AL8807A
HIGH EFFICIENCY LOW EMI WIDE ANALOG
DIMMING RANGE 36V 1A/1.3A BUCK LED DRIVER
Description
Pin Assignments
(Top View)
The AL8807A is a step-down DC/DC converter designed to drive
LEDs with a constant current. The device can drive up to 9 white high
brightness LEDs in series from a voltage source of 6V to 36V.
The AL8807A has an extended CTRL pin voltage range; increasing
its analog dimming range to greater than 10:1. The improved analog
dimming range makes it suitable for a variety of lighting applications
requiring wide analog dimming ranges.
SOT25
The AL8807A switches at frequency up to 1MHz with controlled rise
and fall times to reduce EMI.
(Top View)
This allows the use of small size
external components, hence minimizing the PCB area needed.
Maximum output current of AL8807A is set via an external resistor
connected between the VIN and SET input pins.
Over Temperature Protection is incorporated so that should a fault
occur the device will automatically shut-down and only restart when
SET 1
8 VIN
GND 2
7 N/C
GND 3
6 SW
CTRL 4
5 SW
its junction temperature has cooled down,
MSOP-8EP
Features
Applications
LED Driving Current up to 1A/1.3A
General Illumination Lamps
Better than 5% Accuracy
12V Powered LED Lamps
High Efficiency Up to 96%
Wide Analog Dimming Range LED Lamps
Optimally Controlled Switching Speeds
Operating Input Voltage from 6V to 36V
Wide Analog Input Range for Dimming Control (>10:1)
Built-in Protection Features:
Open-Circuit LED protection
LED Chain Short Circuited
Over-Temperature Protection
MSOP-8EP and SOT25: Available in “Green” Molding Compound
(No Br, Sb) with lead Free Finish/ RoHS Compliant
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 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
AL8807A
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AL8807A
Pin Descriptions
Pin Name
SW
GND
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
CTRL
3
4
SET
4
1
VIN
5
8
EP
—
EP
N/C
—
7
LED current Analog Dimming Control Input. – No PWM dimming function.
Connected to internal 2.5V VREF via 50kΩ resistor. So if left open circuit VCTRL = VREF = 2.5V and 100%
LED current is achieved - giving nominal average output current IOUTnom = 0.1/RS
For Analog dimming drive with analog voltage < 2.5V
(0.25V < VCTRL< 2.5V adjusts output current from 10% to 100% of IOUTnom. Device will dim the LED current
lower than this level but at reduced accuracy. Some devices will not totally turn off the LED current.
Soft-start can be implemented by connecting a capacitor to CTRL pin. The amount of soft-start is
dependent on ramp-up of input supply voltage and capacitor on CTRL pin. See apps section.
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 section for more information.
Exposed pad/TAB.
It should be connected to GND and thermal mass for enhanced thermal impedance.
It should not be used as electrical ground conduction path.
No connection – may be connected to GND.
Functional Block Diagram
AL8807A
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AL8807A
Absolute Maximum Ratings (@TA = +25°C, unless otherwise specified.)
Symbol
ESD HBM
Parameter
Human Body Model ESD Protection
ESD MM
Machine Model ESD Protection
VIN
VSW
VCTRL
ISW-RMS
ISW-PK
TJ
Ratings
2.5
Unit
kV
200
V
-0.3 to +40
V
SW Voltage Relative to GND
-0.3 to +40
V
CTRL Pin Input Voltage
-0.3 to +6.0
V
1.25
1.5
A
Continuous VIN Pin Voltage Relative to GND
SOT25
MSOP-8EP
DC or RMS Switch Current
Peak Switch Current (< 10% duty cycle)
2.5
A
Junction Temperature
150
°C
TLEAD
Lead Temperature Soldering
300
°C
TST
Storage Temperature Range
-65 to +150
°C
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
VIN
VCTRL
fSW
Notes:
Parameter
Min
Max
Unit
Operating Input Voltage
6.0
36
V
CTRL Pin Input Voltage Range for 10% to 100% ANALOG Dimming (Note 4)
0.25
2.50
V
0.7
MHz
1
1.3
A
+125
°C
Maximum Switching Frequency at 100% dimming
ISW
Continuous Switch Current (Note 5)
TJ
Junction Temperature Range
SOT25
MSOP-8EP
-40
4. AL8807A analog dimming range extends below 10% but at reduced LED current accuracies and may not turn completely off. Switching frequencies will
also be increased.
5. Maximum switch current is dependent on power dissipation and junction temperature.
AL8807A
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AL8807A
Electrical Characteristics (@TA = +25°C, unless otherwise specified.)
