Transform an LED driver from buck to boost for enhanced flexibility, reduced BOM

Transform an LED driver from buck to boost for enhanced flexibility, reduced BOM
LIGHTING
CONTENT
Transform an LED Driver from
Buck to Boost for Enhanced
Flexibility, Reduced BOM
With a few extra components and some rearrangement of the topology,
a buck-mode DC-DC converter IC can be made into a boost-mode device,
to drive LED strings with voltages higher than the supply voltage.
By Fons Janssen, Principal Member Technical Staff,
and Field Application Engineer, Maxim Integrated
The hysteretic-buck LED-driver is a popular, easily implemented current source for situations where the voltage across the LED string is
lower than the input voltage. By rearranging the external components,
it is practical to switch this topology from buck mode to boost mode, to
support LED strings where the sum of the diode drops is greater than
the input voltage.
While there are many boost regulators available, this topology allows
a single buck regulator IC to provide both buck and boost functions,
and so may simplify the bill of materials (BOM) and reduce overall
cost. Although using the buck device for boost operation may result in
increased variation in the LED current beyond what is acceptable, an
additional control loop can be added to further regulate the current, if
needed.
MOSFET is on, the current ramps up and flows from input voltage
Vin to GND via the sense resistor, the LEDs, the inductor, and the
MOSFET; when the MOSFET is off, the current ramps down and
flows back to Vin via the sense resistor, the LEDs, the inductor, and
diode D1.
Adding the hysteresis results in a self-oscillating system which generates a sawtooth-shaped LED current, Figure 2. The amplitude of the
sawtooth is determined by the amount of hysteresis. Capacitor C3
acts as a filter, so that the LEDs will mainly see a DC current. This
topology is a known as a high-side buck topology.
This transformation example uses the MAX16822/32 hysteretic buck
converters from Maxim Integrated, which are 2-MHz high-brightness
LED-driver ICs with integrated MOSFET and high-side current sense,
Figure 1. (The MAX16822 and MAX16832 differ only in current rating:
500mA versus 1A, respectively.)
Figure 2: The current waveform of the hysteretic buck LED driver has
a sawtooth LED current due to self-induced oscillation.
Figure 1: Typical application circuit of the MAX16832 as a buckconverter LED driver
This circuit regulates the voltage on sense resistor Rsense so that a
constant current flows through the LEDs that are in series with that
resistor. The MOSFET within the MAX16832 is turned on for currents
below the set point and turned off for currents above it. When the
52
Bodo´s Power Systems®
Going from buck to boost
A buck topology can only be used if the voltage across the LEDs
is less than the input voltage. When voltage across the LEDs is
greater than the input voltage, a boost topology is needed. Since the
boost topology also has the switching MOSFET on the low-side, it is
straightforward to change the high-side buck topology into a boost
topology by rearranging the external components, Figure 3. In this
boost topology, the current is regulated in the same way as in the
high-side buck topology.
The difference is that the LEDs are no longer in series with the sense
resistor and inductor. The result is that the input current is regulated
rather than the LED current. Figure 4 shows the waveforms for the
input and output currents; the LED current is a filtered version of the
output current via C3.
December 2015
www.bodospower.com
LIGHTING
CONTENT
s a hysteretic boost LED driver, the input current is regulated rather than
e waveforms for the input and output currents.>>
The result of this arrangement is that the LED current will depend
not only on the regulated input current (IIN), but also on input voltage
output
voltage not
(VLED),
andthe
theregulated
efficiencyinput
(η) of
the converter:
that the LED(VIN),
current
will depend
only on
current
IN), output voltage (VLED), and the efficiency (η) of the converter:
ηVIN IIN
ILED =
VLED
ED current is greater than acceptable, an extra circuit based on the
unt regulator for isolated DC-to-DC converters) can be added to regulate
Since 1960 your suorce
for film capacitors
based on the MAX8515 shunt regulator can be used to improve LED
amplifier and compares feedback voltage VFB to an internal reference
oportional to the LED current, with VFB = R2 × ILED. Since the output of
m the TEMP_I pin but cannot source current, a small constant current is
.
rents is integrated by capacitor C2. If the MAX8515 sinks more current
e voltage decreases; the reverse is true as well. The set point for the
o this voltage, Figure 6. Therefore, if VFB is smaller than the 0.6V
k and the voltage on TEMP_I increases. This, in turn, will increase the
ED current and VFB. If VFB is greater than the reference, the voltage on
rder to reduce the LED current.
