AL8808
COST EFFECTIVE LOW EMI 30V 1A BUCK LED DRIVER
Description
Pin Assignments
The AL8808 is a step-down DC/DC converter designed to drive LEDs
(Top View)
with a constant current. The device can drive up to 8 LEDs,
depending on the forward voltage of the LEDs, in series from a
SW 1
voltage source of 6V to 30V. Series connection of the LEDs provides
identical LED currents resulting in uniform brightness and eliminating
GND 2
NEW PRODUCT
the need for ballast resistors. The AL8808 switches at frequency up to
1MHz with controlled rise and fall times to reduce EMI. This allows
area needed.
TSOT25
Maximum output current of AL8808 is set via an external resistor
connected between the VIN and SET input pins. Dimming is achieved
by applying either an analog DC voltage or a PWM signal at the
CTRL input pin. An input voltage of 0.4V or lower at CTRL switches
Applications
•
MR16 Lamps
•
General Illumination Lamps
Features
•
LED Driving Current Up to 1A
•
Better Than 5% Accuracy
•
High Efficiency Up to 96%
•
Fast Controlled Falling Edges 7ns
•
Operating Input Voltage from 6V to 30V
•
High Switching Frequency Up to 1MHz
•
PWM/DC Input for Dimming Control
•
Built-In Output Open-Circuit Protection
•
Built-In Over-Temperature Protection
•
Notes:
4 SET
CTRL 3
the use of small size external components, hence minimizing the PCB
off the output MOSFET simplifying PWM dimming.
5 VIN
TSOT25: Available in “Green” Molding Compound (No Br, Sb)
Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2)
Halogen and Antimony Free. “Green” Device (Note 3)
1. No purposely added lead. Fully EU Directive 2002/95/EC (RoHS) & 2011/65/EU (RoHS 2) compliant.
2. See http://www.diodes.com for more information about Diodes Incorporated’s definitions of Halogen and Antimony free, "Green" and Lead-Free.
3. Halogen and Antimony free "Green” products are defined as those which contain <900ppm bromine, <900ppm chlorine (<1500ppm total Br + Cl)
and <1000ppm antimony compounds.
Typical Applications Circuit
AL8808
Document number: DS35648 Rev. 2 - 2
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AL8808
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Pin Descriiptions
Pin Number
1
2
Pin Name
SW
GND
3
CTRL
4
SET
5
VIN
Function
Switch Pin. Connect
C
inductorr/freewheeling dio
ode here, minimiizing track length
h at this pin to red
duce EMI.
GND Pin
Dimming and
d On/Off Control Input.
•
Leave floating
f
for norma
al operation.
(V
VCTRL = 5V, Give
es nominal average output curren
nt IOUTnom = 0.1/R
RS )
•
Drive to
o voltage below 0.4V
0
to turn off ou
utput current
•
Drive with an analog volltage (0.5V < VCTTRL < 2.5V) to ad
djust output curre
ent from 20% to 100% of
IOUTnom
m
•
Drive with an analog volltage > 2.6V outp
put current will be
e 100% of IOUTnoom
•
A PWM signal (low level ≤ 0.4V and high
h level > 2.6; tran
nsition times lesss than 1µs) allow
ws the output
b
the level se
et by the resistorr connected to SET input pin.
current to be adjusted below
Set Nominal Output Current Pin.
P Configure th
he output current of the device.
Input Supply Pin. Must be loccally decoupled to GND with > 2.2
2µF X7R ceramic capacitor – see
e applications
m
information.
section for more
Functiona
al Block Dia
agram
Figure 1. AL8808 Block
k Diagram
Absolute Maximum
M
Ratings (@TA = +25°C, unleess otherwise sppecified.)
Symbol
ESD HBM
ESD MM
VIN
Parameter
Human Body Model
M
ESD Prote
ection
Machine Mode
el ESD Protection
Continuous VIN pin voltage rela
ative to GND
Rating
gs
2.5
200
0
Unit
kV
V
-0.3 to +33
V
VSET
SET pin voltag
ge relative to VIN pin
-5 to +0.3
V
VSW
SW voltage re
elative to GND
-0.3 to +33
V
CTRL pin inpu
ut voltage
-0.3 to +6
V
VCTRL
ISW
Switch currentt
1.25
5
A
TJ
Junction Temp
perature
150
0
°C
TLEAD
Lead Tempera
ature Soldering
300
0
°C
TST
Storage Temp
perature Range
-55 to +150
+
°C
Caution:
Stresse
es greater than the
e 'Absolute Maxim
mum Ratings' specified above, may cause
c
permanent damage
d
to the devvice. These are sttress ratings only;
function
nal operation of the
e device at these or
o any other condittions exceeding tho
ose indicated in thiis specification is not
n implied. Device
e reliability may be
affected by exposure to absolute maximum rating conditions fo
or extended periodss of time.
Semico
onductor devices are
a ESD sensitive and
a may be damag
ged by exposure to
o ESD events. Suittable ESD precautions should be takken when handling
and tra
ansporting these de
evices.
AL8808
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AL8808
Recommended Operating Conditions (@TA = +25°C, unless otherwise specified.)
Symbol
Parameter
VIN
VCTRLH
VCTRLDC
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VCTRLL
Min
Max
Operating Input Voltage Relative to GND
6.0
30
V
Voltage High for PWM Dimming Relative to GND
2.6
5.5
V
Voltage Range for 20% to 100% DC Dimming Relative to GND
0.5
2.5
V
0
0.4
V
Voltage Low for PWM Dimming Relative to GND
Unit
ISW
Continuous Switch Current (Note 4)
1
A
fSW
Maximum Switching Frequency
1
MHz
+125
°C
TJ
Note:
Junction Temperature Range
-40
4. Subject to ambient temperature, input voltage and switching frequency. See applications section for suggested derating.
Electrical Characteristics (@ TA = +25°C, VIN = 12V, CTRL pin floating; unless otherwise specified.)
