AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Description
Pin Assignments
(Top View)
The AL8807 is a step-down DC/DC converter designed to
drive LEDs with a constant current. The device can drive up
to 9 LEDs, depending on the forward voltage of the LEDs, in
series from a voltage source of 6V to 36V. Series connection
of the LEDs provides identical LED currents resulting in
uniform brightness and eliminating the need for ballast
resistors. The AL8807 switches at frequency up to 1MHz with
controlled rise and fall times to reduce EMI. This allows the
use of small size external components, hence minimizing the
PCB area needed.
SW
GND
CTRL
(Top View)
SET
VIN
GND
N/C
GND
SW
CTRL
SW
Features
•
MSOP-8EP
LED driving current up to 1.3A (MSOP-8EP)
Better than 5% accuracy
High efficiency up to 96%
Optimally controlled switching speeds
Operating input voltage from 6V to 36V
PWM/DC input for dimming control
Built-in output open-circuit protection
SOT25: Available in “Green” Molding Compound (No Br,
Sb) with lead Free Finish/ RoHS Compliant (Note 1)
MSOP-8EP Available in “Green” Molding Compound (No
Br, Sb) with lead Free Finish/ RoHS Compliant (Note 1)
Notes:
SET
SOT25
Maximum output current of AL8807 is set via an external
resistor connected between the VIN and SET input pins.
Dimming is achieved by applying either a DC voltage or a
PWM signal at the CTRL input pin. An input voltage of 0.4V
or lower at CTRL switches off the output MOSFET simplifying
PWM dimming.
•
•
•
•
•
•
•
•
VIN
Applications
•
•
•
•
MR16 lamps
General illumination lamps
12V powered LED Lamps
24V powered LED Lamps
1. EU Directive 2002/95/EC (RoHS) & 2011/65/EU (RoHS 2). All applicable RoHS exemptions applied.Please visit our website at
http://www.diodes.com/products/lead_free.html.
Typical Application Circuit
D1
DFLS 2100
R1
ANODE
0 R15
D3
DFLS 2100
D2
SET
100nF
VIN
C5
P1
C2
C3
DFLS 2100
D5
150µF
L1
1µF
C1
SW
CTRL
P2
D4
C4
U1
DFLS 2100
150µF
100 nF
33µH
CATHODE
GND
AL8807
DFLS 2100
GND
AL8807
Document number: DS35281 Rev. 4 - 2
1 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Pin Descriptions
Pin Name
SW
GND
CTRL
SET
VIN
EP
N/C
Pin Number
Descriptions
SOT25 MSOP-8EP
Switch Pin. Connect inductor/freewheeling diode here, minimizing track length at this pin to
1
5, 6
reduce EMI.
2
2, 3
GND Pin
Dimming and On/Off Control Input.
•
Leave floating for normal operation.
(VCTRL = VREF = 2.5V giving nominal average output current IOUTnom = 0.1/RS)
•
Drive to voltage below 0.4V to turn off output current
3
4
•
Drive with DC voltage (0.5V < VCTRL < 2.5V) to adjust output current from 20% to
100% of IOUTnom
•
A PWM signal (low level ≤ 0.4V and high level > 2.6; transition times less than 1us)
allows the output current to be adjusted below the level set by the resistor connected
to SET input pin.
4
1
Set Nominal Output Current Pin. Configure the output current of the device.
Input Supply Pin. Must be locally decoupled to GND with > 2.2µF X7R ceramic capacitor –
5
8
see applications section for more information.
Exposed pad/TAB connect to GND and thermal mass for enhanced thermal impedance.
EP
Should not be used as electrical ground conduction path.
7
no connection
Absolute Maximum Ratings
Symbol
Parameter
ESD HBM
ESD MM
Ratings
Human Body Model ESD Protection
Machine Model ESD Protection
Unit
2.5
200
kV
V
VIN
Continuous VIN pin voltage relative to GND
-0.3~40
V
VSW
SW voltage relative to GND
-0.3~40
V
VCTRL
CTRL pin input voltage
ISW-RMS
DC or RMS Switch current
ISW-PK
Peak Switch current (<10%)
SOT25
MSOP-8EP
-0.3 ~ 6
V
1.25
1.6
A
2.5
A
TJ
Junction Temperature
150
°C
TLEAD
Lead Temperature Soldering
300
°C
Storage Temperature Range
-65 to +150
TST
Caution:
°C
Stresses greater than the 'Absolute Maximum Ratings' specified above, may cause permanent damage to the device. These are stress ratings only;
functional operation of the device at these or any other conditions exceeding those indicated in this specification is not implied. Device reliability may
be affected by exposure to absolute maximum rating conditions for extended periods of time.
