AN3321, High-Brightness LED Control Interface - Application Notes

Freescale Semiconductor
Application Note
Document Number: AN3321
Rev. 0, 10/2007
High-Brightness LED Control
Interface
MCU-Based LED Drivers Using DC-to-DC Converters:
Buck, Boost, and Buck-Boost
by: Philip Drake
US Applications Engineering
High-brightness LED control requires a constant current,
maintained over temperature and voltage. The driver and
control system must be designed to deliver a constant
current to optimize reliability and constancy. Integration
of high-brightness LED control with a low-cost
microcontroller (MCU) affords a control system
increased functionality and flexibility for tomorrow’s
lighting application needs. LED luminosity and
reliability rely on thermal control achieved with an
MCU. Wired and wireless protocols such as DALI,
DMX512, and ZigBee™ can be implemented at a very
low cost with an MCU. Multi-color mixing is practical
with the use of an MCU control device.
The analog subsystem configuration has several options.
In most applications, a DC-to-DC converter is needed to
supply the voltage and constant current that the
high-power or high-brightness LED needs. And in other
applications, the DC-to- DC converter supplies a
constant voltage tot he LED string and the current is
controlled through an active switch in the LED string.
Multi-output buck-boost constant current drivers
© Freescale Semiconductor, Inc., 2007. All rights reserved.
Contents
1
2
3
4
5
6
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Terminology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 LED Specifics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Interface and Control Options . . . . . . . . . . . . . . . . . 3
MCU Control of LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 MCU Control of the DC-to-DC Convertor . . . . . . . . 4
Current Control for LEDs . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Buck, Boost, or Buck-Boost. . . . . . . . . . . . . . . . . . . 5
3.2 Buck Topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 Boost Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4 Buck-Boost Topology . . . . . . . . . . . . . . . . . . . . . . . 8
3.5 Cuk Converter Topology . . . . . . . . . . . . . . . . . . . . 10
3.6 Example of Buck Current Control
With a Simple Comparator . . . . . . . . . . . . . . . . . . 11
3.7 Component Selection Guidelines . . . . . . . . . . . . . 12
3.8 Integrated MCU and Analog Power Solutions . . . . 16
3.9 Serial and Parallel Interface Options . . . . . . . . . . . 17
3.10 DALI Lighting Protocol. . . . . . . . . . . . . . . . . . . . . . 17
3.11 DMX Lighting Protocol. . . . . . . . . . . . . . . . . . . . . . 18
3.12 ZigBee Communication . . . . . . . . . . . . . . . . . . . . . 18
User Interfaces Displays and Buttons . . . . . . . . . . . . . . 19
4.1 LCD Display. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2 Button and Slider Switch Interface . . . . . . . . . . . . 19
Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Overview
optimized for interface with high-brightness LEDs and the MCU are available and cost-effective. The
features of many of the HCS08 MCUs, such as the analog comparator interface, the ADC inputs, timer
channels, and digital I/O options, offer a variety of control and interface options for applications driving
high-brightness LEDs.
DC-to-DC converters for LED drivers come in several varieties: buck, boost, buck-boost, and several other
variants of these circuits. All are inductor-based switch mode power converters with various features. The
buck circuit reduces the supply voltage to a lower output voltage. The boost circuit increases the output
voltage and the buck-boost circuit can either increase or decrease the output voltage over the input voltage
or supply.
1
Overview
1.1
Terminology
The amount of light that a standard incandescent light bulb gives off is traditionally given in watts (W).
The unit of light strength is the lumen (lm). Watt ratings are based on electrical consumption
(IFWD x VFWD), but they are not equivalent to incandescent bulb ratings. A typical light output of an
incandescent light bulb is approximately 10 lm/W or for a 75 W bulb, about 750 lm. The efficiency of the
LED is improving, typically showing 25–40 lm/W in production devices. If we used the low end of the
scale, 25 lm/W as an example using the 5 W LED rating below the output is 125 lm. A set of six
high-power LEDs of this type consumes only 30 watts and give off the equivalent amount of light of that
75 W incandescent light bulb
1-Watt — 350 mA operation at ~3.5 V = ~1.2 Watt electrical power
2-Watt — 700 mA operation at ~3.5 V = ~2.4 Watt electrical power
3-Watt — 1000 mA operation at ~3.5 V = ~3.5 Watt electrical power
5-Watt — 700 mA operation at ~6.8 V = ~4.8 Watt electrical power
– Incandescent 75 W @ 10 lm/W = 750 lm
– High-brightness LED 5W @ 25 lm/W = 125 lm
– Six high-brightness LEDs 5-Watts (30 electrical watts) ~ 75W incandescent ~(6 x 125 lm =
750 lm)
1.2
1.2.1
LED Specifics
LED Intensity and Flux
There is no direct conversion between millicandela (mcd) and lumens. Luminous intensity, measured in
candela or millicandela, is the light output in a particular direction. Luminous Flux, measured in lumens,
is the total light output in all directions.
High-Brightness LED Control Interface, Rev. 0
2
Freescale Semiconductor
MCU Control of LEDs
1.2.2
LED Forward Voltage
The forward voltage (VFWD) of high-brightness and high-power LEDs are not identical. The colors have
different technologies (1.4 V for GaAs, 2 V for GaAsP, 3 V for GaP) and there are differences from part
to part. The perceived color intensity for a uniform field of LEDs of different colors is not uniform either.
An example of forward voltages versus brightness is given below:
• Red: 1400 mcd @ 20 mA @ VFWD = 2.4 V
• Green: 1000 mcd @ 20 mA @ VFWD = 3.4 V
• Blue: 300 mcd @ 20 mA @ VFWD = 3.4 VB
This presents a challenge that can be solved with the use of color mixing algorithms executing on an MCU.