Symbol
Parameter
VINSU
Internal Regulator Start Up Voltage
VINSH
Conditions
Min
Typ
Max
Unit
VIN rising
5.6
V
Internal Regulator Hysteresis Threshold
VIN falling
200
mV
IQ
Quiescent Current
VIN = 30V, output not switching
350
µA
IS
Input Supply Current
fSW = 250kHz
450
VTH
VTH-H
ISET
Set Current Threshold Voltage
95
Set Threshold Hysteresis
SET Pin Input Current
VCTRL
Open Circuit CTRL Pin Voltage
RCTRL
CTRL Pin Input Resistance
VREF
Internal Reference Voltage
VSET = VIN -0.1
Referred to internal 5V regulator.
RDS(on)
On Resistance of SW MOSFET
ISW = 0.35A
ISW_Lkg
Switch Leakage Current
VIN = 30V, VCTRL = 0.4V, VSENSE = 0V
tR
SW Rise Time
tF
SW Fall Time
VSENSE = 100 ±20mV, fSW = 250kHz
VSW = 0.1V to 12V to 0.1V, CL = 15pF
100
µA
105
mV
±20
mV
16
µA
5
V
50
kΩ
2.5
V
0.35
Ω
0.5
µA
7
ns
5
ns
TOTP
Over-Temperature Shutdown
145
°C
TOTP-Hyst
Over-Temperature Hysteresis
10
°C
θJA
Thermal Resistance Junction-to-Ambient
TSOT25 (Note 5)
209
θJL
Thermal Resistance Junction-to-Lead
TSOT25 (Note 5)
57
θJT
Thermal Resistance Junction-to-Top
TSOT25 (Note 5)
13
Notes:
°C/W
5. Device mounted on FR-4 PCB (25mm x 25mm 1oz copper, minimum recommended pad layout on top layer and thermal vias to maximum area
bottom layer ground plane. For better thermal performance, larger copper pad for heat-sink is needed.
…Refer to Figure 42 for the device derating curve.
AL8808
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AL8808
900
350
800
250
200
150
400
L = 68µH
300
200
3
100
9 12 15 18 21 24 27 30
INPUT VOLTAGE (V)
Figure 2. Supply Current vs. Input Voltage
RSET = 100mΩ
TA = +25°C, VIN = 12V
L = 68µH, RSET = 150mΩ
1 LED
0
CTRL PIN CURRENT (µA)
LED CURRENT (A)
20
TA = +25°C
VIN = 12V
L = 68µH
1 LED
0.8
RSET = 150mΩ
0.6
0.4
RSET = 300mΩ
0.2
-20
-40
-60
-80
-100
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
VCTRL (V)
Figure 4. LED Current vs. VCTRL
-120
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
VCTRL (V)
Figure 5. ICTRL vs. VCTRL
0.9
6
100
TA = +25°C, VIN = 12V
90
L = 68µH, RSET = 150mΩ
1 LED, fPWM = 500Hz
80
3
0.3
2
LED CURRENT (A)
0.6
4
DUTY CYCLE (%)
5
L = 100µH
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
VCTRL (V)
Figure 3. Switching Frequency vs. VCTRL
6
1.2
1.0
RSET = 150mΩ
1 LED
500
VCTRL = 0V
VSET = VIN
TA = +25°C
0
70
60
50
40
30
20
1
TA = +25°C, RSET = 150mΩ
L = 33µH, L = 68µH, L = 100µH
10
0
TA = +25°C
VIN = 12V
600
100
0
L = 33µH
700
300
FREQUENCY (kHz)
INPUT CURRENT (µA)
400
50
LED CURRENT ERROR (%)
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Typical Performance Characteristics (@ TA = +25°C, unless otherwise stated.)
0
20
40
60
80
PWM DUTY CYCLE (%)
Figure 6. ILED vs. PWM Duty Cycle
AL8808
Document number: DS35648 Rev. 2 - 2
0.0
100
0
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6
9
12 15 18 21 24 27
INPUT VOLTAGE VCTRL (V)
Figure 7. Duty Cycle vs. Input Voltage
30
January 2013
© Diodes Incorporated
AL8808
Typical Pe
erformance
e Characte
eristics (contt.) (@ TA = +25°CC, unless otherwwise stated.)
400
60
00
SW MOSFET RDS(ON) (mΩ)
SW MOSFET RDS(ON) (mΩ)
350
40
00
30
00
20
00
10
00
300
250
200
150
100
50
0
-40 -25 -10 5 20 35 50 65
5 80 95 110 125
TURE (°C)
AMBIENT TEMPERAT
Figure 8. SW RDS(ON) vs. Temperature
0
6
VCTRL = Open
VSET = VIN
T A = 25°°C
9
12
15
18
8
21 24
27
VIN (V)
age
Figure 9. SW RDS(O
ON) vs. Input Volta
30
0.50
0
VIN = 12V
VSET = VIN
TA = +25°C
CTRL = Floating
0.45
5
0.40
0
SW MOSFET RDS(ON) (Ω)
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50
00
VCTRL = Open
VSET = VIN = 12
2V
0.35
5
0.30
0
0.25
5
0.20
0
0.15
5
0.10
0
0.05
5
0.00
0
0
200
400
600
800
1000
SW
WITCH CURRENT
T (mA)
Figure 10. SW RDS(ON) vs. Switch
S
Current
Figure 12. Steady
S
State Wa
aveforms
AL8808
Document numberr: DS35648 Rev. 2 - 2
Figu
ure 11. SW Outp
put Fall Time
Figurre 13. SW Output Rise Time
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AL8808
Typical Performance Characteristics
100
4 LEDs 5 LEDs
(L = 68µH, TA = +25°C, unless otherwise stated.)