Semiconductor devices are ESD sensitive and may be damaged by exposure to ESD events. Suitable ESD precautions should be taken when
handling and transporting these devices.
Recommended Operating Conditions
Symbol
Parameter
VIN
Operating Input Voltage relative to GND
6.0
36
V
VCTRLH
Voltage High for PWM dimming relative to GND
2.6
5.5
V
VCTRLDC
Voltage range for 20% to 100% DC dimming relative to GND
0.5
2.5
V
VCTRLL
Voltage Low for PWM dimming relative to GND
0
0.4
V
fSW
Maximum switching frequency
ISW
Continuous switch current
TJ
Junction Temperature Range
AL8807
Document number: DS35281 Rev. 4 - 2
Min
Max
Unit
1
SOT25
MSOP-8EP
1.3
-40
2 of 21
www.diodes.com
MHz
1
125
A
°C
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Electrical Characteristics
VIN =12V, TA=25oC, unless otherwise specified.
Symbol
Parameter
Conditions
Min
Typ
Max
Unit
5.9
V
300
mV
VINSU
Internal regulator start up threshold VIN rising
VINSH
Internal regulator hysteresis
threshold
VIN falling
IQ
Quiescent current
Output not switching (Note 2)
IS
Input supply Current
CTRL pin floating f = 250kHz
VTH
Set current Threshold Voltage
VTH-H
Set threshold hysteresis
ISET
SET pin input current
VSET = VIN-0.1
16
RCTRL
CTRL pin input resistance
Referred to internal reference
50
kΩ
VREF
Internal Reference Voltage
2.5
V
RDS(on)
On Resistance of SW MOSFET
ISW = 1A
0.25
tR
SW rise time
12
tF
SW fall time
VSENSE = 100±20mV fSW = 250kHz
VSW = 0.1V~12V~0.1V CL = 15pF
ISW_Leakage
Switch leakage current
VIN =30V
θJA
Thermal Resistance Junction-toAmbient (Note 3)
SOT25 (Note 4)
MSOP-8EP (Note 5)
250
69
ΨJL
Thermal Resistance Junction-toLead (Note 6)
SOT25 (Note 4)
50
θJC
Thermal Resistance Junction-tocase (Note 7)
MSOP-8EP (Note 5)
4.3
Notes:
100
95
350
µA
1.8
5
mA
100
105
mV
±20
mV
22
µA
0.4
Ω
ns
20
ns
0.5
μA
°C/W
2. AL8807 does not have a low power standby mode but current consumption is reduced when output switch is inhibited: VSENSE = 0V. Parameter is
tested with VCTRL ≤ 2.5V
3. Refer to figure 35 for the device derating curve.
4. Test condition for SOT25: Device mounted on FR-4 PCB (25mm x 25mm 1oz copper, minimum recommended pad layout on top layer and thermal
vias to bottom layer ground plane. For better thermal performance, larger copper pad for heat-sink is needed.
5. Test condition for MSOP-8EP: Device mounted on FR-4 PCB (51mm x 51mm 2oz copper, minimum recommended pad layout on top layer and
thermal vias to bottom layer with maximum area ground plane. For better thermal performance, larger copper pad for heat-sink is needed
6. Dominant conduction path via Gnd pin (pin 2).
7. Dominant conduction path via exposed pad.
AL8807
Document number: DS35281 Rev. 4 - 2
3 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Typical Performance Characteristics (TA = 25°C unless otherwise stated)
400
900
350
800
700
FREQUENCY (kHz)
300
IIN (µA)
250
200
150
100
0
3
600
L = 68µH
500
400
300
200
VCTRL = 0V
VSET = VIN
TA = 25°C
50
0
L = 100µH
100
0
6
9 12 15 18 21 24 27 30 33 36
VIN (V)
Figure 1. Supply Current (not switching) vs.
