1 Overview Freescale Semiconductor Application Note

1 Overview Freescale Semiconductor Application Note
Freescale Semiconductor
Application Note
Document Number: AN4105
Rev. 0, 04/2010
Automotive High Brightness LED
Control
Based on the MC9S08MP16 microcontroller
by: Oscar Camacho
Automotive Systems Enablement
Microcontroller Solutions Group
1
Overview
This document describes the implementation of a
lighting control for high-brightness LED strings based
on the MC9S08MP16 family of microcontrollers and a
low-cost discrete solution. The embedded closed-loop
control algorithm that is implemented ensures the
optimal flow of current through the high-brightness LED
strings, maximizing their life and avoiding undesirable
visual blemishes.
The microcontroller implements a closed-loop PID
control, along with some features required for
automotive lighting applications including dimming,
indicating over-temperature, and open-load protections.
The right combination of high performance,
analog-to-digital converters, FlexTimer module, and
connectivity make the MC9S08MP16 the ideal solution
for this application.
© Freescale Semiconductor, Inc., 2010. All rights reserved.
Contents
1
2
3
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Background information. . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 LED basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2 Driving LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Switched-mode power supply basics . . . . . . . . . . . 6
4
Demo application overview . . . . . . . . . . . . . . . . . . . . . . . 8
5
Hardware design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1 Buck-boost switched-mode power supply design . . 9
5.2 Component value calculation . . . . . . . . . . . . . . . . 11
5.3 Current-sense circuitry . . . . . . . . . . . . . . . . . . . . . 14
5.4 Dimming circuitry (special considerations) . . . . . . 15
5.5 Protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.6 Microcontroller . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.7 Thermal management for HBLEDs . . . . . . . . . . . . 21
6
Software design considerations. . . . . . . . . . . . . . . . . . . 23
6.1 Pulse width generation for a switched-mode power
supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2 Analog-to-digital conversion resolution . . . . . . . . . 24
6.3 Analog reading synchronization . . . . . . . . . . . . . . 24
6.4 Constant current control algorithm . . . . . . . . . . . . 25
7
Implementation results . . . . . . . . . . . . . . . . . . . . . . . . . 26
8
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
9
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Appendix A Schematics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Appendix B Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Introduction
2
Introduction
High-brightness light emitting diodes (HBLEDs) are taking the lighting industry by storm. Brilliant colors,
long life, and energy efficiency are only three reasons why high-brightness LEDs are gaining rapid
popularity in the automotive, consumer, and industrial markets. With improved output intensities, drive
circuitry and packaging technologies, high-brightness LED systems offer tremendous opportunities for
dazzling new lighting applications.
In the automotive industry, the HBLED technology differentiates vehicles in terms of styling, safety, and
fuel economy, going from simple switch illumination and LCD backlighting through very high-brightness
headlamp applications.
Some of the potential applications for HBLEDs in a vehicle include:
• Exterior LED lighting
— Daytime running lamps (DRL)
— Headlamps
— Center high-mounted stop lamps (CHMSL)
— Rear stop/turn/tail lamps
— Adaptive brake lamps
— Indicators
— License plate illumination
— Fog lamps
— Cornering lamps
— Static bending lights
• Interior LED lighting
— Map lamps
— Dome lamps
— Puddle lamps
— Reading lamps
— Accent lighting
— Ambient lighting
— Dashboard lighting
• LED backlighting
— Infotainment system (navigation, entertainment) displays
— HVAC backlighting
— Gear indicators
— Switches
— Instrument clusters
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Background information
3
Background information
3.1
LED basics
Figure 1 shows the basic symbol of a common LED; when polarized, the current flowing through the LED
is commonly called forward current and the voltage drop across it forward voltage. In the next paragraphs,
the basic implications of these parameters in system behavior are described.
iF
Anode
Kathod
+ V
F
-
i=
Figure 1. LED symbol
3.1.1
Forward current (IF)
High-power LED systems require a constant current to maintain color integrity and provide luminosity.
The typical forward current for HBLEDs ranges from 100 mA up to 1000 mA; in automotive systems the
control also needs to compensate low or high operational battery conditions to maintain a constant current
through the LED.
3.1.2
Forward voltage (VF)
LEDs with the same part number and same specification will not have exactly the same forward voltage.
If the current IF flowing in two LEDs is the same, their forward voltages VF will not be identical.
Controlling the LEDs intensity by means of a constant voltage might result in variations in intensity from
LED to LED. A current control is required to ensure the same luminosity for all LEDs.
3.1.3
Dimming an LED
Not only does LED luminous intensity depend on the current flowing in the LED, but LED chromaticity
also depends on the LED current (Figure 2). To make sure not to change LED color, it is necessary to drive
the LED with constant current. By using PWM, the average current in the LED (light intensity) can be
changed without changing the real LED current (color).
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Background information
color
Figure 2. True color dimming example
3.1.4
Thermal design
LED luminous intensity is inversely proportional to LED junction temperature — emitter colors can go to
higher wavelengths as temperature (TJ) increases. Thermal design of LED lighting systems is of primary
importance.
3.2
Driving LEDs
There are several strategies for driving an LED, each with pros and cons in terms of cost, functionality,
efficiency, and reliability; below you can find a comparison for each of these strategies:
3.2.1
Parallel vs. LED strings
Figure 3. Parallel LED driving
Using resistors to limit LED current is very common for low-intensity LEDs, but in the case of HBLEDs
the resistors must be rated for higher power, resulting in an inefficient system.
When driving LEDs in parallel, LED current variations are lower in case of VBAT variations. Differences
in light output between LEDs are due to mismatches in the LED forward voltage and resistance variations.
If a control loop is chosen, each LED will require dedicated control, driver, and sense circuitry. This
solution would be expensive if there are a large number of LEDs.
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Background information
Figure 4. LED strings
When using LED strings, the resistors can be rated for lower power. Compared to parallel driving, power
dissipation is lower, but LED current variations are higher in case of VBAT variations.
In a series string only one driver is needed per string — all of the LEDs in the series have the same current
flowing through them to give a relatively constant brightness, and each string requires only a single sense
circuit.
On LED strings, differences in light output are lower than in the parallel driver, because all LEDs have the
same forward current; variations in light output remain proportional to VBAT.