0.36
7 LEDs
3 LEDs
LED CURRENT (A)
EFFICIENCY (%)
0.35
2 LEDs
90
1 LED
85
80
TA = 25°C
L= 68µH
RSET = 150mΩ
75
70
6
9
12
15
18 21 24 27
INPUT VOLTAGE (V)
Figure 14. Efficiency vs. Input Voltage
0.34
2 LEDs
0.33
3 LEDs
4 LEDs
5 LEDs
7 LEDs
0.32
0.30
30
TA = 25°C
L= 68µH
RSET = 300mΩ
6
9
12
15
18 21 24 27 30
INPUT VOLTAGE (V)
Figure 15. 330mA LED Current vs. Input Voltage
0.74
400
0.72
350
LED CURRENT (A)
SWITCHING FREQUENCY (kHz)
1 LED
0.31
450
300
250
200
1 LED
150
0.70
5 LEDs
2 LEDs 3 LEDs
4 LEDs
7 LEDs
0.68
0.66
1 LED
0.64
100
0.62
50
0
2 LEDs 3 LEDs 4 LEDs
6
9
5 LEDs
0.60
12
100
4 LEDs
2 LEDs
90
5 LEDs
L= 68µH, RSET = 150m Ω
6
9
15
18 21 24 27 30
INPUT VOLTAGE (V)
Figure 17. 670mA LED Current vs. Input Voltage
3 LEDs
7 LEDs
3 LEDs
2 LEDs
LED CURRENT (A)
1.05
70
12
1.10
7 LEDs
80
1 LED
60
50
40
30
4 LEDs
1.00
5 LEDs
1 LED
0.95
20
TA = 25°C
L= 68µH
RSET = 100mΩ
TA = 25°C
L= 68µH
RSET = 150mΩ
10
0
T A = 25°C, VIN = 12V
7 LEDs
15
18 21 24 27 30
INPUT VOLTAGE (V)
Figure 16. Switching Frequency vs. Input Voltage
DUTY CYCLE (%)
NEW PRODUCT
95
6
9
12
15
18 21 24 27 30
INPUT VOLTAGE (V)
Figure 18. Duty Cycle vs. Input Voltage
AL8808
Document number: DS35648 Rev. 2 - 2
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0.90
6
9
12
15
18 21 24 27 30
INPUT VOLTAGE (V)
Figure 19. 1A LED Current vs. Input Voltage
January 2013
© Diodes Incorporated
AL8808
Typical Performance Characteristics (670mA LED Current; TA = +25°C unless otherwise stated.)
800
3 LEDs
4 LEDs
3
1
1 LED
-1
7 LEDs
-3
TA = 25°C,
L = 33µH
RSET = 150mΩ
700
600
500
400
1 LED
300
200
100
0
-5
6
DEVIATION FROM TARGET (%)
4
3 LEDs
9
12
4 LEDs
2
1
0
1 LED
-1
-2
-3
TA = 25°C,
L = 68µH
RSET = 150mΩ
-4
-5
6
9
0
6
9
12
9
12
350
300
250
200
150
100
50
15 18 21 24 27
30
INPUT VOLTAGE (V)
Figure 23. Switching Frequency vs. Input Voltage
15 18 21 24 27
30
INPUT VOLTAGE (V)
Figure 22. LED Current Deviation vs. Input Voltage
12
5
250
2 LEDs
3
3 LEDs
4 LEDs 5 LEDs
SWITCHING FREQUENCY (kHz)
4
7 LEDs
2
1
0
1 LED
-1
-2
-3
TA = 25°C,
L = 100µH
RSET = 150mΩ
-4
-5
15 18 21 24
27
30
INPUT VOLTAGE (V)
Figure 21. Switching Frequency vs. Input Voltage
400
7 LEDs
3
7 LEDs
450
5 LEDs
SWITCHING FREQUENCY (kHz)
2 LEDs
2 LEDs 3 LEDs 4 LEDs 5 LEDs
6
9
5
TA = 25°C,
L = 33µH
RSET = 150mΩ
5 LEDs
SWITCHING FREQUENCY (kHz)
DEVIATION FROM TARGET CURRENT (%)
2 LEDs
12 15 18 21 24
27
30
INPUT VOLTAGE (V)
Figure 20. LED Current Deviation vs. Input Voltage
DEVIATION FROM TARGET (%)
NEW PRODUCT
5
6
9
12
15 18 21 24 27
30
INPUT VOLTAGE (V)
Figure 24. LED Current Deviation vs. Input Voltage
AL8808
Document number: DS35648 Rev. 2 - 2
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200
150
100
50
0
6
15 18 21 24 27
30
INPUT VOLTAGE (V)
Figure 25. Switching Frequency vs. Input Voltage
January 2013
© Diodes Incorporated
AL8808
2 LEDs
4
600
3 LEDs 4 LEDs
5 LEDs
3
2
1
1 LED
0
-1
-2
-3
TA = 25°C
L = 33µH
RSET = 100mΩ
-4
-5
9
5
4
1
1 LED
-1
-2
-3
TA = 25°C
L = 68µH
RSET = 100mΩ
-4
-5
9
15
18 21
24 27
30
INPUT VOLTAGE (V)
Figure 28. LED Current Deviation vs. Input Voltage
2 LEDs
4
3 LEDs
4 LEDs
1 LED
-1
-2
-5
TA = 25°C
L = 100µH
RSET = 100mΩ
6
9
12
6
9
12
100
50
160
140
120
100
1 LED
80
60
40
20
15
18 21
24 27
30
INPUT VOLTAGE (V)
Figure 30. LED Current Deviation vs. Input Voltage
Document number: DS35648 Rev. 2 - 2
150
15
18 21
24 27
30
INPUT VOLTAGE (V)
Figure 29. Switching Frequency vs. Input Voltage
12
AL8808
200
180
1
-4
9
7 LEDs
7 LEDs
2
-3
6
5 LEDs
200
5 LEDs
3
0
2 LEDs 3 LEDs 4 LEDs
250
0
12
5
100
15
18 21
24 27
30
INPUT VOLTAGE (V)
Figure 27. Switching Frequency vs. Input Voltage
SWITCHING FREQUENCY (kHz)
6
1 LED
200
7 LEDs
2
0
300
5 LEDs
2 LEDs
3
400
300
4 LEDs
3 LEDs
500
0
12
SWITCHING FREQUENCY (kHz)
DEVIATION FROM TARGET CURRENT (%)
6
TA = 25°C
L = 33µH
RSET = 100mΩ
7 LEDs
SWITCHING FREQUENCY (kHz)
DEVIATION FROM TARGET CURRENT (%)
5
15
18 21
24 27
30
INPUT VOLTAGE (V)
Figure 26. LED Current Deviation vs. Input Voltage
DEVIATION FROM TARGET CURRENT (%)
NEW PRODUCT
Typical Performance Characteristics (1A LED Current) TA = +25°C unless otherwise stated.)
