Texas Instruments | AN-1656 Design Challenges of Switching LED Drivers (Rev. A) | Application notes | Texas Instruments AN-1656 Design Challenges of Switching LED Drivers (Rev. A) Application notes

Texas Instruments AN-1656 Design Challenges of Switching LED Drivers (Rev. A) Application notes
Application Report
SNVA253A – October 2007 – Revised May 2013
AN-1656 Design Challenges of Switching LED Drivers
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ABSTRACT
Using a switching regulator as an LED driver requires the designer to convert a voltage regulator into a
current regulator. Beyond the challenge of changing the feedback system to control current, the LEDs
themselves present a load characteristic that is much different than the digital devices and other loads that
require constant voltage. The LED WEBENCH® online design environment predicts and simulates the
response of an LED to constant current while taking into account several potential design parameters that
are new to designers of traditional switching regulators.
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Contents
Output Voltage Changes when LED Current Changes ................................................................
Designing for VO-MIN and VO-MAX .............................................................................................
Pitfalls of Parallel LED Arrays .............................................................................................
Selecting LED Ripple Current .............................................................................................
Dynamic Resistance ........................................................................................................
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List of Figures
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V-I Curve with Typical VF and IF ........................................................................................... 2
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VIN-MIN > VO-TYP, Buck Regulator Works .................................................................................... 3
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VIN-MIN < VO-MAX, Buck Regulator Fails to Regulate ....................................................................... 3
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Mismatched LEDs in Parallel .............................................................................................. 4
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LED Current (DC and AC) ................................................................................................. 5
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Only LED Ripple Current ................................................................................................... 5
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VF vs IF ........................................................................................................................ 6
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rD vs IF
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WEBENCH is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
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Output Voltage Changes when LED Current Changes
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Output Voltage Changes when LED Current Changes
In the first step of the LED WEBENCH tool, "Choose Your LEDs", an LED is selected with a standard
forward current, IF. This default value is provided by the LED manufacturers, and in most cases it
represents the testing condition for that LED. Typical values for high-power LEDs are 350 mA, 700 mA,
and 1000 mA.
Not all designs will use a standard current, however. The designer can select a different LED current, and
then the forward voltage will change in the VLED box under step 2. The change in voltage comes from
LEDs’ V-I curve. Figure 1 shows a curve from a 5W white (InGaN) LED that differs from the curves
normally found in LED datasheets. LED manufacturers provide these curves, but they are often shown as
I-V curves with voltage as the independent quantity. In Figure 1, forward current is the independent
variable, reflecting the fact that in LED drivers current is controlled, and voltage is allowed to vary. The
cross-hairs intersect at the standard/typical IF and VF values of 350 mA and 3.5V, respectively.
Once the VF of the LEDs has been determined from the V-I curve, the LED driver’s output voltage is
calculated using the following formula:
VO = n × VF + VSNS
where
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•
n is the number of LEDs connected in series
VSNS is the voltage drop across the current sense resistor
(1)
Figure 1. V-I Curve with Typical VF and IF
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AN-1656 Design Challenges of Switching LED Drivers
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Designing for VO-MIN and VO-MAX
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Designing for VO-MIN and VO-MAX
In practice, the typical value of VF changes with forward current. Further analysis of total output voltage is
needed because VF also changes with process and with the LED die temperature. The more LEDs in
series, the larger the potential difference between VO-MIN, VO-TYP and VO-MAX. An LED driver must therefore
be able to vary output voltage over a wide range to maintain a constant current. IF is the controlled
parameter, but minimum and maximum output voltage must be predicted in order to select the proper
regulator topology, IC, and passive components.
A typical example that can lead to trouble is driving three white (InGaN) LEDs from an input voltage of
12V ±5%. In Figure 2, each LED operates at the typical VF of 3.5V, and the current sense adds 0.2V for a
VO of 10.7V. Minimum input voltage is 95% of 12V, or 11.4V, meaning that a buck regulator capable of
high duty cycle could be used to drive the LEDs.
Figure 2. VIN-MIN > VO-TYP, Buck Regulator Works
However, a buck regulator designed for the typical VO will be unable to control IF if VO-MAX exceeds the
minimum input voltage. The same white LEDs with a typical VF of 3.5V have a VF-MAX of 4.0V. Headroom is
tight under typical conditions, and the buck regulator will lose regulation with only a small increase in VF
from one or more of the LEDs (Figure 3).
Figure 3. VIN-MIN < VO-MAX, Buck Regulator Fails to Regulate
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3
Pitfalls of Parallel LED Arrays
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Pitfalls of Parallel LED Arrays
Whenever LEDs are placed in parallel, the potential exists for a mismatch in the current that flows through
the different branches. The forward voltage, VF, of each LED varies with process, so unless each LED is
binned or selected to match VF, the LED or LED string with the lowest total forward voltage will draw the
most current (Figure 4). This problem is compounded by the negative temperature coefficient of LEDs
(and all PN junction diodes). The LEDs that draw the most current suffer the greatest increase in die
temperature. As their die temperature increases, their VF decreases, creating a positive feedback loop.
