AL8807
HIGH EFFICIENCY LOW 36V 1A BUCK LED DRIVER
Description
Pin Assignments
The AL8807 is a step-down DC/DC converter designed to drive LEDs
(Top View)
with a constant current. The device can drive up to 9 LEDs,
depending on the forward voltage of the LEDs, in series from a
SW
voltage source of 6V to 36V. Series connection of the LEDs provides
identical LED currents resulting in uniform brightness and eliminating
VIN
GND
the need for ballast resistors. The AL8807 switches at frequency up to
1MHz with controlled rise and fall times to reduce EMI. This allows
CTRL
the use of small size external components, hence minimizing the PCB
SET
area needed.
SOT25
(Top View)
Maximum output current of AL8807 is set via an external resistor
connected between the VIN and SET input pins. Dimming is achieved
SET
VIN
GND
N/C
GND
SW
CTRL
SW
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.
Features
LED Driving Current up to 1.3A (MSOP-8EP)
Better Than 5% Accuracy
High Efficiency up to 96%
Optimally Controlled Switching Speeds
Applications
Operating Input Voltage from 6V to 36V
MR16 Lamps
PWM/DC Input for Dimming Control
General Illumination Lamps
Built-In Output Open-Circuit Protection
12V Powered LED Lamps
SOT25, MSOP-8EP: Available in “Green” Molding Compound
24V Powered LED Lamps
MSOP-8EP
(No Br, Sb)
Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2)
Halogen and Antimony Free. “Green” Device (Note 3)
Notes:
1. No purposely added lead. Fully EU Directive 2002/95/EC (RoHS) & 2011/65/EU (RoHS 2) compliant.
2. See http://www.diodes.com/quality/lead_free.html 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
D1
DFLS 2100
R1
ANODE
0 R15
D3
DFLS 2100
D2
U1
100nF
SET
C5
P1
VIN
DFLS 2100
C2
C3
D4
DFLS 2100
D5
150µF
L1
1µF
C1
SW
CTRL
P2
C4
150µF
100 nF
33µH
CATHODE
GND
AL8807
DFLS 2100
GND
AL8807
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AL8807
Pin Descriptions
Pin Name
SW
GND
CTRL
SET
VIN
Pin Number
Function
SOT25
MSOP-8EP
1
5, 6
Switch Pin. Connect inductor/freewheeling diode here, minimizing track length at this pin to 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)
3
4
Drive to voltage below 0.4V to turn off output current
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 – see applications
5
8
section for more information.
EP
—
EP
N/C
—
7
Exposed pad/TAB connect to GND and thermal mass for enhanced thermal impedance. Should not be
used as electrical ground conduction path.
No Connection
Absolute Maximum Ratings (@TA = +25°C, unless otherwise specified.)
Symbol
ESD HBM
ESD MM
VIN
VSW
VCTRL
ISW-RMS
ISW-PK
Parameter
Ratings
2.5
200
Unit
kV
V
Continuous VIN Pin Voltage Relative to GND
-0.3 to +40
V
SW Voltage Relative to GND
-0.3 to +40
V
CTRL Pin Input Voltage
-0.3 to +6
V
1.25
1.6
A
2.5
A
Human Body Model ESD Protection
Machine Model ESD Protection
SOT25
MSOP-8EP
DC or RMS Switch Current
Peak Switch Current (<10%)
Junction Temperature
150
°C
TLEAD
Lead Temperature Soldering
300
°C
TST
Storage Temperature Range
-65 to +150
°C
TJ
Caution:
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 (@TA = +25°C, unless otherwise specified.)
Symbol
Parameter
Min
Max
Unit
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
0
0.4
V
VIN
VCTRLL
fSW
Voltage Low for PWM Dimming Relative to GND
Maximum Switching Frequency
ISW
Continuous Switch Current
TJ
Junction Temperature Range
AL8807
Document number: DS35281 Rev. 5 - 2
SOT25
MSOP-8EP
-40
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1
MHz
1
1.3
A
+125
°C
March 2013
© Diodes Incorporated
AL8807
Electrical Characteristics (@VIN = 12, TA = +25°C, unless otherwise specified.)