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0
6
9
12
15
18 21
24 27
30
INPUT VOLTAGE (V)
Figure 31. Switching Frequency vs. Input Voltage
January 2013
© Diodes Incorporated
AL8808
Applicatio
on Informattion
ation
AL8808 Opera
In normal opera
ation, when volta
age is applied at +VIN, the AL880
08 internal switcch is turned on. Current starts to
o flow through se
ense resistor R1,
inductor L1, and
d the LEDs. The
e current ramps up linearly, and
d the ramp rate is determined by
b the input volta
age (+VIN) minu
us the LED chain
n
voltage and the inductor L1.
ent produces a vo
oltage ramp acro
oss R1. The interrnal circuit of the AL8808 sensess the voltage acro
oss R1 and appliies a proportiona
al
This rising curre
voltage to the input of the interna
al comparator.
NEW PRODUCT
When this voltag
ge reaches an in
nternally set upp
per threshold, the
e internal switch is turned off. Th
he inductor curre
ent continues to flow through R1,
L1, the LEDs an
nd the schottky diode
d
D1, and ba
ack to the supplyy rail, but it decayys, with the rate of decay determ
mined by the forw
ward voltage drop
p
of the LEDs and
d the schottky dio
ode.
This decaying current produces a falling voltage
e at R1, which is sensed by the AL8808.
A
A voltag
ge proportional to
o the sense voltage across R1 iss
nput of the internal comparator. When
W
this voltage falls to the inte
ernally set lowerr threshold, the in
nternal switch is turned on again
n.
applied at the in
This switch-on-a
and-off cycle continues to provide
e the average LE
ED current set byy the sense resisttor R1.
Figure 32. Typical Applica
ation Circuit
LED Current Control
C
The LED curren
nt is controlled by
b the resistor R1 (in Figure 32) connected betw
ween VIN and SE
ET pins. The AL8
8808 has an inte
ernal 50k resisto
or
connected from the CTRL pin to
o an internal 5V regulator.
r
When the CTRL pin iss left floating it ge
ets pulled up to 5V
5 - increasing its
i noise rejection
oating. If the CTRL pin is left floa
ating or driven ab
bove 2.5V the no
ominal average output current in the
t LED(s) is deffined as:
with CTRL left flo
ILED =
VTH
R1
Where VTH is nominallyy 100mV.
i driven by an external voltage (higher than 0.5V
V and lower than 2.5V), the avera
age LED current is:
If the CTRL pin is
ILED
=
L
VCTRL VTH
T
VREF R1
Where
e VREF is nomina
ally 2.5V
c
of 660mA
A and VCTRL=2.5V
V or with the CTR
RL pin left open the resulting resistor is:
For example for a desired LED current
R SET =
VTH
0.1
=
≈ 150mΩ
ILED 0.66
L voltage is broug
ght below 0.4V, the output switch
h is turned off which allows PWM dimming.
When the CTRL
AL8808
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AL8808
Application Information (cont.)
Analog Dimming
The CTRL pin can be driven by an external analog voltage (VCTRL), to adjust the output current to a value below the nominal average value
defined by R1. The LED current decreases linearly with the CTRL voltage when 0.5V ≤ VCTRL ≤ 2.5V.
Note that 100% brightness is achieved when either the CTRL pin is left floating or pulled above 2.5V by an external voltage source.
For 2.6V ≤ VCTRL ≤ 5.5V the LED current will not get overdriven and will be set the current according to the equation VCTRL = 2.5V (the internal
2.2
DIMMED : 100% LED CURRENT RATIO (%)
110
100
90
TA = 25°C
VIN = 12V
L = 68µH
2.0
Dimmed LED Current Ratio
1.8
80
1.6
70
1.4
60
1.2
50
1.0
40
0.8
30
0.6
20
TYPICAL ERROR (%)
NEW PRODUCT
reference voltage). See Figure 33 below.
0.4
LED Current Error
0.2
10
0.0
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
VCTRL (V)
Figure 33. LED Current Dimming Ration and
Typical Error vs. Control Voltage
PWM Dimming
LED current can be adjusted digitally, by applying a low frequency Pulse Width Modulated (PWM) logic signal to the CTRL pin to turn the device
on and off. This will produce an average output current proportional to the duty cycle of the control signal. In particular, a PWM signal with a
max resolution of 10bit (~0.1% duty cycle) can be applied to the CTRL pin to change the output current to a value below the nominal average
value set by resistor RSET. To achieve this resolution the PWM frequency has to be lower than 500Hz, however higher dimming frequencies can
be used, at the expense of dimming dynamic range and accuracy.
Typically, for a PWM frequency of 500Hz the accuracy is better than 2% for PWM ranging from 5% to 100%.
0.8
10
0.7
LED CURRENT (A)
0.6
0.5
0.4
0.3
0.2
0.1
0
VIN = 12V
T A = +25°C
L = 68µH
RSET = 150mΩ
1 LED
9
LOAD CURRENT ERROR (%)
VIN = 12V
TA = +25°C
L = 68µH
RSET = 150mΩ
1 LED
8
7
6
5
4
3
2
1
0
10
10
0.0
5.0
7.5
10.0 12.5
15.0
PWM DUTY CYCLE (%)
Figure 35. Low Duty Cycle PWM Dimming at 500Hz
20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE (%)
Figure 34. PWM Dimming at 500Hz
AL8808
Document number: DS35648 Rev. 2 - 2
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January 2013
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AL8808
Applicatio
on Informattion (cont.)
ng (cont.)