ages the sinking and sourcing, as seen in the relation between voltage
t point. >>
Figure 3: Change topology from high-side buck to boost just requires
some rearrangement of the external components.
mizes variations
Design capability
ontrol loop used to regulate the LED current, Figure 7:
e MAX8515 is the input for the control loop;
proportional to the LED current, with ILED = VFB/R2;
and resistor R2 (note that the gain of MAX8515 is actually negative due to
ransistor; this is compensated by swapping the plus and minus signs on
Manufacturing flexibility
Development support
hile G2 is the gain between the TEMP_I voltage and the feedback
ulating the LED current begins by maintaining the feedback voltage VFB
B
www.icel.it
to 0.6V:
0.6V
R2
Figure 4: When configured as a hysteretic boost LED driver, the input
circuit, sense resistor RSENSE should be chosen so that the maximum
current
regulated
thanreduce
the LED
as shown
by the
an needed. The
extraiscontrol
looprather
will then
thecurrent,
input current
to get
waveforms
for
the
input
and
output
currents.
he value of this resistor can be calculated as follows:
ηVIN 200mV
If the<resulting
variation in the LED current is greater than acceptable,
R sense
ILED VLED
an extra circuit
based on the MAX8515 (a wide-Input, 0.6V shunt
regulator
can be calculated
by:for isolated DC-to-DC converters) can be added to regulate
the LED current, Figure 5.
ILED =
The difference between both currents is integrated by capacitor C2.
If the MAX8515 sinks more current than the TEMP_I pin sources, the
voltage decreases; the reverse is true as well. The set point for the
input current IIN is proportional to this voltage, Figure 6. Therefore, if
VFB is smaller than the 0.6V reference, no current will be sunk and
the voltage on TEMP_I increases. This, in turn, will increase the input
power, and therefore the LED current and VFB. If VFB is greater than
the reference, the voltage on TEMP_I will be pulled lower in order to
reduce the LED current.
Figure 5: An additional circuit based on the MAX8515 shunt regulator
can be used to improve LED current regulation, if needed.
The MAX8515 acts as an error amplifier and compares feedback
voltage VFB to an internal reference voltage of 0.6V. VFB is directly
proportional to the LED current, with VFB = R2 × ILED. Since the
output of the amplifier can sink current from the TEMP_I pin but cannot source current, a small constant current is sourced by the TEMP_I
pin itself.
www.bodospower.com
Figure 6: The MAX8515 manages the sinking and sourcing, as seen
in the relation between voltage on TEMP_I and input-current set point.
December 2015
Bodo´s Power Systems®
53
mplifier and compares feedback voltage VFB to an internal reference
uit basedtoonthe
theLED
MAX8515
regulator
can be used to improve LED
portional
current,shunt
with V
FB = R2 × ILED. Since the output of
>>
the TEMP_I pin but cannot source current, a small constant current is
internal
r amplifier
and compares
feedback
VFB to an
ents
is integrated
by capacitor
C2. If voltage
the MAX8515
sinks
morereference
current
proportional
to
the
LED
current,
with
V
FB = R2 × ILED. Since the output of
LIGHTING
CONTENT
voltage decreases; the reverse is true as well. The set point for the
from
the TEMP_I
cannot source
small
his voltage,
Figurepin
6. but
Therefore,
if VFB iscurrent,
smaller athan
theconstant
0.6V current is
self.
and the voltage on TEMP_I increases. This, in turn, will increase the
currents is integrated
by capacitor C2. If the MAX8515 sinks more current
D current and VFB. If VFB is greater than the reference, the voltage on
thetovoltage
the reverse is true as well. The set point for the
er
reduce decreases;
the LED current.
l to this voltage, Figure 6. Therefore, if VFB is smaller than the 0.6V
current-control
loop minimizes
variations
unk the
andsinking
the LED
voltage
on TEMP_I
in turn,
will increase
ges
and sourcing,
as increases.
seen in theThis,
relation
between
voltage the
the reference,
theto
voltage
on the LED
epoint.