3.2.2
Linear regulator vs. switched-mode power supply
There are two strategies to regulate LED current with closed-loop control:
• Linear regulation
• Switched regulation
When using linear regulators, HBLED current can be set by using a resistor — the feedback loop regulates
the current linearly. In case of large VBAT – VOUT delta and/or high LED current, the power dissipation in
the transistor is large.
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Background information
Figure 5. LED driving example using a linear regulator
Differences in light output between LEDs are due only to mismatch in light intensity between the LEDs.
When using switched-mode power supplies, the LED current can be also set by using a resistor. The
feedback loop regulates the current, but in this case, the transistor is being driven in the cutoff and
saturation regimes, switching VBAT voltage on and off. This solution is the most energy-efficient because
there is very low power dissipation on the transistor.
Figure 6. LED driving example using a switched-mode power supply
Depending on the SMPS topology (buck, boost, buck-boost), VOUT can be below and/or above VBAT,
allowing a large number of LEDs.
In terms of cost, switched-mode power supplies are usually more expensive because of the need for energy
storage components (inductors and capacitors); also, switched-mode power supplies might create noise or
EMI problems.
3.3
Switched-mode power supply basics
A switched-mode power supply regulates the output voltage as a proportion of the input voltage,
controlling the average current by means of the duty cycle or on-time. When the load requires a higher
current, the percentage of on time is temporarily increased to accommodate the voltage required to adapt
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Background information
to that change; after enough energy is stored in the inductor the on time will then be back to its previous
value. This results in an average DC current flowing through the inductor that depends on the on time or
the duty cycle, with a ripple current proportional to the switching frequency (FSwitch). The higher the
frequency, the smaller the ripple, and the smaller the inductor can be. The maximum inductor current will
depend on the switching frequency FSwitch .
There are basically three types of switched-mode power supplies: buck, boost, and buck-boost.
3.3.1
Buck converter
Figure 7. Common topology for a buck converter
•
•
A buck (or step-down) converter changes DC voltage from a higher to a lower level.
The transfer function of a buck converter is given by Equation 1, where D is the duty cycle of the
PWM switching frequency.
D≈
3.3.2
VOUT
VIN
1
Eqn. 1
Boost converter
Figure 8. Common topology for a boost converter
•
•
A boost (or step-up) converter changes DC voltage from a lower to a higher level.
The transfer function of a buck converter is given by Equation 2. D is the duty cycle of the PWM
switching frequency.
D≈
VOUT − VIN
VOUT
2
Eqn. 2
1. Equation 1 is only valid for continuous conduction mode.
2. Equation 2 is only valid for continuous conduction mode.
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Demo application overview
•
3.3.3
Input current IIN will be larger than the actual output current IOUT. The relationship is given by the
equation IIN = ε × IOUT / (1-D), where ε is the power supply efficiency (ideally = 1).
Buck-boost converter
Figure 9. Negative topology for a buck-boost converter
•
•
Depending on the duty cycle of the switching signal, in a buck-boost converter VOUT can be lower
or greater than VIN.
Given the fact that output power POUT should ideally be equal to input power PIN, the output
current can be greater or lower than IIN.
Several topologies can be used for buck-boost converters. It is important to have in mind that some provide
a negative output voltage, like the CUK converter.
The transfer function for the buck-boost converter shown in Figure 9 (negative topology for a buck-boost
converter) is given by Equation 3.
Vout
⎛ D ⎞
≈ −⎜
⎟
Vin
⎝1− D ⎠
3.3.4
Eqn. 3
Power supply selection
Depending on the battery voltage VBAT and on the number and type of LEDs composing the string, the
output voltage VOUT can be lower or greater than VBAT.
Output voltage for an LED string is given by the sum of the forward voltage of the LEDs that it contains,
resulting in Equation 4.
VOUT = VF1 + VF 2 + … + VF N ≈ N × (VF)
Eqn. 4
Where:
• N is the number of LEDs in the string
• VF is the typical forward voltage of the LED
4
Demo application overview
The present demo application is a dual string HBLED lighting control based on the MC9S08MP16
microcontroller. The microcontroller is in charge of measuring the current feedback coming from the LED
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strings, processing it with a control algorithm and, as a result, controlling the operation of a discrete
buck-boost switched-mode power supply. This ensures the optimal flow of current through the HBLED
strings, maximizing HBLED life and avoiding undesirable visual blemishes.
The microcontroller is also responsible for monitoring user inputs, battery voltage and temperature
sensors, and diagnosing the status of the LED power supply in real time; extra communication features,
such as LIN, can be also implemented with the same microcontroller.
The switched-mode power supply used to provide power to the HBLEDs is a discrete buck-boost topology,
designed to manage from one to eighteen LED strings (from 0 V to ~65 V continuous) and up to 500 mA
of output current. The design block diagram is found in Figure 10.
Figure 10. Application block diagram
5
Hardware design
This section describes the main hardware design considerations for the demo application, including the
switched-mode power supply and microcontroller blocks.
5.1
Buck-boost switched-mode power supply design
As described in section Section 3.3, “Switched-mode power supply basics,” a buck-boost power supply is
capable of supplying an output voltage VOUT either lower or greater than battery voltage VBAT.
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In the same manner, the output current capability can be lower or higher than the input current; if the
module is designed to keep a constant current at the output, then the power supply’s current consumption
will be variable as well.
The simple buck-boost topology shown in Figure 9 might look like an easy and cheap solution, but as
stated in its transfer function (Equation 3), the output voltage is negative and referenced to VBAT. Because
of this, the application requires a complex and expensive current feedback strategy to ensure proper
voltage to the microcontroller’s ADC.
The same issue can be found in other buck-boost topologies, for example the CUK converter shown in the
left side of Figure 11. An alternative solution could be to use a SEPIC configuration (right side of
Figure 11) in which LEDs are always connected directly to ground and have no need for differential
current feedback measurements. However, this solution does have the disadvantage of added cost because
it requires one extra inductor and one extra capacitor.
Figure 11. Alternative Buck-Boost topologies
For this demo application we explored a different topology, the one shown in Figure 12. This is a
combination of a buck converter and a boost converter sharing the same inductor and capacitor.
