MS-2437: Convert a Buck Regulator into a Smart LED Driver PDF

MS-2437: Convert a Buck Regulator into a Smart LED Driver PDF
Technical Article
MS-2437
.
Convert a Buck Regulator into
a Smart LED Driver
some easy tricks to take a readily available dc-to-dc buck
regulator and transform it into a super smart LED driver.
A buck regulator simply chops up an input voltage, passes it
through an LC filter, and gives a stable output. To do this,
the buck employs two active elements and two passive
elements. The active elements are a switch from the input to
the inductor (labeled A in Figure 3) and a switch (or diode)
from GND to the inductor (labeled B in Figure 3). The
passive elements are the inductor (L) and the output
capacitor (COUT). These form an LC filter, which reduces the
ripple created by the active elements. Their arrangement is
shown in Figure 3.
by Jon Kraft, Applications Engineering Manager,
Analog Devices, Inc.
LEDs promise to change the world, and few doubt that they
will, but a key limiter to more rapid adoption is the cost of
the LED themselves. The cost breakdown of LED luminaires
vary, but it is safe to put the cost of the LED at around 25%
to 40% of the total luminaire cost. It is projected to remain a
significant cost of the total luminaire for many years.
LED Luminaire Cost
% Cost Due to LED
100
90
80
70
60
50
40
30
20
10
0
2020
% of Total Cost Due to LED
Percent of 2010 Luminaire Cost
100
90
80
70
60
50
40
30
20
10
0
2010
2012
2014
2016
2018
Figure 3. Basic Buck Arrangement3
Figure 1. Breakdown of LED Luminaire Costs1
One way to reduce the total luminaire cost is to drive an
LED at its highest possible current. If driven properly, these
LEDs produce a greater lumens/cost.
Figure 2. Potentially Under-Utilized Operating Capacity for the Cree XLamp
XP-G LED2
Doing this requires higher current drivers, and while there
are many solutions to drive LEDs at low currents (<500 mA),
there are fewer options at higher currents (700 mA to 4.0 A).
This is all the more surprising given that the semiconductor
world is rich with <4.0 A dc-to-dc solutions. However, the
problem is that these dc-to-dc solutions are made to control
voltage—not LED current. In this article, we will examine
If the switches are internal, we call it a “regulator.”
Otherwise, it is a “controller.” If both switches are power
transistors (either MOSFETS or BJTs), then it is
“synchronous”; otherwise, it is called “asynchronous.” This
gives several categories of buck circuits, and each has its own
merits and drawbacks. The discussion of which type to use,
and all the trade-offs, would be extensive. But in a very
general sense, the solution that often times gives an optimal
efficiency, BOM count, solution cost, and board area is a
synchronous buck regulator. However, synchronous buck
regulators that drive high current LEDs (up to 4 A) are few
and expensive. So, why not take a standard synchronous
buck regulator and modify it to regulate LED current? We
will use two examples of recently released general-purpose
synchronous buck regulators from Analog Devices: the
ADP2441 and the ADP2384.
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©2013 Analog Devices, Inc. All rights reserved.
MS-2437
Technical Article
The ADP2441 is a high efficiency, 36 V input synchronous
buck regulator, capable of producing an output current of up
to 1.2 A. The ADP2384 is another high efficiency
synchronous buck regulator but with an output current of up
to 4.0 A and an input voltage up to 20 V. Standard output
voltage regulation schematics for both are shown in Figure 4.
For both the ADP2441 and the ADP2384, the output voltage
is resistor divided down to the FB pin. This is compared
against an internal 600 mV reference and used to generate
the proper duty cycle to the switches. In steady state, this FB
pin regulates to exactly 600 mV. So, instead of a resistor
divider, it is easy to put the LEDs there (Figure 5), with a
resistor (RSENSE) in series to set the current.
Figure 4. ADP2384 and ADP2441 Schematics for Regulating Output Voltage
Figure 5. Basic (but Inefficient) LED Driver
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Technical Article
MS-2437
Using a precision resistor from FB to GND sets the LED
current to ILED = 600 mV/RSENSE. This works nicely, but it
produces a lot of power dissipation: PDISS = 600 mV × ILED.
For low LED currents, it is not a big issue, but at high LED
currents, the impact to efficiency adds significantly to the
heat that the luminaire must dissipate (600 mV × 4 A = 2.4 W).