PWM Dimmin
The CTRL pin iss designed to be
e driven by both 3.3V and 5V log
gic levels directlyy from a logic output with either an
a open drain ou
utput or push pull
output stage.
WM dimming reso
olution is determined by the number of full LED switching
s
cycles that
t
can be achie
eved during the PWM on-time. A
At
The ultimate PW
lower switching frequencies and/or higher PWM frequencies the number of full sw
witching cycles that
t
can be achie
eved is reduced thereby reducing
g
NEW PRODUCT
earity of the PWM
M dimming.
the accuracy/line
e
PWM cycle
e the LED curre
ent needs to resstart from zero up
u to the upperr threshold level (nominally 120mV/RSET). If thiss
At the start of each
threshold isn’t re
eached then the accuracy will be greatly affected..
Greater PWM dimming
d
dynamic ranges can be
e achieved by reducing
r
the PW
WM dimming frequency and/or increasing the AL8808
A
switching
g
frequency.
P
pulse resolution with differe
ent PWM frequen
ncies and different inductor value
es driving 2 LEDss
The three figures below show 0.2% duty cycle PWM
a +25°C for a no
ominal LED curre
ent of 670mA.
from a 12V rail at
Figure 36. 0.2% PWM Duty Cycle at 100Hz PWM Frequenc
cy and
H Inductance
68µH
gure 37. 0.2% PWM Duty Cycle
e at 100Hz PWM Frequency and
d
Fig
22µH Indu
uctance
Figure 38. 0.2% PWM Duty Cycle at 500Hz PWM Frequenc
cy and
H Inductance
22µH
As can be observed from Figure
e 37 greater dim
mming accuracy can
c be achieved
d by reducing both the PWM dim
mming frequency and the inducto
or
value.
AL8808
Document numberr: DS35648 Rev. 2 - 2
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AL8808
Applicatio
on Informattion (cont.)
Start-up and Soft
S
Start
On initial power up the device will
w not start switcching until the power supply hass reached approxximately 5.6V orr the CTRL pin voltage
v
is greate
er
s a slight delay (dependent
(
on ra
amp rate of input voltage and inp
put bulk capacita
ance of the AL88
808 circuit). Once
e
than 0.45V (typical). This causes
ED current will build up to the upp
per threshold levvel:
the output startss switching the LE
ILEDS
SSPK =
VCTRL 0.1×
1 1 .2
2.5 V
R1
NEW PRODUCT
This will cause some
s
additional input current to th
hat of charging th
he input bulk cap
pacitance. One way
w of reducing this
t
additional cu
urrent is to reduce
e
the upper LED current
c
threshold level by slowing
g down the rise of the CTRL pin voltage
v
– implementing a soft-start.
The AL8808 doe
es not have in-bu
uilt soft-start actiion allowing veryy fast control of the
t output Powerr MOSFET switcch which improve
es PWM dimming
g
accuracy.
Soft start can be
e easily impleme
ented by adding an
a external capa
acitor from the CTRL
C
pin to groun
nd. The internal pull-up resistor to
t the internal 5V
V
rail on the CTRL
L pin will charge the
t external capa
acitor up to 5V.
The external cap
pacitor slows up the ramp-up of the CTRL pin vo
oltage thereby re
educing the LED current via anallog dimming. To ensure soft-starrt
occurs it is essential that the cap
pacitor is large en
nough to keep th
he CTRL pin volta
age below 2.5V during
d
the ramp--up of the input voltage.
v
This is achieved
d by increasing th
he time taken forr the CTRL volta
age to rise to the upper (turn-off) threshold and byy slowing down the
t rate of rise o
of
the control voltage at the input off the comparatorr.
acitor increases the time taken for
f the output to
o reach 90% of its final value, th
his delay is 25µss/nF, but will imp
pact on the PWM
M
Adding this capa
dimming accuracy depending on
n the delay introd
duced.
Figure 39. Soft Start with
w
100nF Cap
pacitor on CTRL
L pin (VIN = 24V, ILED = 667mA, 1 LED)
Reducing Outtput Ripple
Peak to peak rip
pple current in the
e LED(s) can be reduced, if required, by shunting
g a capacitor C2 across the LED((s) as shown in Figure
F
32.
A value of 1μF will reduce the supply ripple current significan
ntly in the typica
al case. Proportio
onally lower ripp
ple can be achie
eved with highe
er
capacitor valuess.
apacitor will not affect operating
g frequency or efficiency,
e
but it will
w increase sta
art-up delay, by reducing the ratte of rise of LED
D
Note that the ca
voltage. By adding this capacito
or the current wa
aveform through the LED(s) chan
nges from a trian
ngular ramp to a more sinusoida
al version withou
ut
an current value.
altering the mea
AL8808
Document numberr: DS35648 Rev. 2 - 2
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AL8808
Applicatio
on Informattion (cont.)
Inductor Sele
ection
Recommended inductor values for the AL8808 are in the range
e 33µH to 100µH
H. Note that the
e AL8808 Web Calculator
C
provides performance
ed component va
alues. The inducctance used will depend on a combination
c
of In
nput voltage and
d LED chain voltage to set the
data for selecte
required switching frequency. Lower
L
inductor values can be ussed to increase the switching freq
quency and redu
uce solution size
e but may affect
NEW PRODUCT
opagation delayss) and increase power
p
dissipation
n (due to switchin
ng losses).
LED current acccuracy (due to pro
Figure 40. Inductor Valu
ue with Input Volttage and Numbe
er of LEDs
ould be mounted
d as close to the device
d
as possib
ble with low resisstance/stray inducctance connectio
ons to the SW pin.