This circuit works as a buck or boost converter depending on the status of the transistors Q1 and Q2.
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Figure 12. Buck-Boost converter used in the application
For example, if Q1 is switching and Q2 is always open, D2 would be always conducting, resulting in a
common buck converter (see Figure 7); on the other hand, if Q1 is always closed and Q2 is switching, D1
will always be open and the topology is simplified to that of a boost converter (see Figure 8).
This topology reduces the cost of having an extra inductor and an extra capacitor. Also, the transfer
function is reduced to that of a common buck or boost converter depending on the mode at which the
switched-mode power supply is working. This simplifies the design from the standpoint of control.
5.2
Component value calculation
Before doing any component value calculation it is necessary to establish the electrical and functional
conditions for the module to operate. The module considers these operational parameter specifications:
Table 1. Operational demo application parameters
Forward current of LEDs
Input voltage
LEDs per string
Forward voltage of LEDs
Buck-boost switching frequency
Dimming frequency
Symbol
Unit
Minimum
Typical
Maximum
IF
mA
100
350
500
VBAT
V
8
12
18
LED_#
—
1
7
18
VF
V
2.7
3.2
3.7
FSwitch
kHz
—
350
—
FDim
Hz
—
100
—
From the parameters described in Table 1 we can infer that the maximum output voltage VOUT_MAX for
the switched-mode power supply (based on Equation 4) is given by:
VOUT_MAX = LED_# (max) × VF (max) = 66.6
Eqn. 5
Given the chosen buck-boost topology from Figure 12, the component value can be determined in the
same way typically done for a common buck converter and boost converter separately; in the same manner,
the transfer function for the whole circuit depends on the mode at which it is operating.
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A switching frequency of about 350 kHz is initially considered. However, this value can be changed, and
the component calculation can be modified accordingly if a different switching frequency is chosen.
In the next subsections the basic formulas and procedures for component calculation are described.
5.2.1
Buck transistor (Q1)
Using the circuit shown in Figure 12, Q1 will be switching at 350 kHz in buck mode, and remain closed
in boost mode. Therefore this component must sustain at least the maximum input voltage VBAT (~18V).
In terms of current consumption, in buck mode the maximum current flowing through Q1 is nothing more
than the output current or the HBLED string current; however, it is important to consider that in boost
mode the output current is always lower than input current. In other words, if the output current is constant
then the input current will increase as a proportion of the output voltage.
Equation 6 illustrates this behavior:
IIN = I_OUT × D/(1-D)
Eqn. 6
From Equation 6 we can infer that the maximum input current IIn_max is a function of the maximum output
current and the maximum duty cycle.
IIN_MAX = I_OUTMAX × DMAX / (1 – DMAX)
Eqn. 7
You can derive the maximum duty cycle, DMAX, from the boost mode transfer function (Equation 2).
Substituting VIn_min as the minimum input voltage and VOUT_MAX as the maximum output voltage, the
expression yields:
D≈
VOUT _ MAX − VIN _ MIN 66.6v − 8v
VOUT − VIN
∴ DMAX ≈
=
= 0.87
VOUT
VOUT _ MAX
66.6v
Eqn. 8
From Equation 8 we must rate the components, considering an operational range with a maximum 87%
duty cycle.
NOTE
It is important for the application software to limit the maximum duty cycle
to avoid exceeding hardware maximum limits.
Substituting Dmax in Equation 7, the maximum input current results in:
IIN_MAX = I_OUTMAX × DMAX / (1 – DMAX) = 0.5A × (0.87) / (1 – 0.87) = 3.3 A
Please consider that, even though the output current for the LEDs is 500 mA maximum, the
switched-mode power supply components must be rated to sustain 3.3 A.
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5.2.2
Boost transistor (Q2)
Q2 will always be open in buck mode, so the most important electrical considerations for this component
are when the switched mode power supply is working in the boost mode.
When the circuit is in boost mode, the boost transistor Q2 is used to charge the inductor during the positive
duty cycle. We previously calculated that the maximum duty cycle for the PWM is 87%, and that for this
range, the maximum input current IIN_MAX is 3.3 A; this same spec applies for the boost transistor Q2.
In terms of voltage, during the off time the inductor will conduct its charged current to the output by
polarizing D2. Therefore the drain to source voltage (VDS) for the boost transistor is even higher than the
maximum VOUT of the power supply (LED string maximum voltage), considering the voltage drop at
boost diode D2.
Summarizing, the specification for the maximum voltage is 66.6 V + 0.7 V (output diode D2 voltage drop)
= 67.3 V.
It is recommended that the boost transistor have a value of at least 50% of this limit, to avoid the possibility
of voltage spikes created during commutation from damaging the device.
5.2.3
Buck diode (D1)
When in buck mode, D1 is open during on time, then acts as a free-wheel diode during off time.
The specification for this diode is recommended to be:
• Forward current > output current +50%
• Breakdown voltage > VBAT
In boost mode, this diode always has an inverse polarity, resulting in a voltage approximating VBAT across
its terminals.
5.2.4
Boost diode (D2)
In either mode of the power supply, the boost diode D2 will be in series with the load. Therefore its ratings
must be:
• Forward current > IF_MAX = 500 mA
• Reverse Voltage > VOUT_MAX = 66.6 V
5.2.5
Inductor (L1)
The inductor’s simplified calculation for the power supply is given in Equation 9.
L=
(Vin _ max − Vo)(Vo)
(Vin _ max) * f * LIR * Iout _ max
Eqn. 9
Where:
• Vin_max — Maximum input voltage = 18 V
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•
•
•
•
Vo — Typical output voltage, given by the multiplication of the typical amount of LEDs per string
times the forward voltage for each = 16.8 V
f — Switching frequency = 350 kHz
LIR — Inductor current ratio given in percentage — typically 0.1
Iout_max — Maximum output current = 0.5 A
Substituting all these values into Equation 9 results in an inductor value of 22 μH.
It is important to consider a saturation current for the inductor of at least 50% of its maximum current. For
this specific case that would be 3.3 A.
Also notice that the higher the switching frequency of the power supply, the lower will be the value for the
inductor.
5.2.6
Capacitor (C1)
The capacitor complements the output filter. The capacitor is important for keeping the output voltage
level as regulated as possible.