Fortunately, there are two tricks we can use to reduce the FB
reference voltage for most buck regulators: use the SS/TRK
pin or offset the RSENSE voltage.
But for our purposes, we want to just set the SS/TRK pin to a
fixed voltage and use it as our new FB reference. A resistor
divider from a constant voltage to this pin works nicely.
Many of these buck regulator ICs include a controlled low
voltage output, like the VREG pin on the ADP2384 or the
VCC pin of the ADP2441. For greater accuracy, a simple 2terminal precision reference like the ADR5040 can be used.
Either way, a resistor divider from the supply to the SS/TRK
pin forms the new reference. Setting the resistor divider to
give a SS/TRK voltage around 100 mV to 200 mV generally
offers the best compromise between power dissipation and
LED current accuracy. Another benefit to setting your own
feedback reference voltage is that any sense resistor values
(which are found in standard values) can be easily
accommodated. This prevents the expense and inaccuracy of
paralleling multiple RSENSE resistors to set the LED current.
Many general-purpose buck ICs include a soft start (SS) or
tracking (TRK) pin. The SS pin is intended to be used to
provide a controlled inductor current at startup, and the
TRK pin is intended to have the buck regulator follow an
independent voltage. Often times, the pins are combined
into one SS/TRK pin. In most cases, the error amplifier will
use the minimum of the SS, TRK, and FB reference voltages
to change the regulation point. The typical setup is shown in
Figure 6.
COMP
0.6V
+
+
CMP
–
ISS
SS
+ AMP
FB
–
Figure 6. Soft Start and/or Tracking Pin Operation (ADP2384 Shown)
VOUT
L
SW
PGND
PGND
PGND
PGND
COUT
AGND
ILED=150 mV
Rsense
PGND
PGND
VOUT
FB
COMP
BST
GND
ADP2384
VIN
PGND
FREQ
SS/TRK
PVIN
VREG
PVIN
FB
ILED=100 mV
Rsense
100 mV
COMP
Rsense
SS
SYNC
RT
VCC
EN
PVIN
PVIN
150 mV
PGOOD
PGOOD
SW
SW
EN
VOUT
SW
VIN
VIN
ADP2441
Rsense
COUT
BST
Vcc
SW
150 mV
L
ADR5040
100 mV
VREG
Figure 7. Using the SS/TRK Pin to Reduce the FB Reference Voltage
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MS-2437
Technical Article
different trick can be used: offset the RSENSE voltage. A
resistor divider tied to an accurate rail between FB and RSENSE
provides a fairly constant offset voltage between RSENSE and
the FB pin.
Using an SS or TRK pin accommodates many, but not all,
buck regulators. Some ICs do not have an SS or TRK pin.
Also, the SS pin of some buck ICs changes the peak inductor
current—not the FB reference, so review the data sheet
carefully. If either of these situations is the case, then a
VOUT
VCC
SW
Rsense
SW
VIN
600 mV
COUT
VOUT
SW
VIN
BST
ADP2441
COMP
L
SW
BST
Vcc
FB
150 mV
PGND
R2
AGND
PGND
COUT
R1
PGND
PGND
VOUT
PGND
PGND
L
ADR5040
ILED=150 mV
Rsense
ILED=100 mV
Rsense
GND
ADP2384
SW
VIN
PVIN
VREG
PVIN
FB
R1
EN
PGND
100 mV
600 mV
SS/TRK
COMP
SS
Page 4 of 10
RT
Figure 8. Offset the RSENSE Voltage
EN
PVIN
PVIN
SYNC
FREQ
PGOOD
PGOOD
R2
Rsense
Technical Article
MS-2437
The necessary values for the resistor divider can be found in
the following equation:
𝑅1 = 𝑅2 ×
𝑉𝑆𝑈𝑃 − 𝐹𝐵𝑅𝐸𝐹
𝐹𝐵𝑅𝐸𝐹 − 𝐹𝐵𝑅𝐸𝐹(𝑁𝐸𝑊)
So to get an effective feedback reference of 150 mV, with
R2 = 1 kΩ and VSUP = 5 V:
𝑅1 = 1 𝑘Ω ×
5.0𝑉 − 0.6𝑉
= 9.78 𝑘Ω
0.6𝑉 − 0.15𝑉
Now, the LED current is equal to:
𝐼𝐿𝐸𝐷 =
𝐹𝐵𝑅𝐸𝐹(𝑁𝐸𝑊)
𝑅𝑆𝐸𝑁𝑆𝐸
The other key for accurate current regulation is proper
layout routing to the sense resistor. Sensing the resistor at
the wrong point can result in several millvolts of error at the
FB pin, which will degrade the LED accuracy significantly. A
4-terminal sense resistor is ideal but more expensive.