This aspect of control is not addressed in this application note. Refer to the presentation, “HB LED Color
Mixing and LCD Backlight,” on Freescale’s web site.
1.3
Interface and Control Options
In addition to color control options, the MCU offers these interface and control options:
• Control system interface implemented via standard MCU interfaces such as RS232, SPI, DMX,
DALI, and ZigBee.
• High-speed A/D used to sense input level (slider) settings, current sensing voltages, or other circuit
levels such as ambient light and temperature.
• Comparators are used to sense input voltage level changes relative to either internal reference or
external set level.
• Temperature sensors are used for circuit protection of high-brightness LED.
• Push button interface is used for user inputs and inter-network control settings.
• Multiple timer outputs can be used for brightness and color control.
• Display drivers are used for user interface.
2
MCU Control of LEDs
A number of sophisticated MCU peripherals can enable the use in the LED lighting applications market.
When the circuit requires a user display with backlighting, an input key or buttons and communication
with another master/slave unit the microcontroller shows its advantages. The brightness of the LED can
be controlled by the pulse-width modulator (PWM). The current control circuit can use the internal analog
comparator integrated into the MCU. If multiple strings of LED need to be controlled, consider using an
MCU per string to control the current drive of the string with the on-chip comparator and control the
brightness and color of each string with PWM outputs of the same MCU. A good reference to help you
decide which MCU to use can be found in AN3325, “Designing for Migration Among 8-Pin, 8-Bit
MCUs.” It compares three devices: the MC9S08QG8, the MC9S08QD4, and the MC9RS08KA2.
High-Brightness LED Control Interface, Rev. 0
Freescale Semiconductor
3
Current Control for LEDs
2.1
MCU Control of the DC-to-DC Convertor
As demonstrated in the circuits in this application note, MCU control of the LED’s current can be achieved
in several ways. This control can be direct or indirect. You can use an external power mosfet switch to
control the current to the LED and use an on-chip comparator or ADC modules to monitor that current.
When control is incorporated into a software loop, flexibility, and optional features are easily integrated
into a expandable control solution.
Outputs can adjust the duty cycle of the switch in the converter in response to the load current as measured
across the sense resistor. Adjustment of the duty cycle of the control waveform can be made depending on
the intensity or color of the LED string being controlled so that true color generation can be achieved.
A comparator that measures the effective LED current against a set reference voltage can be used to control
the switch of the converter in a buck converter. A timer output can be used to modulate the comparators
current control which in turn controls the brightness of the LED string.
3
Current Control for LEDs
High-brightness and high-power LEDs require precise control over the current to maintain efficient,
reliable, and output color wavelength of the LED. 1 W high-brightness LEDs typically require 350 mA
and the 3 to 5 W ultra-bright LEDs typically require higher than 700 mA with a forward voltage (VFWD)
of 3.4 V. Forward voltage varies between different manufacturers and colors of LEDs. If color control is
desired, separate control over the current for each color LED is required. This is accomplished with a
sensing circuit on the output to control the on-time of the transistor to regulate the output current through
the load.
Systems using high or ultra-bright LEDs often have many LEDs connected. A series connection has
several advantages: only one driver is needed per string, all of the series LEDs have the same current
flowing through them to give a relatively constant brightness, and each string requires only one sense
circuit for each string.
A switching regulator regulates the current flow by dividing the input voltage and controlling the average
current by means of the duty cycle or the on-time. When a higher current is required by the load, the
percentage of on time is increased to accommodate the change. This results in a nominal DC current with
a ripple current proportional to the switching frequency (fSwitch) of the control circuit. The higher the
frequency, the smaller the ripple, and the smaller the inductor can be. The current that the inductor circuit
must provide is inversely probational to the fSwitch. Much smaller inductors are needed when frequencies
above 1 MHz are used.
A pure linear solution is simple and quick. It has the drawback of being inefficient and creates more heat.
But if those factors are not important in your circuit, consider the option. A series current limiting resistor
does the job of limiting the current to the LED. It must be sized and meet the power requirements of the
load.
High-Brightness LED Control Interface, Rev. 0
4
Freescale Semiconductor
Current Control for LEDs
3.1
Buck, Boost, or Buck-Boost
Depending on the needs of the system, a DC-to-DC converter is available to meet those needs. As
mentioned, the LED needs constant current. There are three general categories of switching drivers:
• Buck is used when the LED voltage is always lower than the supply voltage. This is typical of a
simple 3 to 5 V system with one LED to drive.
• Boost is used when the LED voltage is always higher than the supply voltage. This is typical of a
simple 3 to 5V system with more than one LED in series are being driven.
• Buck-boost or cuk (pronounced chook) is used when the LED voltage can be higher or lower than
the supply voltage or the supply voltage varies significantly, as it does in an automobile. The
typical lighting application in the vehicle for high brightness led is the tail lights or interior lighting
where there are LEDs in series driven from the nominal 12 V battery supply. The battery supply
actually varies from 9 to 18 V depending upon the conditions.
Each driver can be designed with a single inductor, a fast-switching diode, and a load capacitor. The higher
the switch frequency, the smaller the current maintained by the inductor, and the smaller the inductor can
be. Typically the switch frequency is in the range of 50 kHz to 250 kHz but higher frequency circuits are
becoming available. The load capacitor supplies current in some of the circuits and aids the circuit in
reducing EMI and smoothing the current surges.