Input Voltage
80
90
60
70
40
ICTRL (µA)
100
LED CURRENT (A)
VIN = 12V
1 LED
RSET = 150mΩ
TA = 25°C
L = 33µH
60
40
0
5
3
4
VCTRL
Figure 2. Switching Frequency vs. VCTRL
1
2
VSET = VIN = 12V
TA = 25°C
20
0
30
-20
20
-40
-60
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.0
VCTRL (V)
Figure 4. ICTRL vs. VCTRL
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
CTRL PIN VOLTAGE (V)
Figure 3. LED Current vs. VCTRL
3
2.52
VCTRL = Open
VSET = VIN = 12V
2.5
2.51
VCTRL (V)
VCTRL (V)
2
1.5
2.50
1
0
2.49
VCTRL = Open
VSET = VIN
T A = 25°C
0.5
0
3
6
9 12 15 18 21 24 27 30 33 36
VIN (V)
Figure 5. VCTRL vs. Input Voltage
(CTRL Pin Open Circuit)
AL8807
Document number: DS35281 Rev. 4 - 2
4 of 21
www.diodes.com
2.48
-40
-15
10
35
60
85
110
AMBIENT TEMPERATURE (°C)
Figure 6. VCTRL VS. TEMPERATURE
June 2012
© Diodes Incorporated
AL
L8807
HIGH EFFICIENCY
Y LOW EM
MI 36V 1A BUCK
B
LED DRIVER
Typical Pe
erformance
e Characte
eristics (con
nt.) (TA = 25°C unless oth
herwise state
ed)
7
240
6
0.5
LE
ED Current
Errror
0.4
LED Currrent
4
270
0.7
0.6
5
300
0.3
3
210
RDS(ON) (m
( Ω)
8
LED CURRENT ERROR (%)
0.8
L = 68H, RS = 150m
TA = 25C, VIN = 12V
C
CTRL
= PWM, fPWM = 500Hz
1 LED
LED CURRENT (A)
9
180
150
120
90
0.2
2
VCTRL = Open
VSET = VIN
TA = 25°C
C
60
0.1
1
0
0
20
40
60
8
80
PWM DUTY
D
CYCLE
Figure 7. ILED vs. PWM Duty Cycle
C
30
0
100
0
400
0
6
9
12 15 18 21 24 27 30 33
3
36
(
VIN (V)
ure 8. SW RDS(ON
ge
Figu
N) vs. Input Voltag
100
90
350
0
80
DUTY CYCLE (%)
RDS(ON) (mΩ)
3 LEDS
300
0
250
0
200
0
VCTRL = Open
VSET = VIN = 12V
150
0
L = 68µH
RS = 100m
mΩ
TA = 25°C
C
VCTRL = Open
O
70
60
2 LEDS
50
40
30
20
10
100
0
-40
-15
10
1
35
60
0
85
110
0
Ambient Temperature
e (°C)
Figure 9. SW
S RDS(ON) vs. Temperature
Te
Figure. 11 SW
W Output Rise
e Time
AL8807
Document numberr: DS35281 Rev. 4 - 2
0
6
9
1 15 18 21 24 27 30 33
12
3 36
INPUT VOLT
TAGE (V)
Figurre 10. Duty Cycle
e vs. Input Voltag
ge
Figure. 12 SW Outpu
ut Fall Time
5 of 21
ww
ww.diodes.com
June 2012
© Diodess Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Typical Performance Characteristics (670mA LED current) (TA = 25°C)
10
350
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
6
4
2
0
-2
-4
-6
-8
-10
250
200
150
100
50
0
6
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 13. LED Current Deviation vs. Input Voltage
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 14. Switching Frequency vs. Input Voltage
10
500
8
450
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
6
300
6
4
2
0
-2
-4
-6
400
350
300
250
200
150
100
50
-8
-10
0
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 15. LED Current Deviation vs. Input Voltage
6
9
6
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 16. Switching Frequency vs. Input Voltage
10
SWITCHING FREQUENCY (kHz)
6
4
1 LED
2 LEDs
2
0
3 LEDs
4 LEDs
5 LEDs
6 LEDs
-2
-4
7 LEDs
-6
8 LEDs
-8
-10
9
800
L = 33µH
RS = 150mΩ
TA = 25°C
VCTRL = Open
8
LED CURRENT ERROR (%)
9
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 17. LED Current Deviation vs. Input Voltage
AL8807
Document number: DS35281 Rev. 4 - 2
6 of 21
www.diodes.com
L = 33µH
RS = 150mΩ
TA = 25°C
VCTRL = Open
700
600
500
400
300
1 LED
200
7 LEDs
100
0
8 LEDs
5 LEDs
3 LEDs
4 LEDs
6 LEDs
2 LEDs
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 18. Switching Frequency vs. Input Voltage
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Typical Performance Characteristics (1A LED current MSOP-8EP) (TA = 25°C)
350
10
L = 100µH
RS = 100mΩ
TA = 25°C
VCTRL = Open
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
6
4
2
0
-2
-4
-6
300
250
200
150
1 LED
100
50
-8
-10
4 LEDs 5 LEDs 6 LEDs
2 LEDs3 LEDs
0
6
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 20. Switching Frequency vs. Input Voltage
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 19. LED Current Deviation vs. Input Voltage
6
9
0
6
9
7 LEDs
8 LEDs
350
10
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
6
4
2
0
-2
-4
-6
300
250
200
150
100
50
-8
-10
6
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 22. Switching Frequency vs. Input Voltage
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 21. LED Current Deviation vs. Input Voltage
600
10
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
6
4
2
0
-2
-4
-6
500
400
300
200
100
-8
-10