LED >>
currentThese
and Vparameters
apply tothan
the control
loop used
regulate
FB. If VFB is greater
n order to reduce
the Figure
LED current.
current,
7:
The 0.6V reference voltage of the MAX8515 is the input for the control
izes variations
anages the sinking and sourcing, as seen in the relation between voltage
ntrol loop used
to regulate the LED current, Figure 7:
loop;
set
point. >>
MAX8515
isVthe is
input
the control
loop; proportional to the LED current, with
the for
output
and is directly
FB
oportional to the LED current, with ILED = VFB/R2;
=V
FB/R2;
dnimizes
resistorvariations
R2ILED
(note
that
the gain of MAX8515 is actually negative due to
ensistor;
controlthis
loop
used
to regulate
the
LED current,
Figure
7:R2 (note
G1
is the
gain ofby
the
MAX8515
and
resistor
is
compensated
swapping
the
plus
and minus
signsthat
on the gain of
the MAX8515
is
the
input
for the negative
control loop;
MAX8515 is actually
due to the inverting action of the NPN
ye proportional
to between
the LED current,
with voltage
ILED = VFB
/R2;
G2 is the gain
the compensated
TEMP_I
the feedback
transistor;
by and
swapping
the negative
plus and due
minus
5 and resistor
R2 (note this
thatisthe
gain of MAX8515
is actually
to signs
on the
adder);
N transistor; this
is compensated
by swapping the plus and minus signs on
Capacitor C2 is the integrator while G2 is the gain between the
ating the LED current begins by maintaining the feedback voltage VFB
while G2 is the
gain between
the the
TEMP_I
voltage
and the feedback
TEMP_I
voltage and
feedback
voltage.
reaches its turn-on threshold, Q2 will pull down the DIM pin on the
converter. This will automatically stop the converter from switching
and the output voltage will slowly drop until Q2 is turned off. The cycle
will repeat so that the output voltage will vary around the overvoltage
threshold, which is chosen to be within the operating range of the
converter.
Reference
Without LED current
regulation
With LED current
regulation
L1
100µH
100µH
RSENSE
470mΩ
300mΩ
R2
R3
C1, C2
to 0.6V:
will regulate
VFB tothe
0.6V:
egulating theThis
LEDcontrol
currentloop
begins
by maintaining
feedback voltage VFB
N.A.
N.A.
0.6V
R 2 = 1µF
ILED
10µF
3Ω
27kΩ
1µF
C3
10µF
0.6V
Overvoltage protection also needed
ILED =
A LED normally fails as a short circuit,
lowering
the output
Table thus
1: key
component
valuesvoltage. If the output voltage remains
R2
VFB to 0.6V:
higher than the input voltage, the circuit will continue to function correctly. However, if the LED fails by
To correctly
configure
boost circuit,
resistor
RSENSE
rcuit, sense resistor
RSENSE
should the
be chosen
so thatsense
the
maximum
becoming
a high
impedance (open circuit) rather than a short circuit, the output current will charge the
0.6V
n needed. The
extra
control
loopsowill
then
the input
to slightly
getC3 tohigher
output
capacitor
a value beyond
the operating
range extend
of the IC,
and cause it to fail.
Measurements
confirm,
analysis
should
be
that
thereduce
maximum
current
is
=chosen
ILED
e value of this
resistor
canR2
be calculated
as follows:
To protect
from such aTo
condition,
few extra components
be added,
8, to the two
basic
verify thea buck/boost
analysis andcan
assess
overallFigure
performance,
than
needed.
The
extra control
loop will then
reduce the
the circuit
input current
ηVIN 200mV
circuit. If the gate voltage of Q2 reaches its turn-on threshold, Q2 will pull down the DIM pin on the
circuits were built and tested, one with the extra LED current regulato get
the correct
value. The
value
this resistor can be
<resistor
st circuit,Rsense
RSENSELED-current
should be chosen
so that
theofmaximum
sense
converter. This will automatically stop the converter from switching and the output voltage will slowly drop
ILED Vcontrol
LED
than needed.calculated
The extra
loop will then reduce the input current to get
tionrepeat
and one
without
The circuits
designed
drive
eight LEDs
as
follows:
until Q2 is turned off. The cycle will
so that
the it.
output
voltagewere
will vary
aroundtothe
overvoltage
. The value of this resistor can be calculated as follows:
at 200
mA from
a 12-V
input.
The efficiency was estimated to
threshold, which is chosen to be (≈24V)
within the
operating
range
of the
converter.
an be calculated by: ηV 200mV
IN
R sense <
be around 95%.