3.1.1
DC-to-DC Converters as Constant Voltage Source
DC-to-DC converters can be used as in their standard configuration, as a constant voltage source instead
of a current source for the LED. If the voltage was maintained just higher than the need forward voltage
Vf drop of the LED, or string of LEDs, a linear control approach of the LED is possible. The switch in the
control path of the LED could switch the LED on and off at much higher rates improving control
granularity thereby increasing the number of dimming steps possible or accuracy of the color wavelength
of the LED.
Other more efficient converters circuits are available, but are not discussed in this application note. The
topologies mentioned above are considered non-synchronous. For more efficient design a synchronous
topology replaces the catch diode in these circuits by a low-drop power mosfet. When the diode is
supposed to conduct, the mosfet is switched on and when the diode is not conducting, you switch the diode
off. Because the drop across this mosfet is much lower than across a diode, you would significantly reduce
the conduction loss in the circuit.
3.2
Buck Topology
The buck circuit, used to reduce the input voltage to a lower level, turns on the voltage to control the
current to the load current when the FET turns on, driving current into and charging the capacitor. While
the FET is off, the current flows through the inductor, but also through the diode. A voltage-sensing circuit
on the load can be used to adjust the switch duty or frequency to maintain a set current within a desired
tolerance. Figure 1 shows a version of the buck converter with a sense resistor providing feedback to the
MCU control system.
High-Brightness LED Control Interface, Rev. 0
Freescale Semiconductor
5
Current Control for LEDs
VIN
Switch
VOUT
L
R3
MCU
D1
DL
CL
R1
sense
R2
Figure 1. Buck Converter Configuration
IL
Time
inductor
current
ton
Figure 2. Current Changes
The operation of this regulator topology has two distinct time periods. The first one occurs when the series
switch is on. The input voltage (VIN) is connected to the input of the inductor (L). The output of the
inductor is the output voltage (VOUT), and the rectifier, or catch diode, is reverse-biased. Because there is
a constant voltage source connected across the inductor during this period, the inductor current begins to
linearly ramp upwards, as described by Equation 1.
IL(on) =[(VIN – VOUT) x tON] / L
Eqn. 1
During the on period, energy is stored within the core material in the form of magnetic flux. If the inductor
is properly designed, there is sufficient stored energy to carry the requirements of the load during the off
period.
The next period is the off period of the power switch. When the power switch turns off, the voltage across
the inductor reverses its polarity and is clamped at one diode voltage drop below ground by the diode. The
current now flows through the diode, maintaining the load current loop. This removes the stored energy
from the inductor. The inductor current during this time is:
IL (off) =[(VOUT – VD) x toff] / L
Eqn. 2
This period ends when the power switch is turned on again. Regulation of the converter is accomplished
by varying the duty cycle of the power switch according to the loading conditions. To achieve this, the
power switch requires electronic control for proper operation. It is possible to describe the duty cycle as:
D = ton / T
Eqn. 3
where T is the switching period.
For the buck converter with ideal components, the duty cycle can also be described as:
D = VOUT / VIN
Eqn. 4
High-Brightness LED Control Interface, Rev. 0
6
Freescale Semiconductor
Current Control for LEDs
3.2.1
Simplified Buck Converter
In a previous promotional tool developed by Freescale, the PROMOTORCH, a simplified version of of
the buck converter was used effectively. The buck circuit shown in Figure 3 eliminates the capacitor and
maintains the LED current by turning on the switch to charge the inductor until the current reaches the
desired level in the LED and then turning it off until the lower current threshold is reached. Refer to the
full demonstration of this circuit on the Freescale website.
VIN
Switch
VOUT
R4
L
R3
MCU
D1
DL
R1
sense
R2
Figure 3. Buck Converter Configuration
3.3
Boost Topology
The boost circuit, used to increase the input voltage to a higher level, shunts the voltage on the output of
the inductor, increasing the current through the inductor and stopping the charging of the capacitor.
L
VIN
272 uh
VOUT
1N5817
DL
1.1A sat
LEDD1
MCU
KA2
100 Ω
IRF520
Switch
6
R3
CL
10 uf
LEDD3
LEDD4
Vdd
15 K
R2
5K
R4
LEDD2
3.3 Ω
VREF
R1
VIN
+
Vf = 2.2V
+_
V = 2.2V
_ f
+
Vf = 2.2V
+_
V = 2.2V
_ f
Rsense
CS
100 Ω
Figure 4. Boost Converter Configuration
High-Brightness LED Control Interface, Rev. 0
Freescale Semiconductor
7
Current Control for LEDs
3.3.1
Circuit Operation
While the switch is on, the supply drives current charging the inductor. While the switch is off, the inductor
current flows through the diode to the capacitor and the load. The equation for the output voltage is given
here.
VOUT / VIN = 1/ (1-D)
Eqn. 5
where D is the duty cycle of the switching waveform.
3.3.2
Boost Circuit Control
To use this circuit with an MCU, you must control the switch with a output pin of the MCU and monitor
the current by sensing the voltage across the sense resistor. If a simple on/off function is needed for the
LED, the comparator inside the MCU can be the monitor. The input voltage is set by selecting an
appropriate resistor value for the current needed. For a 350 mA LED current and a voltage equivalent to
the internal bandgap threshold of 1.218 V, a resistor value of 3.45 Ω can be chosen. The output frequency
and duty cycle can be designed to provide nominal 350 mA current with the comparator adjusting the duty
cycle of the output PWM up and down as the sense voltage dictated. If the sense voltage was higher than
the reference the PWM duty cycle is adjusted to decrease the current supplied by the inductor. If the sense
voltage is lower than the bandgap reference, the PWM duty cycle is adjusted to increase the current supply.
The value of the resistor (Rsense) can be selected to keep the power consumed by the resistor to a minimum.