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 23. LED Current Deviation vs. Input Voltage
AL8807
Document number: DS35281 Rev. 4 - 2
7 of 21
www.diodes.com
0
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 24. Switching Frequency vs. Input Voltage
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Typical Performance Characteristics (1.3A LED current MSOP-8EP) (TA = 25°C)
10
250
L = 100µH
RS = 77mΩ
TA = 25°C
VCTRL = Open
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
6
4
2
0
-2
-4
-6
200
150
100
1 LED
50
-8
-10
2 LEDs 3 LEDs
5 LEDs 6 LEDs7 LEDs 8 LEDs
4 LEDs
0
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 25. LED Current Deviation vs. Input Voltage
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 26. Switching Frequency vs. Input Voltage
300
10
L = 68µH
RS = 77mΩ
T A = 25°C
VCTRL = Open
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
6
4
2
0
-2
-4
-6
-8
-10
250
200
150
100
1 LED
50
2 LEDs 3 LEDs 4 LEDs
6 LEDs
5 LEDs
6
0
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 27. LED Current Deviation vs. Input Voltage
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 28. Switching Frequency vs. Input Voltage
L = 33µH
RS = 77mΩ
TA = 25°C
VCTRL = Open
SWITCHING FREQUENCY (kHz)
8
LED CURRENT ERROR (%)
9
600
10
6
4
2
0
-2
-4
-6
500
400
300
200
1 LED
100
5 LEDs
-8
-10
7 LEDs 8 LEDs
0
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 29. LED Current Deviation vs. Input Voltage
AL8807
Document number: DS35281 Rev. 4 - 2
8 of 21
www.diodes.com
4 LEDs
2 LEDs 3 LEDs
6
6 LEDs
7 LEDs
8 LEDs
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 30. Switching Frequency vs. Input Voltage
June 2012
© Diodes Incorporated
AL
L8807
HIGH EFFICIENCY
Y LOW EM
MI 36V 1A BUCK
B
LED DRIVER
Applicatio
on Informattion
The AL8807 is
i a hysteretic
c (also known as equal ripp
ple) LED driver with integratted power swittch. It is availlable in two
packages that provide a PCB
P
area-pow
wer dissipation
n capability co
ompromise. It is recommen
nded that at higher
h
LED
currents/smalle
er PCBs that the MSOP-8E
EP version is used
u
to maxim
mize the allowa
able LED curre
ent over a wid
der ambient
temperature ra
ange.
AL8807 Ope
eration
In normal operation, when voltage
v
is applied at +VIN, the
e AL8807 interrnal switch is turned
t
on. Currrent starts to flow
f
through
sense resistorr R1, inductor L1,
L and the LE
EDs. The curre
ent ramps up linearly,
l
and th
he ramp rate iss determined by
b the input
voltage +Vin and
a the inductor L1.
This rising currrent produces a voltage ram
mp across R1. The
T internal circuit of the AL
L8807 senses the
t voltage acrross R1 and
applies a proportional voltage
e to the input of
o the internal comparator.
c
When this volttage reaches an
a internally sett upper thresho
old, the interna
al switch is turn
ned off. The ind
ductor current continues
c
to
flow through R1, L1, the LED
Ds and the schottky diode D1,
D and back to
o the supply ra
ail, but it deca
ays, with the ra
ate of decay
determined byy the forward vo
oltage drop of the
t LEDs and the
t schottky dio
ode.
This decaying current produces a falling voltage
v
at R1, which
w
is sense
ed by the AL88
807. A voltage proportional to
o the sense
en this voltage falls to the inte
ernally set lowe
er threshold,
voltage acrosss R1 is applied at the input of the internal comparator. Whe
the internal sw
witch is turned on again. Thiss switch-on-and-off cycle con
ntinues to provvide the averag
ge LED curren
nt set by the
sense resistor R1.
LED Currentt Control
The LED curre
ent is controlled
d by the resisto
or R1 in Figure 30.
Fig. 30 Typic
cal Application Circuit
Connected bettween VIN and SET the nomin
nal average ou
utput current in the LED(s) is defined
d
as:
ILED =
VTHD
R1
For example fo
or a desired LE
ED current of 66
60mA and a de
efault voltage VCTRL=2.5V the
e resulting resisstor is:
R1 =
VTHD
0.1
=
≈ 150mΩ
ILED
0.66
DC Dimming
g
Further control of the LED cu
urrent can be achieved by drivving the CTRL pin with an extternal voltage (between
(
0.4V and 2.5V);
ED current bec
comes:
the average LE
ILED =
VCTRL VTHD
VREF R SET
With 0.5V ≤ VCTRL
≤ 2.5V the
e LED current varies linearlyy with VCTRL, as
a in figure 2. If the CTRL pin is brought higher
h
than
C
VTHD
2.5V, the LED current will be clamped to ap
pproximately 10
00% and follow
ws ILED =
.