The calculation for the output filter capacitor is described in Equation 10.
L( I out _ max + ΔI inductor ) 2
2
C=
2
2
(ΔV + V out ) −V out
Eqn. 10
Where:
• L — Calculated inductor = 22 μH
•
•
•
•
I
ΔI
— Maximum output current = 0.5 A
inductor — Given by LIR × Iout_max = 0.05
ΔV — Maximum output voltage overshoot — typically allowed to be no more than 100 mV; in
this case we are choosing 50 mV
VOUT — Typical output voltage for the power supply
out _ max
Substituting all values into Equation 10 results in approximately 4.4 μF.
Please notice that the higher the capacitor value, the lower the output ripple will be — if possible choose
a larger capacitor value than the one calculated. Also, it is recommended to choose a ceramic capacitor
because such capacitors have lower ESR. This provides better frequency response.
5.3
Current-sense circuitry
In the application, a current-sense resistor monitors the LED current by measuring the voltage drop across
it. The measured value is used by the MCU to calculate, with the control algorithm, the proper duty cycle
for adjusting the current supplied to the LED(s).
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The current sense resistor should be small enough not to dissipate a large amount of power, but large
enough to provide adequate voltage to the analog input of the controller.
On this application, we are measuring a maximum current of 500 mA and a typical current of 350 mA;
given these parameters, a 1 Ω resistor would have a maximum drop of 0.5 V in the resistor, with a
dissipation of 250 mW (1/4 W). With this size resistor, the measurements can have a resolution of
1.22 mA.
VLEDS
RSENSE
Figure 13. Current-sense resistor array
5.4
Dimming circuitry (special considerations)
Dimming is a must in today’s automotive lighting applications; it allows different light intensities and
smooth transitions when turning a light source on and off. Dimming also enlarges a lighting source’s life
by reducing thermal and electrical stress.
As explained in Section 3.1, “LED basics,” dimming an LED is not as simple as merely driving less current
through it, because chromaticity also depends on LED current. Pulse width modulation (PWM) is required
to change the average current in the LED without changing the LED color.
Dimming HBLEDs might look simple, but when using a variable switched-mode power supply, several
challenges arise, such as boosting the output voltage during dimming signal off time. Also, the current
feedback measurements must be synchronized with the PWM signal to avoid measuring during off time
or state transitions.
5.4.1
Boosting output voltage
If the power supply is commutating, and the load is simply disconnected, because of either an open load
condition, a broken LED, or the load being intentionally switched on and off when dimming, then the
output of the switched-mode power supply will consequently create a larger voltage on the output
capacitor. While the switched-mode power supply continues commutating, this voltage continues
increasing, up to a point that can damage the power supply components.
This phenomenon occurs because energy is stored on the power supply inductor and then delivered to the
load, but this energy cannot be delivered to any load when the LED string is disconnected. Therefore the
energy is transformed into a large output voltage.
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Hardware design
The output voltage boosting behavior can be observed in Figure 14.
To avoid this phenomenon, the switched-mode power supply must stop switching each time the load is
disconnected (open load) or during the off period of the dimming PWM signal.
The output voltage boosting can be solved using several methods. For example, it is possible to have a
parallel on-board load that is always connected to the output, limiting the voltage boosting when the main
load is disconnected. This method is not recommended because it makes the power supply inefficient,
dissipating energy on the alternative load at all times.
Hardware protections such as Zener diodes can also be used in a more efficient manner, but this adds cost
to the module. The solution proposed here is a software solution, done by means of an open load detection
routine and synchronization between the power supply switching signal and the PWM dimming signal.
Figure 14. Output voltage boosting during off time of dimming signal
5.4.2
PWM synchronization
It is important to consider that, if the load is continuously connected and disconnected, and consequently
the current is going to be switching from zero to its nominal value, the current feedback measurement must
be synchronized with the PWM signal. Synchronization is necessary for taking accurate measurements
after the LED current is stabilized and only during the positive PWM duty cycle.
The PWM synchronization is shown in Figure 15.
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Figure 15. Current feedback measurement synchronization
NOTE
For the PWM synchronization, the MC9S08MP16 analog-to-digital
converter module includes a hardware trigger function to synchronize ADC
readings with either real time counter (RTC), high speed comparators
(HSCMPx) or the FlexTimer modules (FTMx).
In addition, any delay for this ADC trigger can also be programmed with the
programmable delay block built into the MC9S08MP16.
5.5
Protections
Almost every lighting automotive module requires that any output be protected against conditions such
as:1
• Open load
• Short circuit
• Over-current
• Output over-voltage
• Voltage clamping
Also, to ensure safe operation of the switched-mode power supply, it is necessary to protect against battery
over-voltage, battery under-voltage, output voltage boosting, and high-current operation.
In the next subsections you will find a brief description of how the main protections implemented in this
example module work.
1. This list is not intended to list every such condition, but only to offer examples.
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5.5.1
Open load
An open load condition occurs whenever an attempt is made to turn the load on and no current feedback
is detected; this condition is monitored by the microcontroller approximately every 1 ms by reading the
result from the analog-to-digital converter corresponding to the current feedback. If the current feedback
value is lower than 10% of its nominal value and the voltage (also monitored by the microcontroller ADC)
is above the operational limit, an open load flag is asserted and the power supply is disconnected
immediately.
When an open load condition occurs, an open load retry period starts so as to periodically check the
condition and attempt turning the load back on. The open load flag is also cleared when the load is
manually turned off, so next time it is set on again, the module will try to turn it on normally.
The open load condition is monitored only in the positive period of the dimming signal. In other words, an
open load condition is ignored when the load is intentionally disconnected by the dimming circuitry. In the
off time for the PWM dimming signal, the switched-mode power supply stops switching to avoid output
voltage boosting.
5.5.2
Output short circuit/over current
If a short circuit occurs, the output must be protected (off) to avoid excessive current flowing through the
components of the switched-mode power supply.
This condition is also monitored with the microcontroller by reading the analog voltage across the current
feedback resistor. If the value exceeds the one corresponding to the maximum operational current, then the
power supply is disconnected.
5.5.3
Over-voltage (battery)
The battery voltage is monitored to make sure that the control algorithm and its result values are applied
to the power supply only if the battery voltage is valid (18 V > VBAT > 8 V).