The inductor sho
The chosen coil should have a saturation
s
currentt higher than the peak output currrent and a contin
nuous current ratting above the
o
current.
required mean output
Suitable coils forr use with the AL
L8807 are listed in
i the table below
w:
L
D
DCR
ISATT
Part No.
(
(V)
(µH)
(A))
MSS1038-333
33
0.093
2.3
3
MSS1038-683
68
0.213
1.5
5
NPIS64D330M
MTRF
33
0.124
1.1
1
Manufacturer
oilCraft www.coilcraft.com
Co
NIC www.niccom
mp.com
osen to maintain
n operating duty cycle
c
and switch 'on'/'off' times ovver the supply vo
oltage and load current
c
range.
The inductor value should be cho
quations can be used
u
as a guide, with reference to Figure 1 - Ope
erating waveform
ms.
The following eq
Switch ‘On’ tim
me:
tON =
LΔI
−
−
VIN VLED IAVG x (RS + rL + RSW
W)
Switch ‘Off’ tim
me:
tOFF =
L ΔI
+
+
VLED
VD IAVG
L
G x (RS + rL )
Where:
L is the coil inductance (H)
rL is the coil resistance (Ω)
stance (Ω)
RS is the currrent sense resis
Iavg is the required LED curre
ent (A)
VIN is the su
upply voltage (V)
ΔI is the coill peak-peak ripple current (A) {Intternally set to 0.4
4 x IAVG}
VLED is the total
t
LED forward
d voltage (V)
RSW is the switch
s
resistance (Ω) {=0.35Ω nom
minal}
VD is the dio
ode forward volta
age at the require
ed load current (V
V)
AL8808
Document numberr: DS35648 Rev. 2 - 2
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Figure
e 41. Typical Swiitching Waveform
m
January 2013
© Diodes Incorporate
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AL8808
Application Information (cont.)
Capacitor Selection
The small size of ceramic capacitors makes them ideal for AL8808 applications. X7R type is recommended because it retains capacitance value
over wider voltage and temperature ranges than other types such as Y5V or Z5U. X5R is a useful compromise over a restricted temperature
range. Note that even X7R capacitance reduces significantly with increased DC bias voltage. At 50% of rated voltage, the capacitance loss is
between about 10% to 50% of nominal. Therefore it is often necessary to select a voltage rating which is at least twice the operating voltage.
NEW PRODUCT
Input Capacitor
In Figure 32, the input capacitor C1 is required as a reservoir. Diode D1 switches at a rate of typically up to 400kHz. The power supply has a
finite impedance, often including a wiring inductance value of the order of 100nH to 1uH or more depending upon the system design. C1 is
required to limit the power supply voltage and current ripple both to allow stable regulation of the LED current, and also to meet EMC
requirements.
A 2.2μF input capacitor is sufficient for most DC powered applications of AL8808. This depends upon the operating voltage and current and the
maximum level of ripple required. Additional capacitors may be required in parallel for EMC purposes. This is described below in a separate
section.
However, if operated from a rectified low voltage AC source, such as MR16, then the input capacitance will need to be significantly increased to
provide enough reservoir charge when the input voltage falls below the minimum operating voltage of the AL8808 or the LED chain voltage
Output Capacitor
In Figure 32, the output capacitor C2 is normally required to limit the load voltage and current ripple, in order to meet EMC requirements. A value
of 0.1µF to 1µF is sufficient for many requirements, depending on voltage and current conditions. Additional capacitors may be required in parallel
for EMC purposes. This is described below in a separate section.
Diode Selection
For maximum efficiency and performance, the flywheel rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at
the maximum operating voltage and temperature. The silicon PN diode is not suitable because of its increased power loss, due to a combination
of lower forward voltage and reduced recovery time. The use of a Super-Barrier-Rectifier (SBR) is not recommended for use as a flywheel diode
in this application. (However the SBR provides significant advantages when used with an AC power input as a bridge rectifier driving VIN.)
It is important to select D1 with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output
load current. In particular, it is recommended to have a diode voltage rating at least 15% higher than VIN to ensure
safe operation during the switching and a mean current rating at least 10% higher than the peak diode current. The power rating is verified by
calculating the power loss through the diode. In practice, the voltage rating selection is often increased by up to about 50% to obtain a better
compromise with loss due to reverse leakage current at higher temperature. Also the current rating is typically selected to provide a margin of up
to about 50%.
Schottky diodes, e.g. DFLS240L or DFLS140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8808
applications. Leakage current is sufficiently limited for the application.
AL8808
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© Diodes Incorporated
AL8808
Application Information (cont.)
Thermal Considerations
For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The maximum power
dissipation depends on several factors: the thermal resistance of the IC package (θJC), PCB layout, airflow surrounding the IC, and difference
between junction and ambient temperature.
The maximum power dissipation can be calculated using the following formula:
TJ(MAX) − TA
θ JA
where
TJ(MAX) is the maximum operating junction temperature; for the AL8808 this is +125°C.
TA is the ambient temperature, and
θJA is the junction to ambient thermal resistance.
The major thermal path for the TSOT25 package is pin 2 (GND pin) and it is important for minimizing the θJA that a suitable area and thermal
mass is associated with pin 2. The thermal impedance from the AL8808 junction to pin 2 is approximately 57°C/W.
The AL8808’s θJA on a 25 x 25mm double sided FR4 PCB with minimum recommended pad layout on top layer and thermal vias to maximum
area on bottom layer with 1oz copper standing in still air is approximately 209°C/W. Yielding a maximum power dissipation at 25°C of 0.47W
The AL8808’s θJA on a 25 x 25mm double sided FR4 PCB with maximum area top and bottom with vias is approximately 151°C/W; which gives a
maximum power dissipation at 25°C of 0. 66W.
Figure 42 shows the power derating of the AL8808 on different area PCB with maximum area on bottom of PCB with 1 and 2oz copper standing in
still air.