RSET
When the CTR
RL voltage falls
s below the thre
eshold, 0.4V, th
he output switcch is turned off which allows PWM
P
dimming.
AL8807
Document numberr: DS35281 Rev. 4 - 2
9 of 21
ww
ww.diodes.com
June 2012
© Diodess Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Applications Information (cont.)
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 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 1% for PWM ranging from 1% to 100%.
700
LED CURRENT (mA)
600
500
400
300
200
100
0
0
10
20
30
40
50
60
70
80
90
100
PWM DIMMING (%)
Fig. 31 PWM Dimming @ 500Hz
Zooming in at duty cycles below 10% shows:
DEVIATION FROM IDEAL LOAD CURRENT (%)
4.5
VIN = 12VDC
L = 68µH
RS = 0.15Ω
TA = 25°C
2 LEDs
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
10
30
50
70
90
100
300Hz PWM DUTY CYCLE (%)
Fig. 32 Low Duty Cycle PWM Dimming @ 300Hz
The accuracy of the low duty cycle dimming is affected by both the PWM frequency and also the switching frequency of the
AL8807. For best accuracy/resolution the switching frequency should be increased while the PWM frequency should be
reduced.
The CTRL pin is designed to be driven by both 3.3V and 5V logic levels directly from a logic output with either an open drain
output or push pull output stage.
AL8807
Document number: DS35281 Rev. 4 - 2
10 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Applications Information (cont.)
Soft Start
The AL8807 does not have in-built soft-start action – this provides very fast turn off of the output the stage improving PWM
dimming accuracy; nonetheless, adding an external capacitor from the CTRL pin to ground will provide a soft-start delay. This
is achieved by increasing the time taken for the CTRL voltage to rise to the turn-on threshold and by slowing down the rate of
rise of the control voltage at the input of the comparator. Adding a capacitor increases the time taken for the output to reach
90% of its final value, this delay is 0.1ms/nF, but will impact on the PWM dimming accuracy depending on the delay
introduced.
Fig. 33 Soft start with 22nF capacitor on CTRL pin (VIN = 30V, ILED = 667mA, 1 LED)
Reducing Output Ripple
Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor C2 across the LED(s) as shown
already in the circuit schematic.
A value of 1μF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with
higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up
delay, by reducing the rate of rise of LED voltage. By adding this capacitor the current waveform through the LED(s) changes
from a triangular ramp to a more sinusoidal version without altering the mean current value.
Capacitor Selection
The small size of ceramic capacitors makes them ideal for AL8807 applications. X5R and X7R types are recommended
because they retain their capacitance over wider voltage and temperature ranges than other types such as Z5U.
A 2.2μF input capacitor is sufficient for most intended applications of AL8807; however a 4.7μF input capacitor is suggested
for input voltages approaching 36V.
AL8807
Document number: DS35281 Rev. 4 - 2
11 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL
L8807
HIGH EFFICIENCY
Y LOW EM
MI 36V 1A BUCK
B
LED DRIVER
Applicatio
ons Informa
ation (cont.)
Diode Selection
For maximum efficiency and performance, the rectifier (D
D1) should be a fast low capa
acitance Schotttky diode with low reverse
e maximum ope
erating voltage
e and temperatture. The Sch
hottky diode alsso provides be
etter efficiency than silicon
leakage at the
PN diodes, due to a combina
ation of lower fo
orward voltage and reduced recovery
r
time.
urrent rating ab
bove the peak coil
c current and
d a continuouss current rating higher than
It is important to select parts with a peak cu
the maximum output load current. In particcular, it is recom
mmended to ha
ave a diode vo
oltage rating at least 15% high
her than the
age to ensure safe operation
n during the sw
witching and a current rating
g at least 10%
% higher than the
t
average
operating volta
diode current. The power ratting is verified by
b calculating the
t power loss through the diode.
es, e.g. B240 or B140, with their low forwa
ard voltage dro
op and fast revverse recoveryy, are the idea
al choice for
Schottky diode
AL8807 appliccations.
Inductor Sellection
Recommended
d inductor values for the AL88
807 are in the range 33μH to 100μH.