The battery voltage is monitored using a resistor divider; the resistor divider output is connected to an
analog-to-digital converter (ADC) channel of the microcontroller.
If the battery voltage value is lower than nominal, the control algorithm tries to increase the duty cycle so
as to have accurate output voltage to supply the LEDs at a constant current. However, if it’s below the
lower limit (8 V), the power supply duty cycle would increase above the maximum hardware limits,
overstressing the components.
On the other hand, if the voltage is high the control algorithm compensates the output voltage by
decreasing the duty cycle for the switched-mode power supply. However, the excessive input voltage could
damage the components in the module, so it is recommended to stop the module’s operation.
5.5.4
Over-voltage (output)
The output of the switched-mode power supply is monitored by means of a resistor divider; the resistor
divider voltage is tied to an ADC channel which is read periodically to make sure that the output voltage
is within limits.
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Freescale Semiconductor
Hardware design
In case of over-voltage, the switched-mode power supply period will be decreased to the point at which
the condition is removed.
This over-voltage information is also used to determine the open load condition.
5.5.5
Power supply over-current
The over-current protection is implemented to ensure that the power supply components are not driving a
current higher than the maximum specified, thus ensuring proper power supply operation.
This protection makes sure that regardless of the operating conditions and the internal limitations in the
power supply duty cycle, the switched-mode power supply components are not driving higher current than
expected. This protection monitors, using a current-sense resistor, the main power supply loop that is
created by the buck transistor, boost transistor, and inductor. This protection works only in boost mode
where the input current is higher than the output current.
Figure 16. Power supply over-current protection
5.5.6
Over-temperature
Not only is it important to design a proper heat sink for the board to dissipate the energy generated by the
LEDs, but it is also necessary to have temperature feedback to ensure that the module is working within a
safe temperature region.
In this application, an analog-to-digital converter channel is dedicated to reading the information coming
from a temperature sensor; the temperature sensor is strategically placed in the center of the LED daughter
card, where the heat concentration is typically larger.
Automotive High Brightness LED Control, Rev. 0
Freescale Semiconductor
19
Hardware design
The sensor is rated from –55 °C to 130 °C and its main function is to protect LEDs working out of their
operational range. If the LEDs are at a high temperature (in this case, > 85° C) the module dims the string
of HBLEDs to 50% intensity to avoid damaging the LEDs.
5.6
Microcontroller
The MC9S08MP16 is a low-cost, high-performance HCS08 microcontroller available with a variety of
modules, memory sizes, memory types, and package types.
The characteristics that make this device the ideal solution for this application are described in Table 2.
Table 2. Special features of MC9S08MP16 used in HBLED applications
Special characteristics for HBLED
applications
Module
Channels available
Analog-to-digital converter (ADC)
13
12-bit ADC with hardware trigger from
PWM module allows conversion at any
point in the PWM cycle.
FlexTimer (FTM)
6 PWM
16-bit resolution and up to 40 MHz clock
speed enables high-frequency and
high-resolution PWM generation.
High speed analog comparator
(HSCMP)
3
Analog comparator works in conjunction
with the PWM to enable quick LED current
regulation.
Rail to rail, selectable for rising or falling
edge or both.
Programmable gain amplifier (PGA)
1
Allows the use of a very small external
sense resistor.
Differential readings, triggered by software
or hardware, programmable gain from 1× to
32×.
Programmable delay block (PDB)
2
Dead time insertions, trigger
synchronization for PGA or ADC.
NOTE
The modules described here are only some of the modules available in the
MC9S08MP16; the modules highlighted there are special features useful in
the HBLED control applications. To see the full device description and
features please refer to device reference manual and datasheet.
The MC9S08MP16 block diagram is found in Figure 17.
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20
Freescale Semiconductor
Hardware design
Figure 17. MC9S08MP16 block diagram
5.7
Thermal management for HBLEDs
Thermal management is a very important design consideration in applications involving HBLEDs — not
only for ensuring a module’s operating temperature and therefore its safety and reliability, but also from
the lighting perspective. A thermally oriented design is important to ensure the reduction of temperature
impact to LED luminosity.
The optical performance of HBLEDs also depends on temperature; proper thermal design will guarantee
not only the best optical performance in the application but its long term reliability as well.
It is important to consider the thermal resistance parameters in the LED’s datasheet, then design the proper
heat dissipation methodology via either PCB area or external heatsink.
A basic thermal design starts with the estimation of the temperature increase at a given power dissipation.
This relation is defined in the thermal resistance model described in Figure 18.
Electrical
Domain
Thermal
Domain
TA
VA
RAB
current
power
RΘΑΒ
VB
TB
Figure 18. Basic thermal resistance model
As observed in Figure 18, R(θ)AB, called thermal resistance, is defined as the ratio of temperature
difference at a given power dissipation. This dependence is defined by Equation 11.
R(θ ) AB =
ΔT
PD
Eqn. 11
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Freescale Semiconductor
21
Hardware design
Where:
• R(θ)AB = thermal resistance [°C/W or K/W]
• T = Temperature increase [°C or K]
• PD = Power dissipated [W]
If thermal resistance is reduced, the increment of temperature will be lower at the same power dissipation.
Power dissipation can only be reduced by proper component selection.
In the case of HBLEDs, the power dissipated by an HBLED is defined as the product of VF (forward
voltage) and IF (forward current). For example, if an HBLED is specified at nominal values of VF = 3.2 V
and IF = 350 mA, then the power dissipated by a single LED would be 1.12 W.
The total power dissipation to consider will be the sum of the power dissipation generated by all LEDs in
the string.
As power dissipation increases, the temperature will increase proportionally. Increasing the junction
temperature in an LED will impact the light output; typically, there is a loss of light as LED junction
temperature (Tj) increases. Electrically, the temperature increment also affects the forward voltage and
forward current in the LED.
For HBLEDs, the thermal resistance value (junction to case, typically) is given in the device datasheet;
depending on the heatsink strategy, the thermal resistance can go lower as illustrated in Figure 19, resulting
in a smaller temperature increase at a given power dissipation.
Figure 19. Thermal resistance vs. dissipation area example
As observed in Figure 19, the larger the dissipation area, the lower the thermal resistance.