Elevated die temperature both reduces the light output and decreases the lifetime of the LEDs.
The system in Figure 4 also illustrates a potential over-current condition if one of the LEDs fails as an
open circuit. Without some protection scheme, the entire drive current IO will flow through the remaining
LED(s), likely causing thermal overstress. Likewise, if one of the LEDs fails as a short circuit, the total
forward voltage of that string will drop significantly, causing higher current to flow through the affected
branch.
To maintain safety and reliability in a parallel LED system, forward voltage should be binned or matched.
Fault monitoring should detect LEDs that fail as either short or open circuits. Finally, the entire array
should have evenly distributed heat sinking, to ensure that VF change with respect to die temperature
occurs uniformly over all the LEDs.
Figure 4. Mismatched LEDs in Parallel
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Selecting LED Ripple Current
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4
Selecting LED Ripple Current
LED ripple current, ΔiF, in an LED driver is the equivalent of output voltage ripple, ΔvO, in a voltage
regulator. In general, the requirements for ΔiF are not as tight as output voltage ripple. Where a ripple of a
few millivolts to 4%P-P of VO is typical for ΔvO, ripple currents for LED drivers range from 10% to 40%P-P of
the average forward current, IF.Figure 5 and Figure 6 show a typical ripple current of 25%P-P from a buck
switching LED driver. A wider tolerance for ΔiF is acceptable because the ripple is too high in frequency for
the human eye to see. General illumination applications (Such as lamps, flashlights, signs, and so on) can
tolerate large ripple currents without harming the quality or character of the light. Allowing larger ripple
current means lower inductance and capacitance for the output filter, which in turn translates to smaller
PCB footprints and lower BOM costs. For this reason, ΔiF should generally be made as large as the
application permits.
The true upper limit for ΔiF comes from the nonlinear proportion of heat to light that is generated as the
peak current through the LED increases. Above approximately 40%P-P ripple, the LED can experience
more heating during the peaks than cooling during the valleys, resulting in higher die temperature and
reduction in LED lifetime.
Some high-end applications require tighter control over LED ripple current. These include industrial
inspection, machine vision, and blending of red, green, and blue for backlighting or video projection. The
higher system cost of these applications justifies larger, more expensive filtering to achieve ripple currents
in the sub 10%P-P region.
Figure 5. LED Current (DC and AC)
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Figure 6. Only LED Ripple Current
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5
Dynamic Resistance
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Dynamic Resistance
Load resistance is an important parameter in power supply design, particularly for the control loop. In LED
drivers it is also used to select the output capacitance needed to achieve the desired LED ripple current.
In a standard power supply that regulates output voltage, the load resistance has a simple calculation:
RO = VO / IO
(2)
When the load is an LED or string of LEDs, however, the load resistance is replaced with the dynamic
resistance, rD and the current sense resistor. LEDs are PN junction diodes, and their dynamic resistance
shifts as their forward current changes. Dividing VF by IF leads to incorrect results that are 5 to 10 times
higher than the true rD value.
Typical dynamic resistance at a specified forward current is provided by some manufacturers, but in most
cases it must be calculated using I-V curves. (All LED manufacturers will provide at least one I-V curve.)
To determine rD at a certain forward current, draw a line tangent to the I-V slope as shown in Figure 7.
Extend the line to the edges of the plot and record the change in forward voltage and forward current.
Dividing ΔVF by ΔIF provides the rD value at that point. Figure 8 shows a plot of several rD values plotted
against forward current to demonstrate how much rD shifts as the forward current changes.
One amp is a typical driving current for 3W LEDs, and the following calculation shows how the dynamic
resistance of a 3W white InGaN was determined at 1A:
ΔVF = 3.85V – 3.48V
ΔIF = 1.5A – 0A
rD = ΔVF / ΔIF = 0.37 / 1.5 = 0.25Ω
Dynamic resistances combine in series and parallel like linear resistors, hence for a string of 'n' seriesconnected LEDs the total dynamic resistance would be:
rD-TOTAL = n × rD + RSNS
(3)
A curve-tracer capable of the 1A+ currents used by high power LEDs can be used to draw the I-V
characteristic of an LED. If the curve tracer is capable of high current and high voltage, it can also be used
to draw the complete I-V curve of the entire LED array. Total rD can determined using the tangent-line
method from that plot. In the absence of a high-power curve tracer, a laboratory bench-top power supply
can be substituted by driving the LED or LED array at several forward currents and measuring the
resulting forward voltages. A plot is created from the measured points, and again the tangent line method
is used to find rD.
Figure 7. VF vs IF
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AN-1656 Design Challenges of Switching LED Drivers
Figure 8. rD vs IF
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