Symbol
Parameter
Conditions
VINSU
Internal Regulator Start-Up Threshold
VIN rising
VINSH
Internal Regulator Hysteresis Threshold
VIN falling
IQ
Quiescent Current
Output not switching (Note 4)
IS
Input Supply Current
CTRL pin floating f = 250kHz
VTH
VTH-H
Set current Threshold Voltage
Set Threshold Hysteresis
RCTRL
CTRL Pin Input Resistance
Referred to internal reference
VREF
Internal Reference Voltage
ISW = 1A
tR
SW Rise Time
tF
SW Fall Time
VSENSE = 100±20mV, fSW = 250kHz
VSW = 0.1V to 12V to 0.1V, CL = 15pF
mV
350
µA
1.8
5
mA
105
mV
16
mV
22
50
0.25
V
0.4
ns
0.5
SOT25 (Note 6)
MSOP-8EP (Note 7)
250
69
JL
Thermal Resistance Junction-to-Lead (Note 8)
SOT25 (Note 6)
50
Thermal Resistance Junction-to-case (Note 9)
MSOP-8EP (Note 7)
4.3
Ω
ns
20
VIN =30V
µA
kΩ
12
Thermal Resistance Junction-to-Ambient (Note 5)
JC
V
300
100
JA
Notes:
Unit
5.9
2.5
On Resistance of SW MOSFET
ISW_Leakage Switch Leakage Current
Max
±20
VSET = VIN-0.1
RDS(on)
Typ
100
95
SET Pin Input Current
ISET
Min
μA
C/W
4. 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
5. Refer to figure 35 for the device derating curve.
6. 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.
7. 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
8. Dominant conduction path via Gnd pin (pin 2).
9. Dominant conduction path via exposed pad.
AL8807
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AL8807
Typical Performance Characteristics
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
0
6
9 12 15 18 21 24 27 30 33 36
VIN (V)
Figure 1. Supply Current (not switching) vs.
Input Voltage
90
60
70
40
ICTRL (µA)
80
60
40
L = 100µH
100
100
LED CURRENT (A)
VIN = 12V
1 LED
RSET = 150m
TA = 25°C
L = 33µH
0
1
2
3
4
5
VCTRL
Figure 2. Switching Frequency vs. VCTRL
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
2.52
3
VCTRL = Open
VSET = VIN = 12V
2.5
2.51
VCTRL (V)
VCTRL (V)
2
1.5
2.50
1
0.5
0
2.49
VCTRL = Open
VSET = VIN
T A = 25°C
0
3
2.48
-40
6
9 12 15 18 21 24 27 30 33 36
VIN (V)
Figure 5. VCTRL vs. Input Voltage
(CTRL Pin Open Circuit)
AL8807
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-15
10
35
60
85
110
AMBIENT TEMPERATURE (°C)
Figure 6. VCTRL VS. TEMPERATURE
March 2013
© Diodes Incorporated
AL8807
Typical Performance Characteristics (cont.)
7
240
6
0.5
LED Current
Error
0.4
LED Current
4
270
0.7
0.6
5
300
0.3
3
210
RDS(ON) (m)
8
LED CURRENT ERROR (%)
0.8
L = 68H, RS = 150m
TA = 25C, VIN = 12V
CTRL = PWM, fPWM = 500Hz
1 LED
LED CURRENT (A)
9
150
120
90
0.2
2
180
VCTRL = Open
VSET = VIN
TA = 25°C
60
0.1
1
0
0
20
40
60
80
PWM DUTY CYCLE
Figure 7. ILED vs. PWM Duty Cycle
30
0
100
0
6
9
12 15 18 21 24 27 30 33 36
VIN (V)
Figure 8. SW RDS(ON) vs. Input Voltage
100
400
90
350
80
DUTY CYCLE (%)
RDS(ON) (m)
3 LEDS
300
250
200
VCTRL = Open
VSET = VIN = 12V
150
L = 68µH
RS = 100m
TA = 25°C
VCTRL = Open
70
60
2 LEDS
50
40
30
20
10
100
-40
-15
10
35
60
85
110
Ambient Temperature (C)
Figure 9. SW RDS(ON) vs. Temperature
Figure. 11 SW Output Rise Time
AL8807
Document number: DS35281 Rev. 5 - 2
0
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 10. Duty Cycle vs. Input Voltage
Figure. 12 SW Output Fall Time
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AL8807
Typical Performance Characteristics (cont.) (670LED Current)
350
10
6
4
2
0
-2
-4
-6
-8
-10
300
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
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
250
6
4
2
0
-2
-4
-6
400
350
300
250
200
150
100
50
-8
-10
0
6
6
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 16. Switching Frequency vs. Input Voltage
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 15. LED Current Deviation vs. Input Voltage
9
10
1 LED
2 LEDs
2
0
3 LEDs
4 LEDs
5 LEDs
6 LEDs
-2
-4
7 LEDs
-6
8 LEDs
-8
-10
6
SWITCHING FREQUENCY (kHz)
6
4
9
800
L = 33µH
RS = 150m
TA = 25°C
VCTRL = Open
8
LED CURRENT ERROR (%)
9
600
500
400
300
1 LED
200
7 LEDs
0
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 17. LED Current Deviation vs. Input Voltage
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8 LEDs
5 LEDs
100
9
AL8807
L = 33µH
RS = 150m
TA = 25°C
VCTRL = Open
700
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
March 2013
© Diodes Incorporated
AL8807
Typical Performance Characteristics (cont.) (1A LED Current MSOP-8EP)
10
350
L = 100µH
RS = 100m
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
6
7 LEDs
8 LEDs
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
10
9
350
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
6
4
2
0
-2
-4
-6
300
250
200
150
100
50
-8
-10
6
0
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
9
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
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0
6
9
12 15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 24. Switching Frequency vs. Input Voltage
March 2013
© Diodes Incorporated
AL8807
Typical Performance Characteristics (cont.) (1.3A LED Current MSOP-8EP)
250
10
L = 100µH
RS = 77m