The heatsink area is not the only parameter that can help lower thermal resistance; the parameters listed
here must be considered as well for a proper LED board design:
• Intended heatsink (PCB, finned)
• The board’s thermal characteristics
Automotive High Brightness LED Control, Rev. 0
22
Freescale Semiconductor
Software design considerations
•
•
•
•
•
— Copper thickness
— Pad area
— Number of layers
Location and number of vias
Air speed
Board orientation
Board-to-ambient thermal path
Heat flux in adjacent elements
Figure 20. Thermally enhanced LED string board design example
6
Software design considerations
In this section some of the main considerations in terms of software design for the application are
described.
6.1
Pulse width generation for a switched-mode power supply
Switched-mode power supplies require a fine and accurate switching frequency and duty cycle; jitter in
the PWM signal implies variations in the output voltage. For this application, these variations could result
in flickering of the light source.
Also, because a closed-loop control is used for light intensity, if the PWM resolution is poor a PWM step
increment will be reflected in a large control action. This could cause instability or visible light intensity
changes.
The FlexTimer module (FTM) is an enhanced version and compatible with the previous timer module
(TPM) available in Freescale microcontrollers. The FlexTimer module includes:
• Selectable clock source
• 16-bit counter register
• Input capture
• Output compare edge-aligned or center-aligned PWM
• Channel combinations with:
— Dead time insertion
— Hardware and software triggering
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Freescale Semiconductor
23
Software design considerations
— Configurable polarity
— Interrupt generation
The FlexTimer module brings several configurability advantages, but talking specifically about the
resolution, it doubles the PWM resolution by means of working at 2× the bus speed.
For example, if the bus speed at which the module is operating is 20 MHz, the FlexTimer frequency could
go up to 40 MHz; the PWM duty cycle resolution will be dependent on the chosen PWM frequency. For
the example application described herein, the chosen switching frequency for the switch mode power
supply is 350 kHz. Therefore the resolution is 40 Mhz / 350 kHz 114 steps, or 0.87%.
6.2
Analog-to-digital conversion resolution
We already know that luminosity control will depend on the resolution of the PWM signal controlling the
switched-mode power supply; however, the control algorithm also requires a fine and accurate current
feedback measurement for performing the proper duty cycle calculations.
The analog-to-digital converter module on the MC9S08MP16 microcontroller can work at up to twelve
bits; and given the current-sense circuitry described in Section 5.3, “Current-sense circuitry,” the voltage
drop in the 1 Ω resistor would be 500 mV at 500 mA (maximum operational current) while at the
minimum current of 100 mA the drop will be 100 mV.
The given step resolution for twelve bits at 5 V is 5 V / 212 = 1.22 mV; so, in the operational range of
400 mV (500 mV–100 mV) for the current sense circuitry, there are 327 steps of resolution or 0.3%
resolution.
As an alternative, if the resolution is not enough or if a lower current measurement resistor is used, the
programmable gain amplifier, also available on the MC9S08MP16, can be used to increase the reading
resolution.
6.3
Analog reading synchronization
It is important to synchronize current feedback readings with the dimming PWM signal and with the open
load protection, making sure that current feedback measurements take place only when the load is
connected and only when the current for the LEDs is in a steady state condition.
In this situation, the programmable delay block (PDB) can be used to synchronize the ADC reading with
the rising edge of the PWM dimming frequency, adding also a programmable time delay so that the ADC
reading will not start until the current in the LEDs has reached a steady state value.
This hardware trigger process to get the ADC data is described in Figure 21.
Figure 21. Obtaining ADC sample with hardware trigger
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Freescale Semiconductor
Software design considerations
As an alternative, the interrupt service routines of the FlexTimer can be used to trigger the ADC readings,
having a synchronization either in the timer overflow or in the timer channel interrupt service routines (see
Figure 22). In this scenario, an alternative time base variable can be used to trigger ADC readings
(software trigger).
Figure 22. Obtaining ADC sample with software trigger
The solution used for this demo application was the software trigger synchronization, using the timer
interrupt service routines.
The interrupt service routes, both for timer overflow and for channel value, were enabled so as to disable
the switched-mode power supply frequency during off dimming times, avoiding output voltage boosting
(Figure 14).
6.4
Constant current control algorithm
A simple control loop is usually enough to ensure the proper driving of HBLEDs; with closed-loop control
the module compensates for fluctuations in battery voltage, out-of-range temperatures, or any other
parameter variations that might affect the LED current in an open loop.
If performing constant current control, the module will provide the required voltage to keep the LED string
at the proper current; this also allows a flexible number of LEDs per string without any further calibration
or hardware changes.
The proposed control architecture for this example application is a software
proportional-integral-derivative (PID) control. A block diagram is shown in Figure 23.
Set Point Current
+
Error
-
PID Control
Duty
Cycle
Switched Mode Power
G(S)
Current Feedback
H(S)
Current Feedback resistor
Figure 23. Proposed control-loop block diagram
From the block diagram above:
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25
Implementation results
Error = Set Point Current – Current Feedback
The output of the PID control is given by Equation 12:
DutyCycle [i] = DutyCycle[i–1] + Kp × ProportionalFactor[i] + Ki × IntegralFactor[i] + Kd × DerivativeFactor[i]
Eqn. 12
Where:
• ProportionalFactor [i] = Error [i]
• IntegralFactor[i] = {Error[i] + Error[i-1]} × T/2
• DerivativeFactor[i] = {Error[i]-Error[i-1]} × T
And:
•
•
•
•
•
•
Kp is the proportional constant
Ki is the integral constant
Kd is the derivative constant
i is present time value
i–1 is the previous time value
T is the sampling period (this value is considered = 1 to simplify calculations)
The control loop is integrated into the 8-bit microcontroller without using floating-point mathematics.
16-bit variables were used to perform the calculations.
Also, because Kp, Kd, and Ki are typically fractional numbers, the control loop operates on the
integer-division results of the reciprocals (1/Kp, 1/Kd, and 1/Ki).
The control algorithm constants used for the implementation are:
• 1/Kp = 45 → Kp = 0.022
• 1/Ki = 80 → Ki = 0.0125
• 1/Kd = 100 → Kd = 0.01
7
Implementation results
This section shows some basic oscilloscope graphs from the HBLED controller described in this
document. It was implemented on the MC9S08MP16 8-bit microcontroller using a low-cost
switched-mode power supply.