ILED VLED
<< Figure 8: Over-voltage protection is needed when an LED fails open circuit, thus allowing C3 to
become charged beyond the maximum rating of the IC. >>
With the output at 4.8W (24V × 200mA), input power was 4.8 W/0.95
The additional
R2 can be calculated
by: sense resistor R2 can be calculated by:
5.05 W. Using a 12-V power supply, the input current should be
Measurements confirm, extend≈analysis
0.6V
R2 =
to 5.05W/12V
≈ 421 mA,two
which
results
in a
470-mΩ
value one
To verify the buck/boost analysisregulated
and assess
overall performance,
circuits
were
built
and tested,
ILED
with the extra LED current regulation
and
one without
The circuits were designed to drive eight LEDs
for the
sense
resistor it.
(200mV/421mA).
(≈24V) at 200 mA from a 12-V input. The efficiency was estimated to be around 95%.
ded
Withremains
the output at 4.8W (24V × 200mA), input power was 4.8 W/0.95 ≈ 5.05 W. Using a 12-V power
uit, thus lowering
the output
voltage. Ifalso
the output
voltage
For the circuit with regulation of the LED current, R2 needs to be 3Ω
Overvoltage
protection
needed
supply,
theby
input current should be regulated to 5.05W/12V ≈ 421 mA, which results in a 470-mΩ value
rcuit will continue
functionfails
correctly.
However,
if the
LED
fails
(600mV/200mA). To extend the input voltage down to 8V, the sense
A LEDtonormally
as a short
circuit,
thus
lowering
the
output
voltforcharge
the sense
(200mV/421mA).
circuit) rather
than
short
circuit,
the output
current
the resistor
resistor
the following
condition:
age.
If a
the
output
voltage
remains
higherwill
than
thecircuit
input
voltage,
the
For the
with regulation
of the
LED should
current,meet
R2 needs
to be 3Ω
(600mV/200mA). To extend the input
ond the operating range of the IC, and cause it to fail.
voltage down
to
8V,fails
the by
sense resistor should meet the following condition:
circuit
continue tocan
function
correctly.
However,
if the
LED
ondition, a few
extrawill
components
be added,
Figure
8, to the
basic
0.95 × 8V × 200mV
becoming
a high
(open
than a short circuit,
ches its turn-on
threshold,
Q2impedance
will pull down
thecircuit)
DIM pinrather
on the
= 317mΩ
R sense <
200mA × 24V
op the converter
from switching
and
the output
voltage
will slowly
the output
current will
charge
the output
capacitor
C3 drop
to a value beso a value of 300mΩ was chosen.
repeat so that
thethe
output
voltage
will vary
around
thecause
overvoltage
yond
operating
range
of the
IC, and
it to fail.
thin the operating range of the converter.
so a>>value of 300mΩ was chosen.
<< Table 1: key component values
n is needed when an LED fails open circuit, thus allowing C3 to
To demonstrate the added value of the LED current regulation, the LED current was recorded for an input
mum rating of the IC. >>
voltage range of 8V up to 16V for both circuits, Figure 9. It is clear is that for the circuit without LED
current regulation, the LED current is only at its 200-mA target value when the input voltage is at its
nalysis
nominal value of 12V. For other values, it scales linearly with the input voltage. If the input voltage is
nd assess overall performance, two circuits were built and tested, one
regulated, the variation on VIN may be very small and result in an acceptable LED current variation.
on and one without
circuits
designed the
to drive
LEDs
Figure it.
7: The
Control
loopwere
for regulating
LED eight
current
begins by maint. The efficiency was estimated to be around 95%.
taining the feedback voltage VFB at 0.6V.<< Figure 9: LED current versus input voltage with (red) and without (blue) additional regulation shows
0mA), input power was 4.8 W/0.95 ≈ 5.05 W. Using a 12-V power
the output current's sensitivity to the value the input voltage. >>
regulated to 5.05W/12V ≈ 421 mA, which results in a 470-mΩ value
mA).
To protect the circuit from such a condition,
few extra components
In acomparison,
the circuit with LED current regulation does not show this effect but has a constant value
LED current,
R2be
needs
to be
3Ω (600mV/200mA).
To extend
the input
can
added,
Figure
8, to the basic circuit.
If the
voltage
of Q2 range. The extra control loop clearly shows its value by regulating the LED
over
thegate
entire
input voltage
stor should meet the following condition:
current
to
the
target
value
for the entire input-voltage range; it is slightly lower only with 8-V input. Most
0.95 × 8V × 200mV
likely, the efficiency was slightly lower than the estimated 95% due to losses in R2. A quick measurement
= 317mΩ
R sense <
200mA × 24V
showed that the input current was at the maximum for VIN = 8V. A simple fix would be to lower RSENSE to
270mΩ.