The power dissipated by the 3.45 Ω resistor above is about a 1/2 W, which decreases the efficiency of the
drive circuit. It can be sized as an over-current protection for the LED string. If you need to maintain a
lower power, for example, a standard 1/4 W resistor, then the reference voltage gets much smaller and you
cannot use the internal bandgap voltage as a reference. A separate external reference can be provided by
a resistor divider circuit source for the comparator.
3.3.3
Boost Circuit Power Mosfet Use
The boost circuit requires the use of an N-channel power mosfet switch. An N-channel device turns off
with the input gate at ground. Because the circuit’s purpose is to boost the voltage at the inductor, there are
no restrictions for drain to gate voltage. If a P-channel device was used, the source of the mosfet would
have to be connected to the boosted inductor. The voltage at the source cannot be more than a diode step
above the input gate voltage, which is high at 5 V typically.
3.4
Buck-Boost Topology
The buck-boost is a versatile circuit allowing the most dynamic changes in source voltage. The output can
be higher or lower than the supply voltage and is opposite to that of the input voltage. The switch does not
have a reference to ground. Other very similar topologies, the SEPIC and cuk, offer advantages over the
single-inductor buck-boost design. A single-ended primary inductance converter (SEPIC) is also able to
step-up or step-down the voltage. A typical buck-boost converter is shown in Figure 5. A typical cuk
converter is shown in Figure 6. In the cuk converter ,the input current is continuous, resulting in lower peak
values, and the drive circuit requirements are simplified due to the switch location and the possible use of
a coupled inductor, thereby reducing the cost and layout area of these designs.
High-Brightness LED Control Interface, Rev. 0
8
Freescale Semiconductor
Current Control for LEDs
VIN
IRF9540
P-MOS
Switch
Rsense
1N5817
1.0
AW60
DL
15K
10K
Coilcraft 1260
L
VIN
270 uh
CL
5K
R4
R2
MCU
Vdd
VOUT -
CL
10uf
VREF
100 ohm
LEDD1
Vf = 2.2V
LEDD2
Vf = 2.2V
LEDD3
Vf = 2.2V
LEDD4
Vf = 2.2V
LEDD5
Vf = 2.2V
LEDD6
Vf = 2.2V
R3
Figure 5. Buck-Boost Converter
3.4.1
Buck-Boost Circuit Operation
In the buck-boost converter, while the switch is on, the voltage across the inductor VL is equal to VIN.
IL(on) = (VI * D * T) / L
Eqn. 6
While the switch is off, the current flows through the diode and forms an opposite polarity voltage across
the load.
IL(off) = (–VOUT *(1–D) * T) / L
Eqn. 7
When you prototype this circuit you will be able to make it boost the voltage negative enough to drive the
LEDs. Like the buck converter, the use of a P-MOS power mosfet was chosen.
The supply voltage to the microcontroller must be stabilized with a 7805 type voltage regulator that can
provide the 5 V Vdd with an input voltage between 8 and 18 V. The buck-boost converter can accept the
unregulated input voltage and reduce or increase the output current to the LED string as needed to source
the LED’s.
The size of the sense resistor in the control path can be selected to provide a full scale voltage input to the
AD converter when the maximum current is reached. If you have a MCU ADC power level of 5 V and a
current of 350 ma, then a nominal 2 W precision resistor value of 14.2 Ω can be used. Figure 5 shows a
1 Ω 1/5 watt precision resistor. It provides a fairly accurate representation of the current to the ADC input
of an MCU.
A MC9S08AW60 was used in this test. This device can be powered by a 5 V supply, has 12 channels of
ADC input and up to eight timer channels, and a possible 20 MHz bus rate. This combination is well suited
to drive multiple strings of LED, while monitoring the current on each string, the temperature of the LEDs,
variable control voltage inputs to control brightness of each string, and have enough I/O for a keyboard
interface and a communications control interface.
High-Brightness LED Control Interface, Rev. 0
Freescale Semiconductor
9
Current Control for LEDs
The timer control registers in the S08 have a feature to use a center-aligned positive or negative true PWM
output. For the P-MOS power device, the timer output must be active low with an external pull-up resistor
to disable the switch when the MCU control logic is not active. If you are mixing converter types that
require both P-MOS and N-MOS power devices care should be taken to properly select the polarity of the
timer output and pull devices. A current isolation resistor in series with the MCU outputs to the switch,
which is necessary to minimize injection current into the MCU pin. ADC channel input protection is
provided by a high impedance resistor to minimize injection current into the ADC input pin.
3.5
Cuk Converter Topology
Figure 6 depicts an example circuit of a cuk current controlled source. The cuk converter, like the
buck-boost also provides a negative output voltage and can increase or decrease the output voltage.
VIN
VDD
L1
L2
33 μh
Ccoupling 10 μh
R3
MCU
DL
CL
IRF520
N-MOS
Switch
100 Ω
15K
VOUT LEDD6
Vf = 2.2V
LEDD5
Vf = 2.2V
LEDD4
Vf = 2.2V
LEDD3
Vf = 2.2V
LEDD2
Vf = 2.2V
LEDD1
Vf = 2.2V
Rsense
VIN
13.3 Ω
+
Inverting Op Amp
Figure 6. Cuk Converter
3.5.1
Cuk Circuit Operation
The output voltage for the Cuk converter (VOUT) is given in Equation 8.
D = VOUT/(VOUT +VIN)
Eqn. 8
where D is the duty cycle of the switching frequency.
Use an N-channel power mosfet switch in this instance. The inductors should be the same value. In the
schematic, different values were used without any bad effects.
The use of the inverting operation amplifier is needed in this circuit to ensure the MCU sees a positive
voltage from the sense circuit.