Results are shown for a single LED string of 7 LEDs, rated at a programmable value of 350 mA.
No matter what the input battery voltage or number of LEDs in the string, the switched-mode power supply
duty cycle is automatically adapted to keep an output voltage that ensures constant current in the string.
The graphs below depict the basic behavior of this controller running with a PID control algorithm.
In Figure 24, the power-on response for the module can be observed. The bright green scope trace (scope
trace 1) shows the input battery voltage (~12 V), the turquoise channel (channel 2) shows the LED current
Automotive High Brightness LED Control, Rev. 0
26
Freescale Semiconductor
Implementation results
measured at the 1 Ω current sense resistor (350 mV correspond to 350 mA flowing in the LED string); the
violet scope trace (scope trace 3) shows the output voltage generated by the switched mode power supply.
Figure 24. LED string power-on response
For Figure 24 and Figure 25:
• Bright green (CH 1) — battery voltage
• Turquoise (CH 2) — LED current (measured at 1 Ω current-sense resistor)
• Violet (CH 3) — output voltage
Figure 24 also shows a delay of about 250 ms for the LEDs to reach their nominal value. The length of this
time varies depending on the number of LEDs in the string. This delay is determined by the time it will
take for the control to reach the proper current value.
Also, in the output voltage scope trace (violet), a discontinuity in the rising slope can be observed. This is
caused by:
• The non-linearity inherent in the switched-mode power supply
• The operational mode switching from buck to boost mode
In Figure 25, the control response to a battery voltage change (bright green, scope trace 1) is seen; in this
case, the battery voltage changed from ~8 V to ~17 V. However, the output voltage (violet, scope trace 3)
and the LED string current (turquoise, scope trace 2) remain constant.
The control algorithm latency used in this example application is 10 ms; this latency and/or the control
algorithm constants can be modified for faster response times.
Automotive High Brightness LED Control, Rev. 0
Freescale Semiconductor
27
Implementation results
Figure 25. Control response to a battery voltage variation
Figure 26, Figure 27, and Figure 28 illustrate the control algorithm operation by automatically modifying
the switched-mode power supply duty cycle so as to keep the output current constant. As can be seen in
these three figures, the current flowing through the LED is, on average, the nominal value regardless of
the input battery voltage. It can also be observed that the duty cycle of the switched-mode power supply
is modified to maintain this behavior.
Figure 26, Figure 27, and Figure 28:
• Bright green (CH 1) — Switching frequency output generated by the MCU as a result of the PID
control
• Turquoise (CH 2) — LEDs current (measured at 1 Ω current sense resistor)
• Violet (CH3) — Output voltage of the switched-mode power supply
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Freescale Semiconductor
Implementation results
Figure 26. Control operation at input voltage of ~8V
(duty cycle ~67.68% and LED current ~351 mA)
Figure 27. Control operation at input voltage ~12V
(duty cycle ~48.7% and LED current ~365 mA)
Automotive High Brightness LED Control, Rev. 0
Freescale Semiconductor
29
Implementation results
Figure 28. Control operation at input voltage ~18
(duty cycle ~22.48% and LED current ~352 mA)
Finally, in Figure 29 the operation of the dimming function is shown; the average current flowing in the
LED is given by switching the LED output on and off.
For Figure 29:
• Bright green (CH 1) — Dimming signal from the MCU
• Turquoise (CH 2) — LED current (measured at 1 Ω current sense resistor)
• Violet (CH3) — Output voltage of the switched-mode power supply
Figure 29. LED string dimming function
In Figure 29 we can see that the average LED current is 181 mA. However, this is accomplished by
switching LEDs on and off at a nominal current of 350 mA, thus avoiding chromatic changes. Also, we
Automotive High Brightness LED Control, Rev. 0
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Freescale Semiconductor
Conclusions
can see that there is no output voltage boosting, because the power supply output is disconnected during
dimming off period.
8
Conclusions
This application note has explained the implementation of High Brightness LED control based on an
embedded PID algorithm implemented on the MC9S08MP16 8-bit microcontroller. Examples of
dimming, switched-mode power supply control, and many other lighting features were described in the
document.
Even though this demonstration is intended for automotive applications, the concepts and solution
described herein can be applied to many other industrial and consumer applications that use HBLEDs.
Freescale provides a wide range of products supporting automotive lighting applications — the specific
device used in this demonstration is only one possible solution for these types of applications. For further
information about Freescale solutions for lighting please visit www.freescale.com/lighting.