Another nice feature of the hysteretic-buck LED driver is that the control loop is inherently stable, since
>>
there is no feedback. Adding the additional control loop introduces feedback, which could introduce
instabilities. A Bode plot of the stability of the control loop revealed that the circuit has a gain margin of
the LED current regulation, the LED current was recorded for an input
about 47°, which is sufficient to guarantee stable operation, Figure 10.
oth circuits, Figure 9. It is clear is that for the circuit without LED
Figure 9: LED current versus input voltage with (red) and without
is only at its 200-mA target value when the input voltage is at its
(blue)
regulation
theregulation
output current’s
sensitivity
<< Figure 10: The Bode plot of the
LEDadditional
driver circuit
with theshows
current
confirms
the circuittohas
ues, it scales linearly with the input voltage. If the input voltage is
thestable
value operation.
the input voltage.
sufficient
gain
margin
to
guarantee
>>
be very small and result in an acceptable LED current variation.
References
put voltage with
(red)
without (blue)
additional
regulation
Figure
8: and
Over-voltage
protection
is needed
whenshows
an LED fails open
e value the input voltage. >>
circuit, thus allowing C3 to become charged beyond the maximum
rating of
thenot
IC. show this effect but has a constant value
current regulation
does
The extra control loop clearly shows its value by regulating the LED
ntire input-voltage range; it is slightly lower only with 8-V
® input. Most
Bodo´s
Power
wer than54
the estimated
95% due
to lossesSystems
in R2. A quick measurement
at the maximum for VIN = 8V. A simple fix would be to lower RSENSE to
tic-buck LED driver is that the control loop is inherently stable, since
To demonstrate the added value of the LED current regulation, the
LED current was recorded for an input voltage range of 8V up to 16V
for both circuits, Figure 9. It is clear is that for the circuit without LED
current regulation, the LED current is only at its 200-mA target value
December 2015
www.bodospower.com
15 x
when the input voltage is at its nominal value of 12V. For other values,
it scales linearly with the input voltage. If the input voltage is regulated, the variation on VIN may be very small and result in an acceptable
LED current variation.
more reliable than
previously seen
In comparison, the circuit with LED current regulation does not show
this effect but has a constant value over the entire input voltage
range. The extra control loop clearly shows its value by regulating
the LED current to the target value for the entire input-voltage range;
it is slightly lower only with 8-V input. Most likely, the efficiency was
slightly lower than the estimated 95% due to losses in R2. A quick
measurement showed that the input current was at the maximum for
VIN = 8V. A simple fix would be to lower RSENSE to 270mΩ.
Another nice feature of the hysteretic-buck LED driver is that the
control loop is inherently stable, since there is no feedback. Adding
the additional control loop introduces feedback, which could introduce
instabilities. A Bode plot of the stability of the control loop revealed
that the circuit has a phase margin of about 47°, which is sufficient to
guarantee stable operation, Figure 10.
More power, less size,
longer lasting! Add value
with power of customisation
At Danfoss Silicon Power we combine cutting
edge technologies with the power of customisation to create unique solutions that solve your
business challenges. In a dynamic market that
demands increasingly compact, more powerful and longer lasting power modules – all while
minimising cost. Customisation is the way to stay
ahead of the competition.
Figure 10: The Bode plot of the LED driver circuit with the current
regulation confirms the circuit has sufficient phase margin to
guarantee stable operation.
References
• MAX16832 data sheet: http://datasheets.maximintegrated.com/en/
ds/MAX16832-MAX16832C.pdf
• MAX8515 data sheet: http://datasheets.maximintegrated.com/en/
ds/MAX8515-MAX8515A.pdf
Call us and discover how we
can spice up your next project.
About the author
Fons Janssen is a Principal Member of Technical Staff for Maxim Integrated. Prior to joining the company in 2003, he worked at ThreeFive
Photonics developing integrated optical circuits, and before that at
Lucent Technologies, where he worked on optical-access networks.
He graduated from Eindhoven University of Technology (The Netherlands) with an Electrical Engineering degree. Postgraduate studies at
this university led to a Master’s degree in Technological Design.
Danfoss Silicon Power GmbH
Husumer Strasse 251, 24941 Flensburg, Germany
Tel. +49 461 4301-40, [email protected]
www.siliconpower.danfoss.com
www.bodospower.com
DKSP.PA.400.A2.02
www.maximintegrated.com
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