Symbol
Parameter
Conditions
VINSU
Internal Regulator Start Up Threshold
VINSH
Internal Regulator Hysteresis Threshold VIN falling
IQ
IS
VTH
VTH-H
Output not switching (Note 6)
Input Supply Current
floating
f = 250kHz
VTH-10%
95
CTRL pin floating
Max
Unit
5.9
V
300
mV
350
µA
1.8
5
mA
100
105
mV
100
CTRL pin
Set Threshold Hysteresis
Typ
VIN rising
Quiescent Current
Set Current Threshold Voltage
Min
±20
%
10% Set Current Threshold Voltage
VCTRL = 0.25V
10
15
mV
SET Pin Input Current
VSET = VIN -0.1
16
22
µA
RCTRL
CTRL Pin Input Resistance
Referred to internal reference
50
VREF
Internal Reference Voltage
ISET
RDS(on)
2.5
V
0.25
0.40
MSOP-8EP
0.18
0.35
ISW = 0.3A
tR
SW Rise Time
tF
SW Fall Time
VSENSE = 100 ±20mV, fSW = 250kHz
VSW = 0.1V ~ 12V ~ 0.1V, CL = 15pF
Ω
12
ns
20
ns
VIN = 36V
0.5
μA
150
C
Over-Temperature Hysteresis
25
C
Thermal Resistance Junction-to-Ambient SOT25 (Note 8)
(Note 7)
MSOP-8EP (Note 9)
250
TOTP
Over-Temperature Shutdown
TOTP-Hyst
JA
kΩ
SOT25
On Resistance of SW MOSFET
ISW_Leakage Switch Leakage Current
JL
Thermal Resistance Junction-to-Lead
JC
Thermal Resistance Junction-to-case
Notes:
4
(Note 10)
(Note 11)
69
SOT25 (Note 8)
50
MSOP-8EP (Note 9)
4.3
°C/W
6. AL8807A does not have a low power standby mode but current consumption is reduced when output is not being switched.
7. Refer to Figure 40 for the device derating curve.
8. 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.
9. 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.
10. Dominant conduction path via Gnd pin (pin 2).
11. Dominant conduction path via exposed pad.
AL8807A
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AL8807A
Typical Performance Characteristics (@TA = +25°C, unless otherwise specified.)
80
VSET = VIN = 12V
TA = 25°C
60
ICTRL (µA)
40
20
0
-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 5.5
VCTRL (V)
Figure 1 Supply Current (not switching) vs. Input Voltage
Figure 2 ICTRL vs. VCTRL
2.52
VCTRL = Open
VSET = VIN = 12V
VCTRL (V)
2.51
2.50
2.49
2.48
-40
Figure 3 VCTRL vs Input Voltage
(CTRL pin open circuit)
10
35
60
Ambient Temperature (°C)
85
110
Figure 4 VCTRL vs. Temperature
400
300
VCTRL = Open
VSET = VIN = 12V
SOT25
350
240
300
RDS(ON) (m)
RDS(ON) (m)
-15
180
MSOP-8EP
120
SOT25
250
MSOP-8EP
200
60
0
VCTRL = Open
VSET = VIN
TA = 25°C
6
12
18
24
30
INPUT VOLTAGE
Figure 5 SW RDS(ON) vs. Input Voltage
AL8807A
Document number: DS35990 Rev. 2 - 2
150
36
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100
-40
10
35
60
85
AMBIENT TEMPERATURE (°C)
Figure 6 SW RDS(ON) vs. Temperature
-15
110
March 2013
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AL8807A
Typical Performance Characteristics (cont.) (@TA = +25°C, unless otherwise specified.)
Figure 7 SW Output Rise Time
22%
1.00
0.90
LED Current
16%
0.80
0.70
14%
RS = 150m
12%
VIN = 12V
2 LEDs
RS = 100m
TA = 25°C
1.0
0.60
10%
0.50
8%
0.40
6%
0.30
LED CURRENT (A)
18%
1.2
1.10
LED CURRENT (A)
20%
LED CURRENT ERROR (%)
RS = 100m
VIN = 12V
2 LEDs
L = 68µH
TA = 25°C
Figure 8 SW Output Fall Time
0.8
0.6
L = 33µH
0.4
0.20
4%
RS = 150m
2%
LED Current Error
0.10
0.2
L = 68 ~ 220µH
0.00
0%
RS = 100m
-0.10
2
3
4
5
CTRL VOLTAGE (V)
Figure 9 LED Current (Different Sense Resistor) vs. VCTRL
-2%
0
1
0.0
0
1
2
3
4
5
CTRL VOLTAGE (V)
Figure 10 LED Current (Different Inductor) vs. VCTRL
0.25
VIN = 12V
2 LEDs
RS = 100m
TA = 25°C
LED CURRENT (A)
0.20
0.15
0.10
L = 33µH
0.05
L = 68 ~ 220µH
0.00
0
0.1
0.2
0.3
0.4
0.5
CTRL PIN VOLTAGE (V)
Figure 11 LED Current (Zoomed In) vs. VCTRL
AL8807A
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Figure 12 Switching Frequency vs. VCTRL
March 2013
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AL8807A
Typical Performance Characteristics (cont.) (@TA = +25°C, unless otherwise specified.)