High-Brightness LED Control Interface, Rev. 0
10
Freescale Semiconductor
Current Control for LEDs
3.6
Example of Buck Current Control With a Simple Comparator
Figure 7 is an example of how you could generate a control signal based upon an analog input from the
voltage generated across a sense resistor at the base of the LED. If the design does not need brightness
control, adjust the sense resistor size to give a voltage equivalent to the typical voltage of the internal
bandgap of the part. The comparator input can be used to compare to either the internal bandgap or an
external analog reference. The output of the comparator is isolated from the power mosfet with a 1 KΩ
resistor to limit damaging current injection currents that can enter the MCU from the power mosfet. If you
want to program the part in-circuit you must provide a way to disconnect the power mosfet from the pin
since the ACMP output is shared with the BKGD pin on the KA2. Also, because the power mosfet is a
P-channel device, it turns on when the comparator output is driven low. The reference input voltage is fed
into the V- of the comparator and the VSense is feed back to the V+ letting the comparator to turn off (high
output) when the current through the sense resistor is above limit. The addition of the capacitor on the
sense input smooths out the signal as seen in Figure 7. This hook up avoids a potential issue with the KA2
when PTA0 pin is pulled up (refer to KA2 errata for details).
Vdd
5K
R1
IRF9540
VIN
VREF
220pf
CS
VIN
-
R4
+
J2 1 KΩ
disconnect
for programming
R3 2 kΩ
R6
sense
resistor
J1
measure current
R5
15k
Q1
L
220uh
D1
D1
Vf LED =
2.2 V
ILED <= 400 mA
4.8 Ω
Figure 7. LED Drive with Comparator from a KA2
;*******************************************************************
;* This code drives a single high brightness led with the
*
;*
comparator on the MCU.
*
;*
no special set up of clock will be done.
*
;*
for this demonstration the cop will be disabled.
*
;* The initialization is done followed by a main program
*
;* The main program doesn't need to do anything and just
*
;*
goes into stop.
*
;*******************************************************************
; export symbols
XDEF _Startup, main
; we export both '_Startup' and 'main' as symbols. Either can
; be referenced in the linker .prm file or from C/C++ later on
; Include derivative-specific definitions
INCLUDE 'derivative.inc'
; variable/data section
TINY_RAM_VARS: SECTION RS08_SHORT ; Insert here your data definition
tmp: DS.B
1
; code section
MyCode: SECTION
High-Brightness LED Control Interface, Rev. 0
Freescale Semiconductor
11
Current Control for LEDs
main:
_Startup:
;init the acmp
mov
#HIGH_6_13(SRS),PAGESEL
mov #(mSOPT_COPT|mSOPT_STOPE|!mSOPT_BKGDPE), MAP_ADDR_6(SOPT)
; BKGD disabled,COP disabled, STOP enabled
mov #(mACMPSC_ACME|mACMPSC_ACBGS|mACMPSC_ACO|mACMPSC_ACOPE),ACMPSC
;acmp enabled - output pin enabled - no interrupt
mov #(mSPMSC1_LVDE|mSPMSC1_LVDRE), MAP_ADDR_6(SPMSC1)
; LVI enable
stop
nop
bra *
Figure 8. Assembly Language Code for Comparator Circuit
Figure 10 depicts the resulting control output of the comparator. The comparator output changes the output
frequency and duty cycle when the current through the LED is adjusted. The control signal is on to the
P-MOS device when the signal is low. The circuit is design to have a peak current of less than the LED
maximum current of 400 mA when the switch is on all the time.
IL
COMPARITOR
OUTPUT
PMOS
1
0
ON
OFF
1
0
OFF
1
0
OFF
Figure 9. Comparator Control Waveform
3.7
Component Selection Guidelines
This section describes the function of each of the main components of the DC converter power stage. To
meet the performance requirements of the circuit, each of the application’s specific needs must be studied.
For high-brightness LEDs, constant current and reduced current for dimming control with minimum ripple
current drove each drive component selection.
High-Brightness LED Control Interface, Rev. 0
12
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Current Control for LEDs
3.7.1
Inductor Selection
The inductor is functioning as an energy storage device in these circuits. Figure 9 shows us that the
inductor has AC and DC components. The AC component is at a high frequency and will flow through the
output capacitor because it has a low impedance at high frequencies. For a reasonable compromise
between inductor and capacitor size, a ripple current of 10 to 30 percent of the maximum inductor current
can be chosen. However, the maximum ratings of the high-brightness LED can be as little as 7% higher
than nominal and the circuit should be constrained to stay within the LED specification.
3.7.2
Inductor Saturation Current
As the current in an inductor increases, the inductance decreases. How much the inductor decreases is
important. If it decreases too much, the converter circuit will not function properly. The current at which
the inductor does not work properly is the saturation current and is a fundamental parameter of the
inductor. Practically, the change in inductance is not a drastically changing parameter, the inductance
gradually trails off. The specified saturation current for most inductors is often a soft parameter. Given this
soft parameter is usually adequate to select a inductor with a saturation current greater than your average
plus the max ripple current. The maximum DC current if the switch is turned on continuously dictates the
saturation current you specify for the inductor. Typically a 1A saturation current is sufficient for a 350 mA
high-brightness LED.
3.7.3
Buck Converter Inductor Selection
In a buck converter, while the switch is on, the current in the inductor current ramps up and energy is
stored. When the switch turns off, this energy is released into the LED string. The assumption is that the
current is continuous for the circuit. The amount of energy stored by the inductor is given by:
Energy = 1/2 LIL2 - (Joules)
Eqn. 9
where L is the inductance and I is the peak value of inductor current. The amount the current changes
during switching is called the ripple current. The ripple current is given by the equation:
VL = L di/dt
Eqn. 10
where VL is the voltage across the inductor, di is the ripple current, and dt is the amount of time the voltage
is applied. Therefore the ripple current is dependent upon the inductor value.
