Download LED application notes

Optoelectronics
Light Emitting Diodes
Application
Notes
720 Palomar Avenue
Sunnyvale, CA 94086
Tel: 408.523.8200 Fax: 408.733.1287
Email: [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.
R
VF
R
E
E
LED
IF
IF =
IF
E – VF
R
IF =
(a) DC basic circuit
R
E
LED
R
IF
E – n VF
R
IF =
(b) n pcs LED’s series
connection
E – VF
R
(c) LED’s parallel
connection
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.
R
eac
D1
LED
VF1
VF2
Where
eac ; AC supply voltage
LED
R
D1 ; protective diode
eac
D1
IF
VF1 ; forward voltage of D1
IF
VF2 ; forward voltage of LED
eac – (VF1 + VF2)
IF =
R
(a) Protective diode series
connection
eac – VF2
IF =
R
(b) Protective diode placed
“back-to-back” connection
Figure 2
Logic Circuit Drive Techniques
Figure 3 illustrates different methods to drive LEDs using integrated circuits.
VCC
IF
VCC
R
R
LED
24KΩ
24KΩ
VCE(sat)
IF =
VCC – (VCE(sat) + VF)
R
(a) Lamp lights on “H” level
LED
IF =
VCC – VF
R
(b) Lamp lights on “L” level
Figure 3
2
5/24/00
Purdy Electronics Corporation • 720 Palomar Avenue • Sunnyvale, CA 94086
Tel: 408.523.8200 • Fax: 408.733.1287 • [email protected] • www.purdyelectronics.com
Application Notes
Allowable Pulse Forward Current of LED Lamps
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 (IFP) is
determined from the characteristic curves according to
driving conditions (pulse width, duty ratio).
When ambient temperature rises above 25°C, IFP needs
to be determined from Figure 5.
IFP – Ta
Applied Pulse
IFP = 1.0 at
Ta = 25°C
Allowable Pulse Forward Current IFP
(Relative Value)
1.0
IFP
0
Pw
T
IFP : Allowable Pulse Forward Current
PW : Pulse Width
T
0.8
0.6
Allowable Area
0.4
0.2
: Refresh Rate
DR : Duty Ratio
0
(= PW )
T
20
40
60
80
100
Ambient Temperature Ta (°C)
Figure 4
Figure 5
Characteristic Charts for Determining Allowable Pulse Forward Current
GaP Red LEDs
Ta = 25°C
100
Pw = 10µs
1ms
10ms
20ms
80
5ms
60
Allowable Area
40
20
0
10 -3
Figure 6
3
For DC Forward Current
IF Max = 25 mA
10 -2
10 -1
Duty Ratio DR
1
Allowable Pulse Forward Current IFP (mA)
Allowable Pulse Forward Current IFP (mA)
For DC Forward Current
IF Max = 20 mA
Ta = 25°C
100
Pw = 10µs
10ms
20ms
1ms
80
5ms
60
Allowable Area
40
20
0
10 -3
10 -2
10 -1
Duty Ratio DR
1
Figure 7
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
For DC Forward Current
IF Max = 50 mA
Ta = 25°C
200
160
Pw = 1ms
10ms
120
5ms
20ms
Allowable Area
80
40
0
10 -3
10 -2
10 -1
Allowable Pulse Forward Current IFP (mA)
Allowable Pulse Forward Current IFP (mA)
For DC Forward Current
IF Max = 40 mA
Pw = 10µs
10ms
160
1ms
20ms
5ms
120
Allowable Area
80
40
0
10 -3
1
Ta = 25°C
200
10 -2
Duty Ratio DR
10 -1
1
Duty Ratio DR
Figure 8
Figure 9
GaAsP, GaAlAs, InGaAlP & GaP Green LEDs
For DC Forward Current
IF Max = 30 mA
For DC Forward Current
IF Max = 25 mA
Ta = 25°C
5ms
Ta = 25°C
120
120
Pw = 10µs
20ms
1ms
100
80
Allowable Area
60
40
20
0
10 -3
10 -2
10 -1
Pw = 10µs
Allowable Pulse Forward Current IFP (mA)
Allowable Pulse Forward Current IFP (mA)
10ms
1
10ms
100
4
5ms
80
Allowable Area
60
40
20
0
10 -3
Duty Ratio DR
Figure 10
1ms
20ms
10 -2
10 -1
1
Duty Ratio DR
Figure 11
5/24/00
Purdy Electronics Corporation • 720 Palomar Avenue • Sunnyvale, CA 94086
Tel: 408.523.8200 • Fax: 408.733.1287 • [email protected] • www.purdyelectronics.com