Texas Instruments | DAC53608 Delivers Ultra Low Cost LED Array Biasing Solution | Application notes | Texas Instruments DAC53608 Delivers Ultra Low Cost LED Array Biasing Solution Application notes

Texas Instruments DAC53608 Delivers Ultra Low Cost LED Array Biasing Solution Application notes
DACx3608 Delivers Programmable LED Array Biasing
Solution at Ultra Low Cost and Smallest Footprint
Packed with a simple yet complete feature set the
DAC53608 enables a biasing solution for LED
arrays that is low-cost, compact, and adequate.
Though it may be unobvious, meeting all these
requirements needs careful design trade-offs.
VCC
LED
RZ
ILED =ISET
___________________________________________
Many imaging products that used technologies such
as lasers and lamps are now adapting LED array
based solutions. LEDs need to be forward biased in its
linear region to operate. A resistive voltage divider can
serve this purpose. But the biasing point of an LED
changes with temperature and part-to-part variability.
Hence, programmability of the biasing point is a must
in precision circuits. Digital Potentiometers (DPOT),
PWM, or Precision DACs are obvious options for this.
These applications also need the solution to be low
cost, small size, and high integration. Selecting the
right architecture thus becomes non-trivial. This
application note discusses different LED biasing
topologies and the trade-offs. Only low-side LED
biasing topologies are discussed.
DPOT Based Biasing
An LED is biased with current. The most basic
implementation of a programmable current source is
shown in Figure 1 that uses a DPOT and an
adjustable shunt reference. This circuit changes the
current by changing the voltage across the zener. The
major drawback of this circuit is that it uses more
number of components that will lead to an increased
footprint and cost of the solution. This is especially true
when biasing an array of LEDs. Also the VBE of the
transistor varies with temperature and collector
current.
VZ-ADJ
+
VBE
VSET =VZ-ADJ -VBE
DPOT
ISET =
RSET
VZ-ADJ VBE
R SET
GND
GND
Figure 1. DPOT Based Biasing
Simple Biasing Method Using a Precision DAC
A precision DAC based biasing circuit is depicted in
Figure 2. The DAC53608 family of DACs that come
with 8-channels in a 3x3 QFN package, realizes this
circuit with the smallest size and lowest cost. However,
it still carries forward one draw back from the previous
circuit that is the drift in VBE of the transistor. Further,
the output also requires a headroom near ground rail
due to VBE drop.
VCC
LED
ILED =ISET
PWM Based Biasing
A PWM signal can be used in place of the DPOT and
zener in Figure 1 to program the bias point. The bias
point corresponds to the DC value of the PWM signal.
The problem with this circuit is that it needs one PWM
generator per channel, which might be difficult to get
from an MCU. A continuous PWM also creates
distortion and EMI issues.
DAC536 08
VDAC
+
VBE
-
VSET =VDAC -VBE
RSET
ISET =
VDAC -VBE
R SET
GND
Figure 2. Programmable LED Biasing Circuit
SLAA853 – June 2018
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DACx3608 Delivers Programmable LED Array Biasing Solution at Ultra Low
Cost and Smallest Footprint
Copyright © 2018, Texas Instruments Incorporated
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VCC
VCC
LED
DAC536 08
VDAC
RSET
ILED =ISET
LED
ILED =ISET
VEB +
+
±
DAC536 08
+
VBE
VDAC
+
VBE
VSET =VDAC
RSET
ISET =
VSET =VDAC +(VEB-VBE)=VDAC
VDAC
R SET
RSET
GND
GND
Compensating VBE
A robust way of compensating the VBE variation is by
putting it inside the feedback loop of an amplifier as
shown in Figure 3. This circuit is very suitable for
applications that require high accuracy. The only
drawback of this circuit is that it requires an additional
amplifier.
Another approach for compensating VBE can be
considered as shown in Figure 4 that uses a matched
pair of PNP and NPN transistors in order to cancel out
the voltage variations and headroom. As can be seen
this circuit balances accuracy, solution size, and cost.
DACx3608 Family of Precision DACs
The DACx3608 family of DACs is an 8-channel
buffered voltage output DAC with a tiny 3x3 QFN
package. It has a single supply operation and comes
in 8-bit and 10-bit pin-compatible versions. The DAC
provides an I2C interface whose device address can
be configured to four different values using a single
hardware pin. This will allow use of 32 channels
without using any I2C buffer. All these features
combined with tiny footprint makes DAC53608 an
excellent choice for programmable LED biasing.
ISET =
VDAC
R SET
GND
Figure 4. VBE Compensation Without Buffer
Figure 3. VBE Compensation Using Buffer
The VBE varies with temperature and collector current.
This variation and the headroom may be ignored in
few applications that places this circuit inside a larger
feedback loop with gain. But it can be of concern for
applications that are open-loop and do not employ
temperature calibration. This drift will consequently
lead to gain error and full scale error at system level.
-
Discussion
Table 1 provides a summarized comparison of the
topologies discussed in this document. It can be
noticed that the precision DAC based solutions excel
other approaches in many aspects. General purpose
precision DACs in 8- and 10-bit resolutions have been
flooding the market for a long time. But the DAC53608
comes with the latest semiconductor technologies that
enable the ultra low cost and size, much differentiated
from the competition. Hence, it is perfectly suitable for
LED biasing applications and geared to meet highly
competitive requirements of the end equipment.
Powered with the DAC53608 family, the topologies
discussed above can provide both low precision and
high precision functionality with ease of
implementation.
Table 1. Comparison of LED Biasing Topologies
Topology
Trade-off
DPOT Based Circuit
More components leading to
extra cost and space, No VBE
compensation
PWM Based Circuit
Difficult to generate multichannel PWM. Also creates
distortion and EMI issues
DAC Based Circuit without VBE
Compensation
Very simple implementation
suitable for applications not
concerned about VBE
DAC Based Circuit with Buffer
Suitable for high accuracy
applications, needs an
additional amplifier
DAC Based Circuit with
Matched Transistor Pair
Best trade-off between cost and
accuracy
Related End Equipment
•
•
•
•
2
Printers
Projectors
Electronic Point of Sale (EPOS)
Visible Light Communication (VLC)
DACx3608 Delivers Programmable LED Array Biasing Solution at Ultra Low
Cost and Smallest Footprint
Copyright © 2018, Texas Instruments Incorporated
SLAA853 – June 2018
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