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 = 77m
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
6 LEDs
2 LEDs 3 LEDs 4 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 = 77m
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
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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
March 2013
© Diodes Incorporated
AL8807
Application Information
The AL8807 is a hysteretic (also known as equal ripple) LED driver with integrated power switch. It is available in two packages that provide a PCB
area-power dissipation capability compromise. It is recommended that at higher LED currents/smaller PCBs that the MSOP-8EP version is used to
maximize the allowable LED current over a wider ambient temperature range.
AL8807 Operation
In normal operation, when voltage is applied at +VIN, the AL8807 internal switch is turned on. Current starts to flow through sense resistor R1,
inductor L1, and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage +Vin and the inductor L1.
This rising current produces a voltage ramp across R1. The internal circuit of the AL8807 senses the voltage across R1 and applies a proportional
voltage to the input of the internal comparator.
When this voltage reaches an internally set upper threshold, the internal switch is turned off. The inductor current continues to flow through R1, L1,
the LEDs and the schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined by the forward voltage drop of the
LEDs and the schottky diode.
This decaying current produces a falling voltage at R1, which is sensed by the AL8807. A voltage proportional to the sense voltage across R1 is
applied at the input of the internal comparator. When this voltage falls to the internally set lower threshold, the internal switch is turned on again.
This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor R1.
LED Current Control
The LED current is controlled by the resistor R1 in Figure 30.
Figure 30 Typical Application Circuit
Connected between VIN and SET the nominal average output current in the LED(s) is defined as:
ILED
VTHD
R1
For example for a desired LED current of 660mA and a default voltage VCTRL=2.5V the resulting resistor is:
R1
VTHD
0.1
150m
ILED
0.66
DC Dimming
Further control of the LED current can be achieved by driving the CTRL pin with an external voltage (between 0.4V and 2.5V); the average LED
current becomes:
ILED
VCTRL VTHD
VREF R SET
With 0.5V ≤ VCTRL ≤ 2.5V the LED current varies linearly with VCTRL, as in figure 2. If the CTRL pin is brought higher than 2.5V, the LED current will
V
be clamped to approximately 100% and follows ILED THD .
RSET
When the CTRL voltage falls below the threshold, 0.4V, the output switch is turned off which allows PWM dimming.
AL8807
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AL8807
Application 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 [%]
Figure 31 PWM Dimming at 500Hz
Zooming in at duty cycles below 10% shows:
Figure 32 Low Duty Cycle PWM Dimming at 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
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AL8807
Application 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.
Figure 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
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AL8807
Application Information (cont.)
Diode Selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the
maximum operating voltage and temperature. The Schottky diode also provides better efficiency than silicon PN diodes, due to a combination of
lower forward voltage and reduced recovery time.
It is important to select parts 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 the operating voltage to ensure safe operation
during the switching and a current rating at least 10% higher than the average diode current. The power rating is verified by calculating the power
loss through the diode.
Schottky diodes, e.g. B240 or B140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8807 applications.
Inductor Selection
Recommended inductor values for the AL8807 are in the range 33μH to 100μH.
Higher values of inductance are recommended at higher supply voltages in order to minimize errors due to switching delays, which result in
increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range.
(See graphs).
Figure 34 Inductor value with input voltage and number of LEDs
The inductor should be mounted as close to the device as possible with low resistance/stray inductance connections to the SW pin.
The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean
output current.
Suitable coils for use with the AL8807 are listed in the table below:
Part No.
MSS1038-333
MSS1038-683
NPIS64D330MTRF
L
(µH)
33
68
33
DCR
(V)
0.093
0.213
0.124
ISAT
(A)
2.3
1.5
1.1
Manufacturer
CoilCraft www.coilcraft.com
NIC www.niccomp.com
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times over the supply voltage and load current range.