However, by following good layout techniques, high
accuracy can also be obtained from a traditional 2-terminal
resistor. Marcus O’Sullivan has published an excellent paper
on the subject at
http://www.analog.com/library/analogDialogue/archives/4606/shunt_resistors.pdf
Using this method does not require an SS or TRK pin.
Additionally, the FB pin will still regulate to 600 mV (but the
voltage at RSENSE regulates to the FBREF (NEW) voltage). This
means that other functions of the chip (like soft start,
tracking, and power good) will still function normally.
The downside is that the offset between RSENSE and FB is
strongly influenced by the accuracy of the supply. A
precision reference like the ADR5040 has little problem, but
if the reference was a ±5% reference, it would create a ±12%
variation in the LED current. A comparison is shown in
Table 1.
Table 1.
Option 1: Use SS/TRK to
Reduce FB Reference
±5% supply voltage variation
gives ±5% error on ILED. This is
not impacted by the VSENSE
voltage; therefore, this method
has the best RSENSE power
dissipation.
Very good open/short LED
protection. FB_OVP does not
factor into intermittent open
protection. LED current is
limited by the inductor and
the control loop speed.
PGOOD will always remain
low.
By keeping the SS/TRK pin
lower than normal, some fault
modes may not work properly.
Option 2: Offset RSENSE
Voltage
±5% supply voltage variation
gives ±12% error on ILED.
Higher VSENSE voltages improve
this.
Very good open/short LED
protection. Additionally, some
ICs have another FB reference
(FB_OVP) that immediately
disables switching if FB rises
50 mV to 100 mV above
normal. This gives a guarantee
for the maximum LED
overcurrent during
intermittent faults.
Since FB pin still regulates to
600 mV, the PGOOD pin
functions normally.
All fault modes work normally.
Figure 9. Recommended PCB Trace Routing for RSENSE4
Regulating the LED current is easy. The real challenge comes
in trying to duplicate all the smart features that a dedicated
LED buck regulator gives. Some of the more popular ones
are LED short/open fault protection, RSENSE open/short fault
protection, PWM dimming, analog dimming, and current
foldback thermal protection.
A standard buck converter usually has no problem dealing
with shorted or open LEDs. A shorted LED appears like a
lower voltage load, and the buck IC adjusts its duty cycle to
compensate. An open LED looks like an open load to the
buck IC, so it delivers maximum duty cycle, and VOUT rises
to its maximum (but no higher than VIN)—but no power is
delivered. Either of these conditions keeps everything in the
system safe. The issue comes with intermittently shorted or
open LEDs that occur after startup, and the problem is the
output capacitor. The stored energy in the output capacitor
creates a huge surge of current through the LEDs when an
LED is shorted or an open LED reconnects.
Consider the case of a sudden short across an LED string.
The buck regulator controls the duty cycle to give a constant
FB voltage, which produces a constant current through the
LEDs. This current produces a certain forward voltage (VF)
across the LED string, so the buck output capacitor charges
to VOUT = VF + VFB. If the VF suddenly drops because an
LED in the string is shorted, then the output capacitor sources
current to the LED string. This current is equal to IC = C ×
ΔVf/Δt (Δt is small and C is often large to minimize L in the
Page 5 of 10
MS-2437
Technical Article
buck LC filter, meaning IC can become large). An example is
shown in Figure 10.
Figure 10. Response to a Shorted LED with COUT = 10 µF and L = 47 µH
A similar issue arises for an intermittently open string: VOUT
charges to its maximum value. Then, when the LED
reconnects, a huge inrush of current shoots through the
LEDs. However, if that output capacitor wasn’t there, then
there would be no problem. Dare we remove one of the four
key elements of a buck? Fortunately, LEDs do not care if they
have some ripple on them, and our feedback loop will ensure
that the average LED current remains the same. Therefore,
by removing the capacitor, we solve some potential problems
and save ourselves one of the larger parts on the bill of
materials (BOM). Now, our LED current is exactly equal to
our inductor current, and the ripple on the inductor current
is reflected in the FB voltage.