9
References
•
•
•
Lighting page at Freescale website:
www.freescale.com/lighting
Freescale document AN3321, “High-Brightness LED Control Interface”
www.freescale.com/files/microcontrollers/doc/app_note/AN3321.pdf?fsrch=1
Freescale’s S08MP product summary page:
www.freescale.com/webapp/sps/site/prod_summary.jsp?code=S08MP&tid=m8Hp
Automotive High Brightness LED Control, Rev. 0
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31
Schematics
Appendix A Schematics
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32
Freescale Semiconductor
Schematics
Automotive High Brightness LED Control, Rev. 0
Freescale Semiconductor
33
Schematics
Automotive High Brightness LED Control, Rev. 0
34
Freescale Semiconductor
Schematics
Automotive High Brightness LED Control, Rev. 0
Freescale Semiconductor
35
Bill of materials
Appendix B Bill of materials
Table B-1. Bill of materials
Item
Quantity
number
Reference
designator
Value
Description
Manufacturer
Manufacturer part
number
1
4
BH1, BH2, BH3, BH4
MTG
Mounting hole 0.130 inch,
no part to order
N/A
N/A
2
20
C1, C4, C5, C7, C8,
C10, C12, C18–C20,
C23–C25, C27–C33
0.10 μF
Cap cer 0.10 μF 25 V 10%
X7R 0805
Any
Any
3
1
C2
47 μF
Cap alel 47 μF 50 V SM
6.3 × 6.3 × 5.3
Any
Any
4
1
C3
1 μF
Cap alel 1 μF 50 V 20%
X5R SMT
Any
Any
5
1
C6
10 μF
Cap tant 10 μF 10 V 10%
— 3216-18
Any
Any
6
3
C9, C11, C13
0.01 μF
Cap cer 0.01 μF 16 V 10%
X7R 0603
Any
Any
7
2
C14, C15
8 pF
Cap cer 8.0 pF 50 V
0.5 pF C0G 0603
Any
Any
8
2
C16, C21
1000 pF
Cap cer 1000 pF 100 V
10% X7R 0603
Any
Any
9
2
C17, C22
4.7 μF
Cap cer 4.7 μF 100 V 10%
X7R 2220
Any
Any
10
1
C26
220 pF
Cap cer 220 pF 50 V 10%
C0G 0603
Any
Any
11
1
D1
PDS5100H-13
Diode SCH 5 A 100 V
PowerDI 5
Diodes Inc
PDS5100H-13
12
1
D2
SMCJ18A
Diode TVS 18 V 1500 W
DO-214AB
Any
Any
13
2
D3, D5
B1100-13-F
Diode SCH rect 1 A 100 V
SMA
Diodes Inc
B1100-13-F
14
2
D4, D6
MBR230LSFT
1G
Diode SCH RECT 2 A
30 V SMT SOD-123
On Semiconductor
MBR230LSFT1G
15
4
D7, D8, D9, D10
LED_GREEN
LED GRN SGL 30 mA
SMT 0603
Any
Any
16
1
F1
Holder
Fuse holder SMT for
5 mm fuse RoHS
compliant
Any
Any
17
1
J1
CON 2 TB
CON 1 × 2 TB TH
5.08 MM SP 740H SN
Any
Any
18
1
J2
HDR 2X3
HDR 2 × 3 TH 100 mil
CTR 335H AU 95L
Any
Any
Automotive High Brightness LED Control, Rev. 0
36
Freescale Semiconductor
Bill of materials
Table B-1. Bill of materials (continued)
Item
Quantity
number
Reference
designator
Value
Description
Manufacturer
Manufacturer part
number
19
1
J3
Con plug 4
Con 2 × 2 plug shrd RA
TH 4.2 mm SP 396H SN
140L
Any
Any
20
2
JP1, JP2
Hdr 1X6
Hdr 1 × 6 TH RA 100 mil
SP 268H AU
Any
Any
21
2
L1, L2
22 μH
Ind pwr shield
22 μ[email protected] kHz 3.2 A
20% SMT
Bussmann
DR124-220-R
22
2
Q1, Q4
SI2343DS
Tran PMOS pwr 3.1 A
30 V SOT23
Vishay
Intertechnology
SI2343DS-T1-E3
23
4
Q2, Q3, Q5, Q6
ZXMN10A25G
Tran NMOS gen 3.7 A
100 V SOT223
Zetex Inc
ZXMN10A25GTA
24
1
QZ1
16 MHz
XTAL 16 MHz — 2520
SMD
Any
Any
25
1
R1
39 kΩ
Res MF 39.0 kΩ 1/10 W
1% 0603
Any
Any
26
2
R2, R4
10.0 kΩ
Res MF 10.0 kΩ 1/10 W
1% 0603
Any
Any
27
1
R3
0Ω
Res MF 0 Ω 1/10 W —
0603
Any
Any
28
1
R5
1.0 mΩ
Res MF 1.0 mΩ 1/10 W
1% 0603
Any
Any
29
2
R6, R9
12.7 kΩ
Res MF 12.7 kΩ 1/10 W
1% 0603
Any
Any
30
2
R7, R10
1Ω
Res MF 1 Ω 1/2 W 1%
1206
Any
Any
31
11
R8, R11, R12,
R17—R24
1.0 kΩ
Res MF 1.00 kΩ 1/10 W
1% 0603
Any
Any
32
3
R13, R14, R15
470 Ω
Res MF 470 Ω 1/10 W 1%
0603
Any
Any
33
2
R16, R25
10 kΩ
Res pot 10.0 kΩ 1/2 W
20% TH
Any
Any
34
2
SW1, SW2
KSC221J
Sw SPST SMT 32 V
50 mA J-BEND
Any
Any
35
12
TP1, TP3,
TP5—TP7, TP9,
TP11, TP18,
TP23—TP25, TP27
TP
Test point 36 × 20 mils
TH, not part to order
N/A
Not part to order
36
1
U1
LM2937-5.0
IC VREG LDO 5 V
500 mA 26V SOT-223
Any
Any
Automotive High Brightness LED Control, Rev. 0
Freescale Semiconductor
37
Bill of materials
Table B-1. Bill of materials (continued)
Item
Quantity
number
Reference
designator
Value
Description
Manufacturer
Manufacturer part
number
37
1
U2
MC9S08MP16
IC MCU 8-bit 16 K flash
1 K RAM 2.7–5.5 V
LQFP48
Freescale
Semiconductor
MC9S08MP16
38
2
U3,U5
FAN3268
IC PMOS-NMOS bridge
DRV 2 A 4.5–18 V SOIC 8
Fairchild
FAN3268TMX
39
1
U4
FAN3227
IC DRV NON INV TTL IN
DUAL 4.5—18 V SOIC8
Fairchild
FAN3227TMX
40
1
U6
MCZ33661EF IC XCVR LIN 10–20 Kbit/s
6.0–18 V SO8
Freescale
Semiconductor
MCZ33661EF
41
4
J4, J5, J6, J7
HDR TH 1 × 3 HDR 1 × 3 TH 100 mil SP
339H AU 118L
Any
Any
42
2
SW3, SW4
EG1257
SW slide SM right angle
0.3A
Any
Any
43
5
TP2, TP8, TP19,
TP21, TP22
Test point red
Test point red 40 mil drill
180 mil TH
Any
Any
44
8
TP4, TP10,
TP12–TP17
Test point
white
Test point white 40 mil drill
180 mil TH
Any
Any
45
2
TP20, TP26
Test point
black
Test Point Black 40 mil drill
180 mil TH
Any
Any
Automotive High Brightness LED Control, Rev. 0
38
Freescale Semiconductor
THIS PAGE IS INTENTIONALLY BLANK
Automotive High Brightness LED Control, Rev. 0
Freescale Semiconductor
39
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Document Number: AN4105
Rev. 0
04/2010
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