APPLICATION NOTE
S3F84A5
An LED Lighting System
January 2010
Revision 0.00
Confidential Proprietary of Samsung Electronics Co., Ltd
Copyright © 2010 Samsung Electronics, Inc. All Rights Reserved
Revision History
Revision No.
Date
Jan. 20, 2010 - Initial draft
Description
Author(s)
0
Wei Ningning
Table of Contents
2 Hardware Implementation ..........................................................................11
2.2.2 Select the Capacitance (C2) for the Requirement of Voltage Ripple.................................................12
3 Software Implementation............................................................................13
4 System Validation .......................................................................................15
5 Appendix......................................................................................................17
5.2 Appendix 2: Source Code..........................................................................................................................17
List of Figures
Figure
Title
Page
Number
Number
Figure 1-1 S3F84P4 Pin Assignment.....................................................................................................................7
Figure 1-2 Simplified Buck Circuit..........................................................................................................................9
Figure 1-3 Current on load.....................................................................................................................................9
Figure 1-4 HKP-D1W1 Forward Voltage, Forward Current, and Relative Luminous Flux...................................10
Figure 2-1 Control Circuit .....................................................................................................................................11
Figure 3-1 Software Implementation Diagram .....................................................................................................13
Figure 3-2 Way to change PWM duty cycle.........................................................................................................14
Figure 4-1 Waveform for HPLED forward voltage and current ............................................................................15
List of Tables
Table
Title
Page
Number
Number
Table 4-1 System validation of efficiency.............................................................................................................16
Table 5-1 BOM list of Key Circuit.........................................................................................................................17
S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
1 OVERVIEW OF HPLED LIGHTING CONTROL SYSTEM
1
OVERVIEW OF HPLED LIGHTING CONTROL
SYSTEM
A light-emitting diode (LED) is a semiconductor light source that presents several advantages over traditional light
(like incandescent) sources such as lower energy consumption, longer lifetime, improved robustness, smaller size,
faster switching, and greater durability and reliance. It renders “green” light and does not contribute towards
material pollution or radiations. Usually, an LED can also be referred to as HPLED (high power LED) if the NRP
(normal rated power) is greater than 1W. It can be driven at currents that vary from hundreds of mA to more than
This document presents a simple HPLED lighting control system implemented with Samsung’s 8-bit MCU
S3F84P4.
1.1 PIN ASSIGNMENT IN S3F84P4
Figure 1-1 shows the pin assignment in S3F84P4.
Figure 1-1 S3F84P4 Pin Assignment
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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
1 OVERVIEW OF HPLED LIGHTING CONTROL SYSTEM
1.2 KEY FEATURES OF S3F84P4
The key features in S3F84P4 include:
4 kbyte Flash ROM or 208 Byte SRAM
6+6 PWM x 1
10-bit ADC x 4
8-bit Basic Timer (can be used as watchdog timer)
16-bit Timer0 (can be used as Timer A or B, the two 8-bit Timers )
EXINT X 2
Supports Configurable LVR (2.2/ 3.0/ 3.9V)
Supports Configurable RC (1M/ 8MHz @5V)
Supports six IOs (maximum) when using internal LVR and internal RC
1.3 SYSTEM PRINCIPLE
The two considerations for HPLED are:
1. Forward voltage
2. Constant control current
Different LED applications have different characteristics. For instance, LEDs come in different colors. In some
cases, manufacturers of the LED applications might also differ. Even if the LED applications come from the same
manufacturer, it can lead to differences in forward voltage. In such cases, constant voltage power cannot work.
Different LED applications should select different power suppliers according to its characteristics. For instance, by
considering efficiency, switch module power supplier (SMPS) can be chosen for different LED applications. SMPS
consists of Buck, Boost, or Buck-Boost circuits.
VO
D
VI
The duty cycle of Buck circuit is
. It is only used when the power supply is higher than the forward
VO VI
voltage, that is,
.
VO VI
D
VO
The duty cycle of Boost circuit is
. It is only used when the forward voltage is higher than the power
VO VI
supply, that is,
.
VO
D
VO VI
The duty cycle of Buck-Boost circuit is
supply and forward voltage.
. It can be used without considering the relationship of power
In this application, buck circuit is chosen to power a HKP-D1W1 white LED (forward voltage 3.5V) with a DC
power source of 5V.
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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
1.3.1 BUCK CIRCUIT
1 OVERVIEW OF HPLED LIGHTING CONTROL SYSTEM
Figure 1-2 Simplified Buck Circuit
Buck circuit works when a switch signal turns on the transistor (Q). The DC power then starts to charge the coil
(L). When the current reaches a predefined level, change the transistor state from On to Off using the switch
signal. At this time, since the coil will have inertia to keep the current direction, the load still can be powered with a
freewheeling diode until the switch signal turns on the transistor again. The resulting current is continuous but
Figure 1-3 Current on load
1.3.2 SUMMARY
The average current over load is determined by the duty cycle of switch signal.
SMPS can lead to current ripple. But it could be alleviated by increasing the PWM frequency or coil
inductance value, or by adding extra filtering circuits.
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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
1.3.3 CONSTANT CURRENT CONTROL
1 OVERVIEW OF HPLED LIGHTING CONTROL SYSTEM
Refer to the HKP-D1W1 datasheet to see the relationship of forward voltage, forward current, and relative
luminous flux.
Figure 1-4 HKP-D1W1 Forward Voltage, Forward Current, and Relative Luminous Flux
As shown in Figure 1-4, describing the luminous flux as a function of current is better than describing it as a
function of voltage. Even a slight change of voltage might lead to significant current shift. Therefore, constant
current control is used in HPLED applications.
