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artigo leds
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ApplicationNotes
Opto
elec
tron
ics
720 Palomar AvenueSunnyvale, CA 94086
Tel: 408.523.8200 Fax: 408.733.1287Email: [email protected]
www.purdyelectronics.com
Light Emitting Diodes
2
Purdy Electronics Corporation • 720 Palomar Avenue • Sunnyvale, CA 94086
5/24/00
Tel: 408.523.8200 • Fax: 408.733.1287 • [email protected] •
www.purdyelectronics.com
Application Notes
Drive Methods of LED Lamps
Drive Methods of LED Lamps
DC Drive Techniques
LEDs are current driven devices which means that when using a constant voltage power, a current limiting resistor must be used. Figure 1 shows circuit examples for the DC drive.
Figure 1
AC Drive Techniques
A reverse bias protective circuit is necessary for operating LEDs in an AC mode to prevent exceeding the maximum reverse voltage. Protective diode circuits are shown in Figure 2.
Figure 2
Logic Circuit Drive Techniques
Figure 3 illustrates different methods to drive LEDs using integrated circuits.
Figure 3
LEDIF
IF =E – VF
R
R
E LEDIF
VF
IF =E – n VF
R
RE
IF
IF =E – VF
R
R RE
(a) DC basic circuit (b) n pcs LED’s series connection
(c) LED’s parallel connection
IF
VF2
LED
IF =eac – (VF1 + VF2)
R
Reac
VF1
D1
IF
LED
D1
IF =eac – VF2
R
Reac
(a) Protective diode series connection
(b) Protective diode placed“back-to-back” connection
Whereeac ; AC supply voltage
D1 ; protective diode
VF1 ; forward voltage of D1
VF2 ; forward voltage of LED
IFLED
IF =VCC – (VCE(sat) + VF)
R
R
VCC
VCE(sat)
24KΩ
IF =VCC – VF
R
24KΩ
LED
R
VCC
(a) Lamp lights on “H” level (b) Lamp lights on “L” level
Application Notes
Allowable Pulse Forward Current of LED Lamps
3
Purdy Electronics Corporation • 720 Palomar Avenue • Sunnyvale, CA 94086
5/24/00
Tel: 408.523.8200 • Fax: 408.733.1287 • [email protected] •
www.purdyelectronics.com
Allowable Pulse Forward Current of LED Lamps
When a design requires an LED lamp to be operated in the pulse mode, allowable pulse forward current (I
FP
) is determined from the characteristic curves according to driving conditions (pulse width, duty ratio).
Applied Pulse
Figure 4
When ambient temperature rises above 25°C, I
FP
needs to be determined from Figure 5.
Figure 5
IFP
Pw
T
0
IFP : Allowable Pulse Forward Current
PW : Pulse Width
T : Refresh Rate
DR : Duty Ratio (= )PTW
1.0
0.8
0.6
0.4
0.2
0 20 40
Ambient Temperature Ta (°C)
Allo
wab
le P
ulse
For
war
d C
urre
nt I
FP
(Rel
ativ
e V
alue
)
Allowable Area
IFP = 1.0 atTa = 25°C
IFP – Ta
60 80 100
Characteristic Charts for Determining Allowable Pulse Forward Current
GaP Red LEDs
Figure 6 Figure 7
100
80
60
40
20
0
Duty Ratio DR
Allo
wab
le P
ulse
For
war
d C
urre
nt I
FP
(m
A)
10-110-210-3 1
Allowable Area
Ta = 25°C
Pw = 10µs
1ms
5ms
10ms20ms
For DC Forward CurrentIF Max = 25 mA
100
80
60
40
20
0
Duty Ratio DR
Allo
wab
le P
ulse
For
war
d C
urre
nt I
FP
(m
A)
10-110-210-3 1
Allowable Area
Ta = 25°C
Pw = 10µs
1ms
5ms10ms20ms
For DC Forward CurrentIF Max = 20 mA
4
Purdy Electronics Corporation • 720 Palomar Avenue • Sunnyvale, CA 94086
5/24/00
Tel: 408.523.8200 • Fax: 408.733.1287 • [email protected] •
www.purdyelectronics.com
Application Notes
Characteristic Charts for Determining Allowable Pulse Forward Current
GaAsP, GaAlAs, InGaAlP & GaP Green LEDs
Figure 8 Figure 9
GaAsP, GaAlAs, InGaAlP & GaP Green LEDs
Figure 10 Figure 11
200
160
120
80
40
0
Duty Ratio DR
Allo
wab
le P
ulse
For
war
d C
urre
nt I
FP
(m
A)
10-110-210-3 1
Allowable Area
Ta = 25°C
Pw = 1ms
5ms10ms
20ms
For DC Forward CurrentIF Max = 40 mA
200
160
120
80
40
0
Duty Ratio DR
Allo
wab
le P
ulse
For
war
d C
urre
nt I
FP
(m
A)
10-110-210-3 1
Allowable Area
Ta = 25°C
Pw = 10µs
1ms
5ms
10ms
20ms
For DC Forward CurrentIF Max = 50 mA
100
120
80
60
40
20
0
Duty Ratio DR
Allo
wab
le P
ulse
For
war
d C
urre
nt I
FP
(m
A)
10-110-210-3 1
Allowable Area
Ta = 25°C
Pw = 10µs
1ms
5ms
10ms
20ms100
120
80
60
40
20
0
Duty Ratio DR
Allo
wab
le P
ulse
For
war
d C
urre
nt I
FP
(m
A)
10-110-210-3 1
Allowable Area
Ta = 25°C
Pw = 10µs
1ms10ms
20ms 5ms
For DC Forward CurrentIF Max = 30 mA
For DC Forward CurrentIF Max = 25 mA