The parameters needed to determine the inductance are the input voltage (VIN), output voltage (VOUT),
maximum DC and ripple currents (IMAX percent of ripple), duty cycle (D), and switching frequency
(FS=1/T).
L = (VIN - VOUT) X (D x T)/IRipple
Eqn. 11
High-Brightness LED Control Interface, Rev. 0
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13
Current Control for LEDs
5V
Switch
V=3.8
I = 350 mA
L
R3
MCU
DL
D1
CL
R1
sense
R2
Figure 10. Buck Converter Components
For example, for a buck converter with an input voltage range of 5 V ± 10% (VIN) – input voltage of 5 + .5
or 5.5, and an output voltage of 3.8 V (VOUT), the duty cycle needed is:
D = VOUT/VIN = 3.8/5.5 = 0.69
Eqn. 12
VL-on = VIN - VOUT = 5 – 3.8 = 1.2 V while the switch is on
Eqn. 13
VL-off = VOUT= 3.8 while the switch is off
Eqn. 14
The voltage across the inductor:
With a switching frequency (F) of 200 KHz and a max ripple current of 30 mA, gives a inductor value of:
3.7.4
L = (VL-on x D/F)/IRipple Max
Eqn. 15
L = (1.2 x 0.69/200 x 103)/.030 = 138 uH (50uH)
Eqn. 16
Boost Converter Inductor Selection
For a boost converter, the process is different in the equations for duty cycle and inductor voltage.
This example uses a maximum input voltage of 5.5 V, a switching frequency of 70 KHz, and a maximum
ripple current of 350 mA × 25% or 87.5 mA.
For an output voltage of 22 V, the duty cycle is:
D = 1–(VIN / VOUT) = 1 – (5.5/22) = 0.25
Eqn. 17
The voltage across the inductor is:
VL = VIN= 5.5 V while the switch is on
Eqn. 18
VL = VOUT - VIN = 22 - 5.5 = 16.5 V while the switch is off
Eqn. 19
Using Equation 15, the inductance is:
L = VL (dt/di) = (16.5 x 0.25/70 x 103)/.0875 = 673.5 uH
Eqn. 20
High-Brightness LED Control Interface, Rev. 0
14
Freescale Semiconductor
Current Control for LEDs
For a buck-boost converter, consider the use of two inductors of equal size. For example, if we have an
output voltage of 36 V, a maximum input voltage of 18 V, a switching frequency of 200 KHz, and a
maximum ripple current of 700 mA × 30 percent or 210 mA, we have
Duty cycle:
or D = VOUT/(VOUT +VIN) = 36/(36+18) =.666
Eqn. 21
VL = VIN= 18 V while the switch is on
Eqn. 22
VL = VOUT = 36 V while the switch is off
Eqn. 23
Inductor voltage of:
Resulting in inductor values of:
L = VL (dt/di) = (36 x 0.666/200 x 103)/.210 = 571.4 uH
3.7.5
Eqn. 24
Output Capacitor
The function of the output capacitor is to store energy, and to maintain a constant voltage. The value is
usually selected to minimize the ripple current of the converter. For our discussion, consider only
continuous inductor current mode of operation. To determine the capacitance needed, we use Equation 25
and our knowledge of the inductor current (IL), switching frequency (FS), and the desired output voltage
ripple (VRipple). The typical output ripple is between 25 and 30 percent.
C >= IL / 8 x FS x VOUT
Eqn. 25
Consideration of the capacitor’s equivalent series resistance (ESR) and equivalent series inductance (ESL)
guides the selection a bit further. The ripple current in the circuit causes a temperature increase internally
in the capacitor. Excessive temperatures result in a long-term reliability issue. To limit the effects of the
ESR and ESL of the capacitor, a value much larger than described in Equation 11 is required to maintain
the ripple current. Tantalum capacitors offer low ESR and ESL and are available in various packages.
3.7.6
Power Switch
For good switching performance, a N or P-channel MOSFET device with a low on-state resistance, and
low gate charge must be used. Typically the current rating must be 20% larger than the maximum load
current or the LED string and the drain-to-source breakdown voltage should be at least 25 percent larger
than the maximum input voltage. N-channel power mosfets are required for all boost or buck-boost circuits
due to the inherent gate to drain limits of the P-channel device. The voltage on the drain of the P-channel
device is typically a diode drop above the driving gate voltage so it is not possible to control the gate with
logic levels and expect the output voltage to boost. For the purpose of demonstration, IRF9540, and
IRF520 devices were used in our circuits.
High-Brightness LED Control Interface, Rev. 0
Freescale Semiconductor
15
Current Control for LEDs
3.7.7
Current Sense Resistor
In our control circuit, a current sense resistor monitors the LED current by monitoring the voltage across
the sense resistor. This enables the MCU to change the duty cycle in the control loop to adjust the current
to the LED(s). The resistor should be small enough not to dissipate a lot of power, but large enough to
provide an adequate voltage to the analog input of the controller. If we want maintain a standard 1/4 W
precision resistor a value of 1 to 5 Ω can achieve our design goals. If the resistor is used as a current
limiting element, then a .5 to 2 W resistor must be used, but a lot of energy will be lost and heat generated
by the resistor.
3.8
Integrated MCU and Analog Power Solutions
.