1.0
2
0.9
(50mm) with 2oz Cu max area top
2
(50mm) with 1oz Cu max area top
0.8
POWER DISSIPATION (W)
NEW PRODUCT
PD(MAX) =
2
(25mm) with 2oz Cu max area top
2
0.7
(25mm) with 1oz Cu min area top
0.6
(25mm) with 2oz Cu min area top
2
0.5
2
(50mm) with 1oz Cu min area top
0.4
2
(25mm) with 1oz Cu min area top
0.3
0.2
0.1
0
-40
-25
-10
5
20
35
50
65
80
AMBIENT TEMPERATURE (C)
95
110
125
Figure 42. Derating Curve for Different PCB
AL8808
Document number: DS35648 Rev. 2 - 2
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AL8808
Applicatio
on Informattion (cont.)
out Considerattions
EMI and Layo
The AL8808 is a switching regulator with fast edges and mea
asures small diffferential voltagess; as a result off this care has to be taken with
h
CB.To help with these effects the
e AL8808 has been developed to
t minimise radia
ated emissions by
b controlling the
e
decoupling and layout of the PC
ds of the internal power MOSFET
T. The rise and
d fall times are controlled
c
to get the right compro
omise between power
p
dissipation
n
switching speed
due to switching
g losses and radia
ated EMI.
NEW PRODUCT
ge (falling edge) dominates the ra
adiated EMI which is due to an interaction
i
betwe
een the Schottkyy diode (D1), Sw
witching MOSFET
T
The turn-on edg
and PCB tracks. After the Scho
ottky diode reversse recovery time of around 5ns has
h occurred; the
e falling edge of the SW pin seess a resonant loop
p
between the Sch
hottky diode capa
acitance and the
e track inductance, LTRACK, See Figure
F
43.
t Anode of the
e Schottky diode
e, D1, and then from
f
D1’s cathod
de to the decoup
pling capacitors C1 should be ass
The tracks from the SW pin to the
e.
short as possible
There is an indu
uctance internally
y in the AL8808 this
t
can be assumed to be aroun
nd 1nH. For PCB
B tracks a figure of 0.5nH per mm
m can be used to
o
estimate the prim
mary resonant frequency. If the track is capable of
o handling 1A in
ncreasing the thicckness will have
e a minor effect on
o the inductance
e
and length will dominate
d
the size
e of the inductancce.
equency of any oscillation is dettermined by the combined inducctance in the track and the effecctive capacitance
e of the Schottkyy
The resonant fre
diode. An exam
mple of good layout is shown in Figure 44 - the stray track inductan
nce should be less than 5nH.
Figure 44. Recommen
nded PCB Layout
Figure 43. PC
CB Loop Resonance
Summary:
1.
on with copper fo
oil on top and bo
ottom. Provide maximum
m
coverag
ge of copper ground plane on bo
oth sides. Ensure
e
Use a PCB constructio
ed together using
g plated via hole
es placed at regular intervals. This
T
is required both for low EM
MI
the grround areas are tightly connecte
2.
(EMC) operation and also
a
to minimize device temperattures by spreadin
ng the dissipated
d heat.
e of D1. The sep
paration of these nodes should be
e
Place capacitor C1 as close as possible to VIN, and as close as possible to the cathode
less th
han about 5mm. To ensure the best
b
possible EM
MI filtering (greate
est attenuation), place the capaciitor and its coppe
er trace such tha
at
the input current pass
ses directly throu
ugh the capacitor mounting pad. This minimizess common imped
dance coupling due
d to the added
d
nductance conne
ection between th
he capacitor and
d its ground conn
nection. Use 2 o
or
parasiitic inductance off the copper tracce. Ensure low in
3.
more ground
g
via holes
s close to the gro
ound pad.
Place sense resistor R1
R as close as po
ossible to VIN and
d SET.
4.
Place D1 anode, the SW
S pin and the in
nductor as close together as posssible to avoid ringing.
5.
s close as possible to L1 and SET.
S
To ensure
e the best possib
ble EMI filtering (greatest attenu
uation), place the
e
Place capacitor C2 as
per trace such tha
at the input curre
ent passes direcctly through the capacitor
c
mountin
ng pad. This minimizes common
n
capaccitor and its copp
imped
dance coupling due to the added parasitic inducta
ance of the coppe
er trace.
AL8808
Document numberr: DS35648 Rev. 2 - 2
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January 2013
© Diodes Incorporate
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AL8808
Application Information (cont.)
EMI and Layout Considerations (cont.)
EMC Design
In addition to the layout instructions above, it may be necessary to take further measures to reduce electromagnetic interference (EMI) and meet
EMC requirements. This depends on the speed of the switching transitions. The fast switching edges include spectral harmonics spreading into
the UHF frequency range towards 500MHz. In this respect, AL8808 has been optimized to shape the switching current waveform to minimize EMI
NEW PRODUCT
while maintaining fast enough switching for high power efficiency. However, depending on the physical system design it may be necessary to add
additional filtering to reduce radiated and conducted emissions. The required circuit changes depend on a number of system design aspects
including the PCB size, the housing design and the length of external connecting wires.
Radiated Emission
Typically, the filtering required to control radiated emission consists of one or two additional capacitors placed close to the connecting points of the
wires. Very often the frequency range requiring most attenuation is in the region of 100MHz to 500MHz. In order to provide best attenuation in
this frequency range, use a capacitor of 1000pF to 2200pF with COG dielectric type, rated 50V or 100V. This capacitor provides very low ESR in
this frequency range. Place two such capacitors, one near the VIN wire connection and one near the output connection to L1. Again, to ensure
the best possible EMI filtering (greatest attenuation), place the capacitor and its copper trace such that the input or output current passes directly
through the capacitor mounting pad. This minimizes common impedance coupling due to the added parasitic inductance of the copper trace.
