Samsung Work Light S3F84A5 User Guide

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  
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Revision History  
Revision No.  
Date  
Jan. 20, 2010 - Initial draft  
Description  
Author(s)  
0
Wei Ningning  
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Table of Contents  
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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  
an ampere. LEDs can produce hundreds of lumens, and find extensive usage in lighting systems.  
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  
alternating (see Figure 1-3 for more details).  
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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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)  
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)  
(MCULED)  
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) 1.0RS (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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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)  
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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