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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
2 HARDWARE IMPLEMENTATION
2
HARDWARE IMPLEMENTATION
2.1 SYSTEM DIAGRAM AND CIRCUIT
Figure 2-1 Control Circuit
As shown in Figure 2-1, the Buck Circuit comprise of Q1, L1, D1, and C2. The output of PWM turns on/off the
transistor (Q1). The current over HPLED is sensed by a 1ohm power resistor. It then goes into S3F84P4’s ADC
module after passing through a filter composed of R4 and C3.
Brightness can be obtained by changing the PWM duty cycle after comparing the actual sensing value and target
forward current. This application uses two external interrupts (“ENINT” and “GPIO” as shown in Figure 2-1) as
keys to control the turn-on/off and brightness. A normal LED indicates the current brightness as full or half
brightness.
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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
2 HARDWARE IMPLEMENTATION
2.2 COMPONENTS SELECTION
Assume the following two conditions:
If non-divided system clock is selected as the clock source of the 6+6 PWM, its base frequency
fOSC
26
125KHz
is
.
2.2.1 SELECT THE INDUCTANCE (L1) FOR THE REQUIREMENT OF CURRENT RIPPLE
L 100uH
1
Therefore, 20% current ripple means
.
2.2.2 SELECT THE CAPACITANCE (C2) FOR THE REQUIREMENT OF VOLTAGE RIPPLE
To reduce the voltage ripple and power loss, a capacitor with small ESR like Tantalum Capacitor should be
chosen as C2. When ESL and ESR are negligible, then,
C2 47uF
Therefore, 1% voltage ripple means
.
The freewheeling diode should be a Schottky diode, as the system requires low turn-on voltage and fast switching.
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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
3 SOFTWARE IMPLEMENTATION
3
SOFTWARE IMPLEMENTATION
Figure 3-1 shows the software implementation.
Initialization
ADC sampling
Value > max?
Decrease PWM
duty cycle
Increase PWM
duty cycle
Vale < min?
Figure 3-1 Software Implementation Diagram
Since the PWM in S3F84P4 is 6+6 type, it affects the software in two ways.
Way to change the PWM duty cycle: The duty cycle is the result of both the register values of PWMDATA and
PWMEX. Therefore, any increase or decrease in register from PWMDATA will not change the duty cycle. For
more details on register PWMEX, refer to the S3F84P4 User’s Manual. Figure 3-2 shows the right way to
change the PWM duty cycle.
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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
3 SOFTWARE IMPLEMENTATION
Start
Start
Y
Y
PWMEX>=
11111100B?
PWMEX>=100B?
N
N
PWMEX = 0xFF
PWMDATA - =1
PWMEX = 0
PWMDATA + =1
PWMEX - =1
PWMEX + = 1
End
End
Decrease PWM duty cycle
Increase PWM duty cycle
Figure 3-2 Way to change PWM duty cycle
Change rate of PWM duty cycle: If PWM in S3F84P4 is 6+6 type, the PWM basic frequency is 6-bit, that
fPWM 8MHz
125KHz
26
64
fPWM 8MHz
. The overall cycle is still 12-bit to make the 12-bit
is,
when
212
4096
0.512ms
fPWM 8MHz
resolution fully valid, that is,
. So Therefore, every change of the duty cycle will
take effect after 0.512ms. Considering the AD conversion duration is 25us, duty cycle can be updated every
21 times of AD conversion.
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4 SYSTEM VALIDATION
4
SYSTEM VALIDATION
Figure 4-1 shows the current ripple and voltage ripple test waveform. Red and blue colors specify the current and
voltage, respectively.
Figure 4-1 Waveform for HPLED forward voltage and current
Based on the above formulas, Table 1 shows the values of Ips, Imcu, Vfd, Vs, and efficiency (%).
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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)
4 SYSTEM VALIDATION
Table 4-1 System validation of efficiency
#1 #2 #3
#4
#5
BD1
300
18
BD2
288
19
BD1
306
18
BD2
285
19
BD1
300
18
BD2
288
19
BD1
309
18
BD2
290
19
BD1
308
18
BD2
304
19
Ips (power supply) (mA)
Imcu (mA)
(MCU供电LED)
3.82
357
3.6
3.85
359
3.60
340
3.86
351
3.6
340
81.6
3.90
359
3.59
341
3.90
358
3.65
360
Vfd V）
340
Vs mV）
87.68 81.6 87.03
82.5
87.35
87.37 80.95 87.45 81.11
Efficiency（%）
NOTE: BD1 and BD2 represent two boards, where the basic difference lies in the value of sensing resistance,
RS (BD1) 1.0 ； RS (BD2) 1.1 . Due to the same reason, the efficiency of BD2 is always better than that
of BD1. The nominal tolerance of VISHAY WSR2 is 1%.
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5 APPENDIX
5
APPENDIX
5.1 BOM LIST OF KEY CIRCUIT
Table 5-1 shows the BOM list of key circuit.
Table 5-1 BOM list of Key Circuit
Description Manufacturer
Reference
Part number
R1
R2
Rs
C1
C2
C3
C4
R4
Q1
D1
L1
1K ohms
10 ohms
Current sensing Resistor
104
WSR21R000FEA
VISHAY
47uF Tantalum Capacitor
2nF
100uF
33K ohms
HEXFET Power MOSFET
Schottky Diode
100 uH inductance
1W1 HPLED
IRF9540
IR
IN5819
VLF12060-101M1R0
TDK
HPLED
HangKe
5.2 APPENDIX 2: SOURCE CODE
For more information, refer to Source_Code_LED_S3F84P4_V10.
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