Conducted Emission
Conducted emission limits sometimes require filtering in the lower frequency range, from the switching frequency itself (Typically 200kHz) up to
about 30MHz. Usually the requirement only applies on the input side. The existing power supply may already include suitable measures. If
necessary add an input capacitor to reduce the ripple in this frequency range. Again the capacitors and their copper traces should be carefully
placed to avoid inductive common impedance coupling. Sometimes an additional series filter inductor may be added to achieve the desired
attenuation. An additional shunt capacitor to ground is connected resulting in a pi-filter configuration.
AL8808
Document number: DS35648 Rev. 2 - 2
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January 2013
© Diodes Incorporated
AL8808
Applicatio
on Informattion (cont.)
Fault Conditio
on Operation
The AL8808 hass by default open
n LED protection
n. If the LEDs sh
hould become op
pen circuit the AL
L8808 will stop oscillating;
o
the SE
ET pin will rise to
o
VIN and the SW pin will then fall to GND. No exce
essive voltages will
w be seen by th
he AL8808.
If the LEDs shou
uld become shorrted together the AL8808 will con
ntinue to switch and
a the current through the AL88
808’s internal sw
witch will still be a
at
NEW PRODUCT
the expected cu
urrent - so no excessive
e
heat will
w be generated
d within the AL8
8808. However, the duty cycle at which it operrates will change
e
dramatically and
d the switching frequency will mo
ost likely decrease. See Figure 45
4 for an examp
ple of this behavior at 24V input voltage driving 3
LEDs.
t internal power MOSFET swiitch is significanttly reduced beca
ause almost all of
o the input volta
age is now deve
eloped across the
e
The on-time of the
inductor. The offf-time is significa
antly increased because the reverse voltage acrosss the inductor iss now just the Scchottky diode volltage (See Figure
e
32) causing a much slower deca
ay in inductor currrent.
Figure 45. Sw
witching Charac
cteristics (norma
al open to shortt LED chain)
High Tempera
ature Operatio
on and Protection
The AL8808 is a high efficiency switching LED driver capable of operating junctio
on temperatures up to +125°C. This allows it operate with ambien
nt
temperature in excess of 100°C given the co
orrect thermal im
mpedance to fre
ee air. If a fault should occur that
t
leads to inccreased ambien
nt
nd hence junctio
on temperature then
t
the Over-Te
emperature Prottection (OTP) off the AL8808 willl cut in turning the
t
output of the
e
temperatures an
AL8808 off. Thiss will allow the junction temperatu
ure of the AL8808 to cool down and
a potentially givving an opportun
nity for the fault to
o clear itself.
own junction tem
mperature of the
e AL8808 is approximately +145°C with a hysterresis of +10°C. This means thatt the AL8808 will
The OTP shutdo
never switch-off with a junction temperature
t
belo
ow +125°C allow
wing the designer to design the system
s
thermally to fully utilize th
he wide operating
g
junction tempera
ature of the AL88
808.
AL8808
Document numberr: DS35648 Rev. 2 - 2
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January 2013
© Diodes Incorporate
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AL8808
Ordering Information
AL8808 XX - XX
Package
Packing
Part Number
Package Code
Packaging
AL8808WT-7
WT
TSOT25
7 : 7” Tape & Reel
7” Tape and Reel
Quantity
3000/Tape & Reel
Part Number Suffix
-7
Marking Information
TSOT25
(Top View)
5
4
7
NEW PRODUCT
WT : TSOT25
XX : Identification code
Y : Year 0~9
W : Week : A~Z : 1~26 week;
a~z : 27~52 week; z represents
52 and 53 week
X : A~Z : Internal code
XX Y W X
1
2
3
Part Number
AL8808WT-7
Package
TSOT25
Identification Code
B9
Package Outline Dimensions (All dimensions in mm.)
Please see AP02002 at http://www.diodes.com/datasheets/ap02002.pdf for latest version.
D
e1
E
E1
L2
c
4x θ1
e
L
θ
5x b
A
A2
A1
AL8808
Document number: DS35648 Rev. 2 - 2
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www.diodes.com
TSOT25
Dim Min Max Typ
A
1.00
−
−
A1
0.01 0.10
−
A2
0.84 0.90
−
D
2.90
−
−
E
2.80
−
−
E1
1.60
−
−
b
0.30 0.45
−
c
0.12 0.20
−
e
0.95
−
−
e1
1.90
−
−
L
0.30 0.50
L2
0.25
−
−
θ
0°
8°
4°
θ1
4°
12°
−
All Dimensions in mm
January 2013
© Diodes Incorporated
AL8808
Suggested Pad Layout
Please see AP02001 at http://www.diodes.com/datasheets/ap02001.pdf for latest version.
C
C
Dimensions Value (in mm)
C
0.950
X
0.700
Y
1.000
Y1
3.199
NEW PRODUCT
Y1
Y (5x)
X (5x)
IMPORTANT NOTICE
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INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
(AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION).
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Should Customers purchase or use Diodes Incorporated products for any unintended or unauthorized application, Customers shall indemnify and
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Products described herein may be covered by one or more United States, international or foreign patents pending. Product names and markings
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This document is written in English but may be translated into multiple languages for reference. Only the English version of this document is the
final and determinative format released by Diodes Incorporated.
LIFE SUPPORT
Diodes Incorporated products are specifically not authorized for use as critical components in life support devices or systems without the express
written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body, or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labeling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the
failure of the life support device or to affect its safety or effectiveness.
Customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support devices or systems, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any
use of Diodes Incorporated products in such safety-critical, life support devices or systems, notwithstanding any devices- or systems-related
information or support that may be provided by Diodes Incorporated. Further, Customers must fully indemnify Diodes Incorporated and its
representatives against any damages arising out of the use of Diodes Incorporated products in such safety-critical, life support devices or systems.
Copyright © 2013, Diodes Incorporated
www.diodes.com
AL8808
Document number: DS35648 Rev. 2 - 2
20 of 20
www.diodes.com
January 2013
© Diodes Incorporated