Higher values of inductance are recommen
nded at higher supply voltage
es in order to minimize
m
errorss due to switch
hing delays,
which result in
n increased rip
pple and lowerr efficiency. Hig
gher values off inductance allso result in a smaller chang
ge in output
current over th
he supply voltag
ge range. (See
e graphs).
Fig. 34 Inductor value witth input voltag
ge and numbe
er of LEDs
The inductor should be moun
nted as close to
o the device ass possible with low resistance
e/stray inductan
nce connections to the SW
pin.
The chosen co
oil should have a saturation cu
urrent higher th
han the peak output
o
current and
a a continuou
us current ratin
ng above
the required mean
m
output currrent.
Suitable coils for
f use with the
e AL8807 are liisted in the table below:
L
(µH)
DCR
D
(
(V)
ISA
AT
(A))
MSS1038
8-333
33
0.093
2.3
3
MSS1038
8-683
68
0.213
1.5
5
NPIS64D
D330MTRF
33
0.124
1.1
1
Parrt No.
M
Manufacturer
CoilCrafft www.coilcrafft.com
NIC ww
ww.niccomp.com
m
v
should be
e chosen to ma
aintain operating duty cycle an
nd switch 'on'/'o
off' times over the
t supply voltage and
The inductor value
load current ra
ange.
AL8807
Document numberr: DS35281 Rev. 4 - 2
12 of 21
ww
ww.diodes.com
June 2012
© Diodess Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Applications Information (cont.)
The following equations can be used as a guide, with reference to Figure 1 - Operating waveforms.
Switch ‘On’ time
Switch ‘Off’ time
tON =
L ΔI
tOFF =
VIN − VLED − IAVG x (RS + rL + RSW )
LΔI
VLED + VD + IAVG x (RS + rL )
Where:
L is the coil inductance (H)
rL is the coil resistance (Ω)RS is the current sense resistance (Ω)
Iavg is the required LED current (A)
ΔI is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}
VIN is the supply voltage (V)
VLED is the total LED forward voltage (V)
RSW is the switch resistance (Ω) {=0.5Ω nominal}
VD is the diode forward voltage at the required load current (V)
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 θJA, PCB layout, airflow
surrounding the IC, and difference between junction and ambient temperature.
The maximum power dissipation can be calculated using the following formula:
PD(MAX) = (TJ(MAX) − TA) / θJA
where
TJ(MAX) is the maximum operating junction temperature,
TA is the ambient temperature, and
θJA is the junction to ambient thermal resistance.
The recommended maximum operating junction temperature, TJ, is 125°C and so maximum ambient temperature is
determined by the AL8807’s junction to ambient thermal resistance, θJA and device power dissipation.
θJA, is layout dependent and package dependent; the AL8807W5’s θJA on a 25x25mm single layer PCB with 1oz copper
standing in still air is approximately 250°C/W (160°C/W on a four-layer PCB).
The maximum power dissipation at TA = 25°C can be calculated by the following formulas:
PD(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for single-layer PCB
PD(MAX) = (125°C − 25°C) / (160°C/W) = 0.625W for standard four-layer PCB
Figure 35, shows the power derating of the AL8807W5 on two (one single-layer and four-layer) different 25x25mm PCB with
1oz copper standing in still air and the AL8807MP on an FR4 51x51mm PCB with 2oz copper standing in still air.
1600
POWER DISSIPATION (mW)
1400
MSOP-8EP
1200
1000
800
600
400
SOT25_4 Layer
SOT25_1 Layer
200
0
-40 -25 -10 5 20 35 50 65 80 90 110 125
AMBIENT TEMPERATURE (°C)
Fig. 35 Derating Curve for Different PCB
AL8807
Document number: DS35281 Rev. 4 - 2
13 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL
L8807
HIGH EFFICIENCY
Y LOW EM
MI 36V 1A BUCK
B
LED DRIVER
Applicatio
ons Informa
ation (cont.)
EMI and Lay
yout Conside
erations
The AL8807 iss a switching re
egulator with fa
ast edges and measures
m
sma
all differential vo
oltages; as a re
esult of this carre has to be
taken with deccoupling and la
ayout of the PC
CB.To help with
h these effects the AL8807 has been developed to minimise radiated
emissions by controlling
c
the switching spee
eds of the interrnal power MOSFET. The risse and fall time
es are controlle
ed to get the
right comprom
mise between power
p
dissipattion due to sw
witching lossess and radiated EMI. The tu
urn-on edge (fa
alling edge)
dominates the
e radiated EMI which is due to an interaction between th
he Schottky dio
ode (D1), Swittching MOSFE
ET and PCB
tracks. After the
t
Schottky diode
d
reverse recovery
r
time of around 5ns has occurred; the falling ed
dge of the SW
W pin sees a
resonant loop between the Schottky
S
diode capacitance
c
an
nd the track ind
ductance, LTRACK, See figure 36.
