. Both
features also require direct connection to the battery, which
calls for a 5V-compatible process. The output voltage of a
white LED driver can go up to
20V, a voltage needed for
charger inputs in power-management systems. Integrating
the white LED driver within the
power-management system can
further reduce system costs.
Driver similarities
Several functional blocks used
in a typical white LED driver are
identical to those that powermanagement circuits require.
For example, both need a reference voltage, current biasing,
an oscillator and digital control.
A current-controlled boost
does not sense the output voltage for regulation; it senses the
voltage generated by a current
flowing through a sense resistor. This sense voltage is compared with an internal refer-
with 40mA output current.
In making these calculations, a typical efficiency of
90 percent for the boost converter is taken into account. For
a voltage-controlled boost, the
output voltage must be high
enough to support LEDs with
maximum forward voltage. This
means that for four LEDs, the
output voltage must be above
18.5 V to guarantee that the
LEDs will be driven correctly in
all operating conditions.
However, it is unlikely each
LED will demonstrate its worstcase forward voltage. Typically,
for four LEDs, only 14.4V
would be required. Therefore
4.1V x 20mA = 0.08W is dissipated in the current sink—
power that is not converted into
emitted light.
ence voltage and the converter
automatically adjusts the boost
output voltage until the reference voltage and the sense voltage are equal.
By using a small reference
voltage (100 mV), the dissipated
power in the sense resistor is
negligible. With this approach,
the boost converter creates a
voltage just high enough to ensure that the current is driven
through the LEDs, independent
of the number and forward voltage of the diodes. Therefore, all
transferred energy from the
boost is dissipated mainly in the
LEDs—plus a small amount
needed in the sense resistor.
An additional advantage of
the current-controlled approach is the boost converter’s
efficiency. Higher output volt-
Voltagecontrolled
boost
Currentcontrolled
boost
Typical
efficiency
overall (%)
64, 80
Vforwardmax
of LED
4, 5
Vforwardtyp
of LED
3, 6
Number
of LEDs
1
Vboost
N
5
4, 5
3, 6
2
9, 5
90
68, 21
4, 5
3, 6
3
14
90
69, 43
4, 5
3, 6
4
18, 5
90
70, 05
4, 5
3, 6
1
3, 7
90
87, 57
4, 5
3, 6
2
7, 3
90
88, 77
4, 5
3, 6
3
10, 9
90
89, 17
4, 5
3, 6
4
14, 5
90
89, 38
Efficiency
boost (%)
90
Source: Dialog Semiconductor
The efficiency for a white LED driver is higher using a current-controller boost converter
than a voltage-controlled converter.
ages normally result in lower efficiency. However, in this case,
as the converter only boosts to
the voltage needed, the efficiency of the boost is optimized
to the actual operating mode.
In most applications, the
brightness of the backlight is
controlled via pulse-width
modulation, which can also be
used for dimming. A serial interface is needed to program
the start value, end value and
dim time. If a fixed-voltage
boost converter is compared
with a current-controlled boost
converter, it is possible to highlight the differences in efficiency. In this case, the consumed power out of the battery
to the dissipated power in the
LEDs has to be compared.
Figure shows a comparison
of the overall efficiency of the
current-controlled and voltagecontrolled white LED driver
In addition, a voltage-controlled boost converter needs to
be programmed to the number
of LEDs used. This can be done
with a serial interface or dedicated devices for the different
numbers of LEDs needed.
The current-controlled boost
converter eliminates the need
for output-voltage programming as the circuit automatically boosts the required voltage
to ensure LED current f low.
Hence, this approach consumes
less power than a standard circuit high-voltage step-up converter with fixed-output voltage, which needs to boost to the
worst-case voltage. It is only in
the worst case—when the LEDs
have maximum forward voltage—that the current-controlled
boost has to create the high voltage. In all other operating
modes, however, it will always
adjust to the actual needs.