IL
M1
VIN
L
DH
IL
IL=ILED
AVG
M2
Time
DL
FB
Vsense
COMP
FB_REF
Rsense
Vsense
ILED
AVG
Time
Buck Regulator IC
Figure 11. Buck LED Driver Operation without COUT
Figure 12. Response to Intermittently Shorted LED with COUT = 10 µF and
COUT = Open
Figure 13. Response to Intermittently Open LED with COUT = 10 µF and
COUT = Open
Eliminating COUT has another benefit: since ILED = IL, and
IL is well controlled (since buck ICs go through great effort
to limit the maximum inductor current), we now have
another level of LED safety. The LED current will never rise
above the buck’s peak current limit value: for the ADP2441,
this is 1.7 A (typ) and for the ADP2384, it is 6.1 A (typ). This
is useful for another fault case: an intermittently shorted
RSENSE.
Shorting RSENSE, or shorting the FB pin to GND, can be
catastrophic: duty cycle rises to maximum value, which
delivers maximum power, and you essentially have VIN
applied directly to the LEDs. This gives an uncontrolled and
very high current through the LEDs. Most bucks will limit
this maximum current, their peak inductor current level, or
enter a hiccup protection mode, but by eliminating the
output capacitor, we also protect the LEDs against RSENSE
shorts that occur after startup.
Therefore, our slightly modified synchronous buck topology
helps quite a bit with common LED fault cases. However, as
with anything, there are trade-offs; the trade-off here is
ripple current through the LED. This ripple current is at a
very high frequency (the switching frequency—generally
>300 kHz), so visible flickering and acoustic noise is not an
issue. The only real issue is to ensure that the LEDs do not
exceed their maximum current rating. This is somewhat
vague on most LED data sheets. Many LED data sheets will
Page 6 of 10
Technical Article
MS-2437
For low PWM frequencies (<1 kHz) this can still give great
accuracy (Figure 14).
specify a surge current rating and a dc current rating, but the
area of a high frequency ripple on top of a dc value isn’t
usually covered. The safest approach is to make the peak of
the LED ripple below the dc rating of the LED. This means
picking a higher value inductance to keep the ripple small.
These inductors are either physically larger or will have a
higher resistance (DCR). Alternatively, we can leave the
inductor the same and choose a higher switching frequency
to keep the current ripple low. However, this increases the
switching power losses. So, if the maximum rated LED
current is close to the maximum average desired out current,
then there may be some trade-offs between board area, cost,
or power loss.
Figure 14. ADP2384 PWM Dimming Linearity
Another key requirement for a smart LED driver is dimming
controls. There are two types of LED dimming: PWM and
analog. PWM dimming pulses the LED current. If the
frequency is above ~120 Hz, then the human eye averages
these pulses to produce a perceived average luminosity.
Analog dimming scales the LED current and the current is
always constant (dc).
The ADP2384, like all general-purpose bucks, doesn’t have a
pin to apply a PWM dimming signal, but we can manipulate
the FB pin to enable and disable switching. If we pull FB
high, then the error amplifier goes low, and the buck
switching stops. If we connect FB to RSENSE, then it resumes
normal regulation. We can do this with either a small signal
(low current) NMOS, or just a general-purpose diode. In the
case of an NMOS a high PWM signal shorts RSENSE to FB,
enabling LED regulation. A low PWM signal turns the
NMOS off, and a pull-up resistor brings FB high. In the case
of the diode, a high PWM signal produces no LED current,
which is a little backwards from the typical convention. Both
solutions are illustrated in Figure 15.
To achieve PWM dimming, we could insert an NMOS in
series with RSENSE and open and close it. That would
definitely pulse the output current, but at these current
levels, we would need to use a power NMOS. Adding one of
those defeats the solution size and cost benefits of having the
power switches internal to the buck regulator. Instead, it is
easier to PWM dim by quickly shutting the buck on and off.
VOUT
L
SW
PGND
PGND
PGND
AGND
PGND
PWM
PGND
VOUT
PGND
ILED=150 mV
Rsense
SW
FB
150 mV
COMP
PGND
SS/TRK
PVIN
VREG
PVIN
FB
100 mV
COMP
SS
RT
SYNC
VCC
EN
150 mV
PVIN
PVIN
PGOOD
FREQ
ILED=100 mV
Rsense
GND
ADP2384
VIN
PGOOD
SW
BST
SW
EN
VOUT
VIN
VIN
ADP2441
Rsense
L
SW
BST
Vcc
PWM
ADR5040
100 mV
VREG
Figure 15. PWM Dimming Options
Page 7 of 10
Rsense
MS-2437
Technical Article
PWM dimming is very popular, but sometimes noiseless
analog dimming is required. Analog dimming scales the
LED current, whereas PWM dimming chops it up. Analog
dimming is also required if two dimming inputs are used.