Integrated high-power analog sub-systems can be used to drive single or multiple LED strings. The control
of these high-power current drivers can incorporate sophisticated serial channel controls. For example, the
MM908E625 is Quad Half H-Bridge with P/S + HC08 + LIN. The 908E625 is a highly integrated
single-package solution that includes a high-performance HC08 MCU with a SMARTMOSTM analog
control IC. The HC08 includes flash memory, a timer, enhanced serial communications interface (ESCI),
an analog-to-digital converter (ADC), serial peripheral interface (SPI), and an internal clock generator
module. The analog control die provides fully protected quad H-Bridge/high-side outputs, voltage
regulator, watchdog, and local interconnect network (LIN) physical layer.
The MM908E625 has current regulators built into each of its four half-bridge low-side outputs. Current
limit circuitry turns off the mosfet when programmed limit is reached. Current trip points levels are
programmable at six different levels: 55, 260, 370, 550, and 740 mA. The circuitry regulates the duty cycle
of the switching frequency. The switching frequency is a maximum of 25 kHz provided by a timer output
of the MCU.
When the control of the LED is combined in an MCU-based system, other options for control and interface
arise. Simple button inputs can be used to set the mode of the high-power LED to improve marketability
of a simple device or allow unassisted safety alarm systems.
3.8.1
LED Lighting Control Using the MC9S08AW60
Refer to a reference design of a multi-color LED lighting control solution using the MC9S08AW60
microcontroller, available on the Freescale website.
Using an MCU to control the red/green/blue (RGB) color LEDs increases system flexibility and
functionality for the next generation of lighting applications, architectural/entertainment lighting, or LCD
back lighting, that require a smart and adaptive control methodology to ensure optimized color space
rendering for various display contents, excellent color contrast for realistic display scene and a consistent
color setting in manufacturing. In many cases, these new applications are controlled by a central control
unit that requires a connectivity interface that can be implemented at a low cost using an MCU-based
lighting controller.
In general, LEDs have a nonlinear I-V behavior and current limitation is required to prevent the power
dissipation to exceed a maximum limit. Therefore, the ideal source for LED driving is a constant current
source. As mentioned earlier the major advantage of linear driver is fast turn ON and OFF response times
High-Brightness LED Control Interface, Rev. 0
16
Freescale Semiconductor
Current Control for LEDs
to support high frequency PWM dimming method and wide range control on dimming level. An integrated
DC-to-DC boost converter (MC34063) generates the high voltage required for LED driving in series and
is shared with RGB channels, but the drawback id the power loss on R channel is higher than G or B
channels. Individual DC-to-DC blocks must be used for each channel in power sensitive applications.
3.9
Serial and Parallel Interface Options
Because most Freescale MCUs have hardware modules that support serial interfaces, you have several
options for implanting control interfaces.
3.9.1
The SCI Interface
The SCI is an asynchronous serial interface similar to a UART using two wires, a TxD and RxD. The SCI
allows full-duplex, asynchronous, NRZ serial communication between the MCU and remote devices,
including other MCUs.
3.9.2
The SPI
The SPI is a 4-wire synchronous high-speed interface. The SPI module pins are master in–slave out
(MISO), master out – slave in (MOSI), SPI synchronous clock (SPSCK) and slave select (SS). The most
common uses of the SPI system include connecting simple shift registers for adding input or output ports
or connecting small peripheral devices, such as serial A/D or D/A converters. Although a system can
exchange data between two MCUs, many practical systems involve simpler connections where data is
unidirectionally transferred from the master MCU to a slave or from a slave to the master MCU.
3.9.3
The Inter-Integrated Circuit—IIC
The inter-integrated circuit (IIC) provides a method of communication between a number of devices. The
interface is designed to operate up to 100 kb/s with maximum bus loading and timing. The device is
capable of operating at higher baud rates, up to a maximum of clock/20, with reduced bus loading. The
maximum communication length and the number of devices that can be connected are limited by a
maximum bus capacitance of 400 pF.
The IIC bus system uses a serial data line (SDA) and a serial clock line (SCL) for data transfer and are
bidirectional. All devices connected to it must have open drain or open collector outputs. A logic AND
function is exercised on both lines with external pull-up resistors.
3.10
DALI Lighting Protocol
Digital Addressable Lighting Interface (DALI) protocol is set out in the technical standard IEC 60929 for
the controlling of lighting in buildings. DALI is a bidirectional, digital protocol developed by lighting
manufacturers for the control of light source levels. A DALI controller can query and set the status of each
light by the bidirectional data exchange. DALI can be operated as a subsystem or as a stand alone system
with maximum of 64 devices. The DALI web site provides for detailed specification and overview
presentations. The interface has been standardized by IEC (EN) 60929 E4 AC Supplied Electronic Ballasts
for Tubular Fluorescent Lamps — Performance Requirements Annex E, “Control Interface for
High-Brightness LED Control Interface, Rev. 0
Freescale Semiconductor
17
Current Control for LEDs
Controllable Ballasts.” The logo is registered by AG-DALI, the international supporter of this standard.
Freescale Semiconductor has created a DALI reference design using the MC68HC908KX8.
Documentation for the design is found in “Digitally Addressable Lighting Interface (DALI) Unit Using
the MC68HC908KX8.” This is a good starting point to learn more about DALI.
3.11
DMX Lighting Protocol
DMX512 is designed around the industry standard EIA485 interface. The system is based on balanced
transmission down a twisted pair of shielded conductors. Data transmission is based on an 8-bit
asynchronous serial protocol with one start bit (low), two stop bits (high), and no parity. This gives a data
frame of 11 bits. Because each bit is 4s wide it takes 44s to send a frame. If the line were to transmit a
continuous data stream this would result in a data rate of 250,000 bits per second, or 250k baud. For more
information on the implementation of this protocol refer to “DMX512 Protocol Implementation Using
MC9S08GT60 8-Bit MCU” (document number AN3315).