Fig. 36 PC
CB Loop Res
sonance
m the SW pin to
t the Anode of
o the Schottkyy diode, D1, and then from D1
1’s cathode to the decoupling
g capacitors
The tracks from
C1 should be as short as po
ossible. There is an inductan
nce internally in
n the AL8807 this can be assumed to be around
a
1nH.
For PCB trackks a figure of 0.5nH
0
per mm can be used to
o estimate the primary reson
nant frequency. If the track iss capable of
handling 1A in
ncreasing the thickness will have a minorr effect on the inductance and length will dominate the size of the
inductance. The resonant fre
equency of anyy oscillation is determined
d
by the combined inductance in the track and the
t effective
capacitance off the Schottky diode.
d
An exam
mple of good la
ayout is shown
n in figure 37 - the
t stray track inductance sho
ould be less
than 5nH.
VIN
SW
SET
ND
GN
CTRL
Place D1 anode
a
, SW pin and
Inductor ass close as possible to
t
minimize rringing
F 37 Recommended PCB Layout
Fig.
AL8807
Document numberr: DS35281 Rev. 4 - 2
14 of 21
ww
ww.diodes.com
June 2012
© Diodess Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Applications Information (cont.)
Recommendations for minimising radiated EMI and other transients and thermal considerations are:
1. The decoupling capacitor (C1) has to be placed as close as possible to the VIN pin and D1 Cathode
2. The freewheeling diode’s (D1) anode, the SW pin and the inductor have to be placed as close as possible to each
other to avoid ringing.
3. The Ground return path from C1 must be a low impedance path with the ground plane as large as possible
4. The LED current sense resistor (R1) has to be placed as close as possible to the VIN and SET pins.
5.
6.
The majority of the conducted heat from the AL8807 is through the GND pin 2. A maximum earth plane with thermal
vias into a second earth plane will minimise self-heating
To reduce emissions via long leads on the supply input and LEDs low RF impedance capacitors (C2 and C5) should
be used at the point the wires are joined to the PCB
A Typical application for the AL8807 is an LED MR16 lamp (schematic shown in Figure 38).
Fig. 38 MR16 Circuit Schematic
An evaluation board for the AL8807 (named the AL8807EV2) for MR16 is available on request from your local Diodes’ sales
representative. This board follows Diodes’ recommendations for low EMI. Images of the top layer and bottom layers are
shown in Figure 39.
Fig. 39 Recommended MR16 PCB Layout
AL8807
Document number: DS35281 Rev. 4 - 2
15 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Applications Information (cont.)
The associated EMI measurements for this board using the AL8807 is shown in figure 40.
Fig 40. AL8807EV2 Radiated EMI Performance
The EMI performance was measured at 12VDC driving two white LEDs (VF = 3.1V at 660mA) on the AL8807EV2. The red bold
line is for EN55022 class B used for domestic equipment including lighting. The bottom magenta line is the noise floor of the
test chamber. The middle purple line is the EMI emitted radiation of the AL8807 over 30MHz to 1000MHz. This shows that the
AL8807 passes the standard with at least 16dB margin.
MR16 lamps typically operate from 12VDC or 12VAC, using conventional electromagnetic transformers or electronic
transformers.
In enclosed lamps such MR16 the ability for the device to operate at high ambient temperatures is critical and figure 41 shows
the surface temperature of the AL8807 on AL8807EV2 in operation under the same conditions as the EMI tests at an free air
temperature of 25ºC. It is anticipated that the internal junction temperature is approximately 6 ºC hotter than the surface
temperature.
Fig 41. Thermal picture of AL8807EV2 at 12VDC 2 white LEDS at 660mA
The thermal image shows that components increasing the board temperature are the inductor, Schottky diodes and the
AL8807.
AL8807
Document number: DS35281 Rev. 4 - 2
16 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Applications Information (cont.)
An inductor choice of 33µH with saturation current higher than 1.1A, will limit the frequency variation between 180kHz and
400kHz over the whole input voltage variation (8V to 18V), and therefore represent the best choice for an MR16 solution also
taking into account the size constraint of the lamp.
The AL8807 guarantees high performance levels with both 12VAC and 12VDC power supplies.
The efficiency is generally higher than 81% and current regulation is better than 0.1mA/V in for a DC input voltage in the range
from 8V to 18V.