PWM dimming on top of another PWM dimming signal
can create beat frequencies, which could result in light
flicker or audible noise. So, we might PWM for one
dimming control and analog dim for another dimming
control. On a general-purpose buck regulator, the easiest
way to implement analog dimming is to manipulate the FB
reference that we constructed earlier. This is accomplished
by making the analog dimming control voltage the supply
for our FB reference adjustment circuits.
Figure 16. PWM Dimming Waveform
VOUT
COMP
SW
VIN
VOUT
SW
BST
ADP2441
L
SW
SW
VIN
600 mV
PGND
BST
Vcc
FB
Vsense
Rsense
AGND
PGND
R2
R1
PGND
VOUT
PGND
ILED=Vsense
Rsense
PGND
PGND
L
DIM
GND
ADP2384
SW
VIN
PVIN
VREG
PVIN
FB
ILED= Vss
Rsense
EN
COMP
Vss
Figure 17. Analog Dimming Circuits
Page 8 of 10
Rsense
SS
RT
EN
SYNC
PVIN
SS/TRK
PGOOD
FREQ
PVIN
PGOOD
PGND
DIM
Technical Article
MS-2437
drops below the reference voltage, then the FB reference
voltage, and hence, the LED current drops.
Since the lifetime of the LEDs is heavily dependent on their
operating junction temperature, it is sometimes necessary to
monitor the LED temperature and respond if the
temperature is too high. An abnormally high temperature
could be caused by a poorly connected heat sink, an
unusually hot ambient, or some other extreme condition. A
common solution is to reduce the LED current if the
temperature exceeds some threshold. This method is called
LED thermal foldback.
We can also implement the desired temperature dimming
profile with the RSENSE offset method. For this method, we
just construct a resistor divider with the NTC. This resistor
divider attaches to the FB pin with a small signal diode. The
diode prevents the resistor divider from acting on the FB pin
until the divider reaches about 600 mV + VF. There is some
degradation in dimming accuracy, but the results are fairly
good (Figure 20).
LED Current
100%
T1
LED Board Temperature (C)
Figure 18. Desired LED Thermal Foldback Curve
In this type of dimming, we wish to keep the LEDs at full
current until some temperature threshold (labeled T1 in
Figure 18) is reached. At that threshold, we start to decrease
the LED current with increasing temperature. This limits the
junction temperature of the LEDs and preserves their
lifetime. A low cost NTC (negative temperature coefficient)
resistor is commonly used to measure the LED heat sink,
and a small modification to our analog dimming scheme can
easily make use of this NTC. If we are using the SS/TRK pin
to control the FB reference, then a simple method is to place
the NTC in parallel with the reference voltage (Figure 19).
Figure 20. LED Thermal Foldback Using RSENSE Offset Technique
These tips should be taken as general guidelines for
implementing comprehensive LED features into a standard
buck regulator. Since these features are a little outside of the
intended application for the buck IC, it is always best to
contact the semiconductor manufacturer and ensure that the
IC can handle these modes of operation. For more
information on the ADP2441 and ADP2384, or for demo
boards of these LED driver solutions, visit
http://www.analog.com/lighting.
REFERENCES
1
DOE SSL 2011 Manufacturing Roadmap (ssl.energy.gov).
David Cox, Don Hirsh, and Michael McClintic. “Are you
using all of the lumens that you paid for?” LEDs Magazine,
Feb. 2012.
2
Ken Marasco. “How to Apply DC-to-DC Step-Down
(Buck) Regulators Successfully.” Analog Dialogue, Vol. 45,
June 2011.
3
Marcus O’Sullivan. “Optimize High-Current Sensing
Accuracy by Improving Pad Layout of Low-Value Shunt
Resistors.” Analog Dialogue, Vol. 46, June 2012.
4
Figure 19. LED Thermal Foldback Using the SS/TRK Pin
The NTC forms a resistor divider with R3. As the heat sink
temperature rises, the NTC resistance drops. If the NTC
resistor divider’s voltage is above the reference voltage, then
maximum current is delivered. If the NTC resistor voltage
RESOURCES
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MS-2437
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Trademarks and registered trademarks are the property of
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