3.12
ZigBee Communication
ZigBee technology, a network layer protocol designed to use the IEEE 802.15.4 standard. is a wireless
solution available to LED control systems. The 802.15.4 standard is a specification for a cost-effective,
low-data rate (<250 kbps), 2.4 GHz or 868/928 MHz wireless technology designed for personal-area and
device-to-device wireless networking. ZigBee technology is designed to replace costly and complicated
proprietary solutions currently existing in the market and is targeted at applications that already use an
MCU. ZigBee technology requires a smaller stack size than Bluetooth™ wireless technology. It occupies
less memory on a chip, keeping costs low. Because they are standard-based, 802.15.4 and ZigBee
technologies help reduce development time for the OEM and offer reliability, security, inter-operability
and certification. ZigBee can be used with the standard lighting control protocols, DMX, or DALI, if
desired.
3.12.1
Integration Brings Reduced Size and Cost
The latest edition to the ZigBee family of ICs, the MC1321x system in package (SiP) integrates the
MC9S08GT MCU with the MC1320x transceiver into a single 9x9 mm LGA package. This reduces the
external component count up to 40 percent, lowers the solution size up to 33 percent, and reduces the
overall cost by up to 29 percent. The platform provides for scalable flash memory size from 16 K to 60 K,
providing solutions for wireless sensing and control applications that require networks that support simple
point-to-point solutions, to complete ZigBee compliant mesh networks.
For example, the MC13192 is an ultra low cost 2.4 GHz transceiver designed for low-power proprietary
wireless applications. Its features include:
• Low cost with printed antenna options, programmable output clock, 5x5 QFN-32 package
• Ideal for simple networks with multiple-end devices linked to one controller
• Compatible to any processor that provides a 4-wire SPI and three GPIO
• Packet mode used for post data processing in non-timing critical applications
• Sixteen channels, 250 kbps over the air data rate
• DSSS O-QPSK spreading and modulation
High-Brightness LED Control Interface, Rev. 0
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Freescale Semiconductor
User Interfaces Displays and Buttons
•
•
Sensitivity of –91 dBm and output power of –16.6 dBm to +3.6 dBm
Low-power modes include hibernate 3 μA and doze 40 μA
SMAC is a software layer used to control the MC13192. It resides on the application processor. The
SMAC:
• Supports simple star and point-to-point networks
• Requires low-memory footprint, less than 2.5 k
• Includes only 16 primitives
• Provides in source code and developed in ANSI C
• Is easily portable to any MCU
4
User Interfaces Displays and Buttons
Lighting solutions are becoming more sophisticated as designers differentiate their products. The
incorporation of MCU-based lighting systems are becoming the standard rather than the exception due to
the expanding need for more features such as button or slider inputs, status or control output state
information, color and brightness control, ambient condition feedback, and interface protocols.
4.1
LCD Display
Several MCUs have LCD hardware interfaces that suit the needs of lighting control systems. The
MC68HC908LJ24 has device circuitry to driver to 4/3 backplanes and static with maximum 32/33 front
planes for liquid crystal display (LCD). The MC689S08LC60 is offering an integrated LCD controller
with the low-power and feature-rich capabilities of the S08 family. It is the first LCD S08 8-bit
microcontroller for battery-powered and handheld applications. It is specifically crafted to provide high
segment count that is easy on the batteries. A larger segment display of up to 160 segments offers total
flexibility with a graphical display and sufficient memory to act as application and LCD controller without
added cost of a dot matrix or chip-on-glass, fulfilling the need for a broad spectrum of applications with
displays.
4.2
Button and Slider Switch Interface
Keyboard interrupt inputs and A/D inputs of the MCU provide the direct sensing of user controls. The
typical slider input is a variable resistor that can provide a variable voltage input to an ADC channel.
Capacitive touch sensors and resistive touch screens interfaces to the MCU are already present with most
of Freescale’s MCU selection.
5
Conclusion
The realm of high-brightness LED drivers is sometimes a daunting task for an MCU designer. These
designs cross over into a mix of power analog and MCU software/digital control design. With the tips and
direction given in this application note, the digital MCU designer can design and test any number of
applications that use the higher voltage and currents to drive high-brightness and high-power LEDs.
High-Brightness LED Control Interface, Rev. 0
Freescale Semiconductor
19
References
6
References
Jim Holdahl, “Simple Inductors, Simple Solutions.” ECN Magazine, May 1, 2001,
http://www.ecnmag.com/article/CA82608.html
Freescale Semiconductor, “MR16 Form Factor Lighting Module Design Example.”
http://www.freescale.com/webapp/sps/site/application.jsp?nodeId=02430Z904043388501
Dugald Campbell and Oliver Jones. “Connecting with High Brightness LEDs.” Presentation, Freescale
Technology Forum, Orlando, Florida, June 20–23, 2005.
Freescale Semiconductor, “ZigBee Standards Overview.”
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=01J4Fs25657725
Wikipedia - DC to DC converters:
http://en.wikipedia.org/wiki/DC_to_DC_converter#Switched-mode_conversion
Power Electronics—Converters, Applications, and Design, 2nd Edition.
by Mohan, Undeland and Robbins
MM908E625: Quad Half H-Bridge with P/S + HC08 + LIN device specification.
http://www.freescale.com
AN3315 “DMX512 Protocol Implementation Using MC9S08GT60 8-Bit MCU”
http://www.freescale.com
HB LED Color Mixing and LCD Backlight presentation at FTF America by John Suchyta
http://www.freescale.com
LED Lighting Control Using the MC9S08AW60 Designer Reference Manual
www.freescale.com
High-Brightness LED Control Interface, Rev. 0
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