AL8807
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AL8807
Application Information (cont.)
The following equations can be used as a guide, with reference to Figure 1 - Operating waveforms.
Switch ‘On’ time
Switch ‘Off’ time
L I
tON
VIN VLED IAVG x RS rL RSW
tOFF
LI
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 JA, 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) / JA
where
TJ(MAX) is the maximum operating junction temperature,
TA is the ambient temperature, and
JA 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, JA and device power dissipation.
JA, is layout dependent and package dependent; the AL8807W5’s JA 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.
Figure 35 Derating Curve for Different PCB
AL8807
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AL8807
Application Information (cont.)
EMI and Layout Considerations
The AL8807 is a switching regulator with fast edges and measures small differential voltages; as a result of this care has to be taken with
decoupling and layout of the PCB.To help with these effects the AL8807 has been developed to minimise radiated emissions by controlling the
switching speeds of the internal power MOSFET. The rise and fall times are controlled to get the right compromise between power dissipation due
to switching losses and radiated EMI. The turn-on edge (falling edge) dominates the radiated EMI which is due to an interaction between the
Schottky diode (D1), Switching MOSFET and PCB tracks. After the Schottky diode reverse recovery time of around 5ns has occurred; the falling
edge of the SW pin sees a resonant loop between the Schottky diode capacitance and the track inductance, LTRACK, See figure 36.
Figure 36 PCB Loop Resonance
The tracks from the SW pin to the Anode of the Schottky diode, D1, and then from D1’s cathode to the decoupling capacitors C1 should be as short
as possible. There is an inductance internally in the AL8807 this can be assumed to be around 1nH. For PCB tracks a figure of 0.5nH per mm can
be used to estimate the primary resonant frequency. If the track is capable of handling 1A increasing the thickness will have a minor effect on the
inductance and length will dominate the size of the inductance.
The resonant frequency of any oscillation is determined by the combined
inductance in the track and the effective capacitance of the Schottky diode. An example of good layout is shown in figure 37 - the stray track
inductance should be less than 5nH.
Figure 37 Recommended PCB Layout
AL8807
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AL8807
Application 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.
4.
The Ground return path from C1 must be a low impedance path with the ground plane as large as possible
The LED current sense resistor (R1) has to be placed as close as possible to the VIN and SET pins.
5.
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
6.
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).
Figure 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.
Figure 39 Recommended MR16 PCB Layout
AL8807
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AL8807
Application Information (cont.)
The associated EMI measurements for this board using the AL8807 is shown in figure 40.
Figure 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 25°C. It is
anticipated that the internal junction temperature is approximately 6°C hotter than the surface temperature.
Figure 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.
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
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AL8807
Ordering Information
AL8807 XX - XX
Package
Packing
W5 : SOT25
MP : MSOP-8EP
7 : 7” Tape & Reel
13 : 13” Tape & Reel
Part Number
Status
Package Code
Packaging
AL8807W5-7
AL8807MP-13
New Product
New Product
W5
MP
SOT25
MSOP-8EP
7” Tape and Reel
Quantity
Part Number Suffix
3000/Tape & Reel
-7
2500/Tape & Reel
-13
Marking Information
(1)
SOT25
(Top View)
4
7
5
XX Y W X
1
2
Part Number
AL8807W5-7
(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
Package
SOT25
Identification Code
B6
MSOP-8EP
AL8807
Document number: DS35281 Rev. 5 - 2
Part Number
Package
AL8807MP-13
MSOP-8EP
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Package Outline Dimensions (All dimensions in mm.)
Please see AP02002 at http://www.diodes.com/datasheets/ap02002.pdf for latest version.
(1)
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°
All Dimensions in mm
B C
H
K
J
(2)
M
N
D
L
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
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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
March 2013
© Diodes Incorporated
AL8807
Suggested Pad Layout
Please see AP02001 at http://www.diodes.com/datasheets/ap02001.pdf for the latest version.
(1)
SOT25
C2
Z
C2
Dimensions Value (in mm)
Z
3.20
G
1.60
X
0.55
Y
0.80
C1
2.40
C2
0.95
C1
G
Y
X
(2)
MSOP-8EP
X
C
Dimensions
C
G
X
X1
Y
Y1
Y2
Y
G
Y2
Y1
X1
AL8807
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Value
(in mm)
0.650
0.450
0.450
2.000
1.350
1.700
5.300
March 2013
© Diodes Incorporated
AL8807
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
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website, harmless against all damages.
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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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indirectly, any claim of personal injury or death associated with such unintended or unauthorized application.
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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.
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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
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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
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