i agree with the caps acting as a filter for ac noise here texas instruments take on the matter
High-brightness LEDs are available today with forward currents more than
100 times greater than their predecessors. These new devices are not just high
brightness, but are high power as well. Single die with dissipations of 5W and
multi-die modules with power in excess of 25W are now available. The
requirements of high efficiency and low dissipation dictate a switching power
supply for this new generation of High-Brightness (HB), High-Power (HP)
LEDs, as a voltage regulator and a current limiting resistor are no longer
appropriate. High-brightness, high-power LEDs require a constant-current
source to take full advantage of their ever-increasing luminous efficiency and
vibrant, pure color. The topology of choice for this new breed of switching
constant current sources is the basic buck converter. The most convincing
argument for using a buck converter is the ease with which this simple
DC-DC converter can be turned into a constant-current source. This article
will explain the selection of, or possible exclusion of, an output capacitor
when designing a buck regulator for constant-current drive of HB LEDs.
No. 116
NEXT ISSUE:
Design Challenges In Step-Down
Regulator Applications
POWER designer
Expert tips, tricks, and techniques for powerful designs
Feature Article................1-7
LED Drivers for
1W/3W LEDs ......................2
60V Low-Side MOSFET
Controller............................4
Switched Capacitor
Flash LED Driver..................6
Power Design Tools..........8
Driving LEDs: To Cap or Not to Cap
— By Chris Richardson, Applications Engineer
VIN
L
+
D +
– CO
RFB2
IO
RFB1
+
VO
–
VIN
L
+
D +
– CO
IO
RSNS
+
VF
–
Figure 1a. Traditional Buck Voltage Regulator Figure 1b. Buck Current Regulator
2
New LED Drivers Power 1W/3W LEDs up to 1A
LM3402/04 Regulators Offer Low Current-Sense Voltage to Reduce Power Loss
and Deliver High Efficiency up to 95%
For FREE samples, datasheets, and more information, visit
www.national.com/pf/LM/LM3402.html
www.national.com/pf/LM/LM3404.html
CB L1 VIN
CIN RON
IF
VIN
RON
RSNS
CF
BOOT SW
CS
DIM GND VCC
D1
LM3402/02HV
LM3404/04HV
Wide VIN range:
LM3402/04 = 6V to 42V
LM3402HV/04HV = 6V to 75V
LED drive currents up to:
500 mA for the LM3402/02HV
and 1.0A for the LM3404/04HV
Current sense voltage of 200 mV
reduces the power loss in RSNS
DIM pin for fast PWM dimming
LM3402/04 Features
• Easy-to-use and low external parts count
• Switching frequency up to 1 MHz
• Constant on-time control with voltage feed-forward
for nearly constant switching frequency over the full
line voltage range
• Low 0.2V feedback reference minimizes power losses
• Low shutdown current when RON pin held low
• High voltage capability supports long strings of LEDs
• Choice of 0.5A or 1.0A peak current capability
(LM3402/LM3404)
DIM
ILED
4 µs/DIV
5V/DIV
100 mA/DIV
T
Dim Response
Typical Application Diagram
Ideal for use in automotive, industrial lighting, gaming/vending machines, general illumination, and
architectural lighting applications
Controlled Current
The buck regulator is uniquely suited to be a
constant current driver because the output
inductor is in series with the load. Regardless of
whether a buck regulator is used as a voltage source
or a current source, selection of the inductor forms
the cornerstone of the system design. With an
inductor in series with the output, the average
inductor current is always equal to the average output
current, and the buck converter naturally
maintains control of the AC-current ripple. By
definition, the LED drive is a constant load system;
hence a large amount of output capacitance is not
necessary to maintain VO during load transients.
No Output Cap Yields High Output Impedance
In theory, a perfect current source has infinite
output impedance, allowing the voltage to slew
infinitely fast in order to maintain a constant
current. For switching regulator designers who
have concentrated on voltage regulators, this
concept may take a moment to sink in. Completely
removing the output capacitor from a buck
regulator forces the output impedance to depend
on the inductor. Without any capacitance to
oppose changes in VO, the output current (referred
to as forward current, or IF) slew rate depends
entirely upon the inductance, the input voltage,
and the output voltage. (VO is equal to
the combined forward voltage, VF, of each
series-connected LED)
LED manufacturers generally recommend a ripple
current, ΔIF, of ±5% to ±20% of the DC forward
current. Over the typical switching regulator
frequency range of 50 kHz to 2 MHz the ripple
itself is not visible to the human eye. These limits
come from increasing thermal losses at higher
ripple current (a property of the LED semiconductor
PN junction itself) and a practical limit to the
inductance used. The percentages are similar to the
recommended current ripple ratio in buck voltage
regulators. Inductor selection for a fixed-frequency
current regulator is therefore governed by the same
equations as a voltage regulator:
One difference is that the inductance used for
current regulators without output capacitors tends
to be higher because the drive currents for the
emerging standards of 1W, 3W, and 5W HB LEDs
are 350 mA, 700 mA, and 1A respectively. Modern
buck voltage regulators tend to use
inductors in the range of 0.1 µH to 10 µH with
saturation currents from 5A to 50A. Current
drivers at similar switching frequencies tend to
require inductors ranging from 10 µH to 1000 µH
and saturation currents ranging from 0.5A to 5A.