AL8807
Document number: DS35281 Rev. 4 - 2
17 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Ordering Information
AL8807 XX - XX
Package
Packing
W5 : SOT25
MP : MSOP-8EP
7 : 7” Tape & Reel
13 : 13” Tape & Reel
Device
Status
Package Code
Packaging
(Note 8)
AL8807W5-7
AL8807MP-13
New Product
New Product
W5
MP
SOT25
MSOP-8EP
Notes:
8.
7” Tape and Reel
Quantity
Part Number Suffix
3000/Tape & Reel
2500/Tape & Reel
-7
-13
Pad layout as shown on Diodes Inc. suggested pad layout document AP02001, which can be found on our website at:
http://www.diodes.com/datasheets/ap02001.pdf .
Marking Information
(1) SOT25
(Top View)
4
7
5
XX Y W X
1
2
3
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
Part Number
Package
Identification Code
AL8807W5-7
SOT25
B6
(2) MSOP-8EP
8
7
Logo
AL8807
1
Document number: DS35281 Rev. 4 - 2
5
YWXE
Part Number
AL8807
6
2
3
a~z : Lead Free
A~Z: Green
MSOP-8EP
Y : Year : 0 - 9
W : Week : A~Z : 1 ~ 26 week;
a~z : 27~52 week
z represents 52 and 53 week
4
Part Number
Package
AL8807MP-13
MSOP-8EP
18 of 21
www.diodes.com
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Package Outline Dimensions (All Dimensions in mm)
(1) Package Type: SOT25
A
SOT25
Dim Min Max Typ
A
0.35 0.50 0.38
B
1.50 1.70 1.60
C
2.70 3.00 2.80
D
⎯
⎯ 0.95
H
2.90 3.10 3.00
J
0.013 0.10 0.05
K
1.00 1.30 1.10
L
0.35 0.55 0.40
M
0.10 0.20 0.15
N
0.70 0.80 0.75
0°
8°
α
⎯
All Dimensions in mm
B C
H
K
M
N
J
L
D
(2) MSOP-8EP
D
4X
10
°
0.25
D1
x
E
E2
Gauge Plane
Seating Plane
a
y
1
4X
10
°
8Xb
e
Detail C
E3
A1
A3
L
c
A2
A
D
E1
See Detail C
AL8807
Document number: DS35281 Rev. 4 - 2
19 of 21
www.diodes.com
MSOP-8EP
Dim Min Max Typ
A
1.10
A1
0.05 0.15 0.10
A2
0.75 0.95 0.86
A3
0.29 0.49 0.39
b
0.22 0.38 0.30
c
0.08 0.23 0.15
D
2.90 3.10 3.00
D1
1.60 2.00 1.80
E
4.70 5.10 4.90
E1
2.90 3.10 3.00
E2
1.30 1.70 1.50
E3
2.85 3.05 2.95
e
0.65
L
0.40 0.80 0.60
a
0°
8°
4°
x
0.750
y
0.750
All Dimensions in mm
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
Suggested Pad Layout
1) Package Type: SOT25
C2
C2
C1
G
Z
Dimensions Value (in mm)
Z
3.20
G
1.60
X
0.55
Y
0.80
C1
2.40
0.95
C2
Y
X
2) MSOP-8EP
X
C
Y
G
Y2
Dimensions
C
G
X
X1
Y
Y1
Y2
Y1
X1
AL8807
Document number: DS35281 Rev. 4 - 2
20 of 21
www.diodes.com
Value
(in mm)
0.650
0.450
0.450
2.000
1.350
1.700
5.300
June 2012
© Diodes Incorporated
AL8807
HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER
IMPORTANT NOTICE
DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT,
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).
Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or other changes
without further notice to this document and any product described herein. Diodes Incorporated does not assume any liability arising out of the
application or use of this document or any product described herein; neither does Diodes Incorporated convey any license under its patent or
trademark rights, nor the rights of others. Any Customer or user of this document or products described herein in such applications shall
assume all risks of such use and will agree to hold Diodes Incorporated and all the companies whose products are represented on Diodes
Incorporated website, harmless against all damages.
Diodes Incorporated does not warrant or accept any liability whatsoever in respect of any products purchased through unauthorized sales
channel.
Should Customers purchase or use Diodes Incorporated products for any unintended or unauthorized application, Customers shall indemnify
and hold Diodes Incorporated and its representatives harmless against all claims, damages, expenses, and attorney fees arising out of,
directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized application.
Products described herein may be covered by one or more United States, international or foreign patents pending. Product names and
markings noted herein may also be covered by one or more United States, international or foreign trademarks.
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 systemsrelated 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 © 2012, Diodes Incorporated
www.diodes.com
AL8807
Document number: DS35281 Rev. 4 - 2
21 of 21
www.diodes.com
June 2012
© Diodes Incorporated