The main goal of high output impedance is to
create a system capable of responding to PWM
dimming signals, the preferred method of controlling
the light output of LEDs. The dimming signal
might be applied to the enable pin of the regulator,
in which case the output current can slew from zero
to the target and back to zero without the delay of
CO being charged and discharged. For even faster,
higher resolution dimming, a shunt switch, usually
a MOSFET, can be placed in parallel with the LED
array, allowing the continuous flow of current at all
times. Again, with no output capacitor to slow the
slew rate, dimming frequencies into the 10’s of kHz
are possible. This is a critical requirement in applications
such as backlighting of flat-panel displays,
and the creation of white light using an RGB arrayUsing an Output Capacitor Reduces Size and Cost
Some amount of output capacitance can be useful
as an AC current filter. Applications such as retrofitting
of incandescent and halogen lights often
require that the LED and driver be placed in a
small space formerly occupied by a light bulb.
Invariably the inductor is the largest, most expensive
component after the LEDs themselves. For the
sake of efficiency (especially important in cramped
quarters), the designer generally chooses the lowest
switching frequency that allows the solution
(mostly the inductor) to fit. Allowing a large ripple
current in the inductor and filtering the LED
current results in a smaller, less expensive solution.
For example, to drive a single white LED
(VF ≈ 3.5V) at 1A with a ripple current ΔiF of ±5%
from an input of 12V at 500 kHz would require a
50 µH inductor with a current rating of 1.1A. A
typical ferrite core device that fits this application
might be 10 mm square and 4.5 mm in height. In
contrast, if the inductor ripple current is allowed to
increase to ±30% (typical for a low-current voltage
regulator) then the inductance required is less than
10 µH, and an inductor measuring 6.0 mm square
and only 2.8 mm in height size can be used. The
output capacitance required is calculated based on
the dynamic resistance, rD, of the LED, the sense
resistance, RSNS, and the impedance of the
capacitor at the switching frequency, using the
following expressions:
Typical values for output capacitors range from
0.1 µF to 10 µF, a perfect fit for ceramic capacitors.
In many applications, the addition of an output
capacitor reduces both the size and the cost of the
total solution.
Output Capacitor Placement
For buck regulators that use PWM-based control,
such as Voltage Mode (VM) and Current Mode
(CM) the output capacitor should be connected
from the regulator output to system ground,
power.national.coidentical to a normal buck regulator. (Figure 3a)
This way, the control-to-output transfer function
of the system can be analyzed with the same
equations used when designing a voltage regulator.
When using comparator-based control, such as
hysteretic or Constant on-Time (COT), the output
capacitor should be placed in parallel with the LED
array. (Figure 3b) In hysteretic voltage regulator
circuits, this technique is often used to increase the
percentage of in-phase voltage ripple at the
feedback node. For the current regulator, it forces
both the ripple current through CO and the
forward current through the LEDs to sum at the
input to the switching comparator. The voltage
waveform across RSNS is therefore in-phase with the
switching node waveform, and the result is
predictable operation with high noise rejection.
The combination of low output capacitance and
high inductor current ripple actually makes
hysteretic and COT current regulators more
reliable and easier to design than voltage regulators.
Conclusion
The high brightness, high power LED represents the
biggest change in lighting design since the introduction
of fluorescent bulbs. Using LEDs requires a
fundamental change in the complexity of electronics
used for lighting systems. Currently a large portion
of LED lighting design is retrofitting of incandescent,
halogen, and fluorescent installations. Such
systems rarely include sophisticated dimming
control, and place a high value on small size. These
are the applications where an output capacitor is a
welcome addition to the driver circuit.
In the future, the higher cost of LEDs for general
lighting will be balanced by new levels of control
over brightness, tone, and color. Lighting in homes
and businesses will require fast PWM dimming,
requiring current drivers to minimize or eliminate
their output capacitance. These systems will draw
upon experience from today’s fast-dimming
applications which have already shed the output
capacitor to provide the best response time. ■
7
POWER designer
Driving LEDs: To Cap or Not to Cap
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