100%
RS = 150m
L = 33µH
TA = 25°C
2 LEDs
CTRL Open
90%
DUTY CYCLE (%)
80%
70%
60%
50%
40%
30%
20%
10%
0%
6
9
12
15 18 21 24 27 30 33
INPUT VOLTAGE (V)
Figure 13 Duty Cycle vs. Input Voltage
36
Figure 14 Efficiency vs. Input Voltage
800
0.36
RS = 300m
L = 68µH
TA = 25°C
CTRL Open
SWITCHING FREQUENCY (kHz)
700
0.35
LED CURRENT (A)
600
500
400
300
1 LED
7 LEDs
200
0.33
0.32
8 LEDs
5 LEDs 6 LEDs
0.31
3 LEDs 4 LEDs
100
0.34
2 LEDs
0
6
9
0.3
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 15 Switching Frequency vs. Input Voltage
0.74
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 16 330mA LED Current vs. Input Voltage
R S = 100m
L = 68µH
T A = 25°C
VCTRL = Open
2 LEDs
4 LEDs
3 LEDs
1.05
0.70
2 LEDs
0.68
9
1.10
RS = 150m
L = 68µH
TA = 25°C
CTRL Open
LED CURRENT (A)
LED CURRENT (A)
0.72
6
1 LED
3 LEDs 4 LEDs
0.66
5 LEDs
6 LEDs
7 LEDs
8 LEDs
0.64
6 LEDs
5 LEDs
7 LEDs
8 LEDs
1 LED
1.00
0.95
0.62
0.60
6
0.90
9
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 17 670mA LED Current vs. Input Voltage
AL8807A
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6
9
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 18 1A LED Current vs. Input Voltage
March 2013
© Diodes Incorporated
AL8807A
Typical Performance Characteristics (670mA LED Current) (@TA = +25°C, unless otherwise specified.)
10
350
L = 100µH
RS = 150m
TA = 25°C
VCTRL = Open
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
8
6
4
2
0
-2
-4
-6
250
200
150
1 LED
100
-10
6
0
9
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 19 LED Current Deviation vs. Input Voltage
6
500
8
450
SWITCHING FREQUENCY (kHz)
10
6
4
2
0
-2
-4
-6
-8
-10
9
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 20 Switching Frequency vs. Input Voltage
L = 68µH
RS = 150m
TA = 25°C
VCTRL = Open
400
350
300
250
200
1 LED
150
100
7 LEDs
8 LEDs
5
LEDs
50
6 LEDs
3 LEDs 4 LEDs
2 LEDs
0
6
9
6
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 21 LED Current Deviation vs. Input Voltage
12
10
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 22 Switching Frequency vs. Input Voltage
L = 33µH
RS = 150m
TA = 25°C
VCTRL = Open
SWITCHING FREQUENCY (kHz)
700
6
4
2
0
-2
-4
-6
600
500
400
300
1 LED
200
7 LEDs
100
-8
-10
9
800
8
LED CURRENT ERROR (%)
7 LEDs 8 LEDs
5 LEDs
6 LEDs
3 LEDs 4 LEDs
2 LEDs
50
-8
LED CURRENT ERROR (%)
300
6
9
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 23 LED Current Deviation vs. Input Voltage
AL8807A
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0
6
8 LEDs
5 LEDs
3 LEDs
4 LEDs
6 LEDs
2 LEDs
9
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 24 Switching Frequency vs. Input Voltage
March 2013
© Diodes Incorporated
AL8807A
Typical Performance Characteristics (1A LED Current) (@TA = +25°C, unless otherwise specified.)
10
350
L = 100µH
RS = 100m
TA = 25°C
VCTRL = Open
2 LEDs
3 LEDs
LED CURRENT ERROR (%)
6
4
SWITCHING FREQUENCY (kHz)
8
6 LEDs
4 LEDs
5 LEDs
8 LEDs
7 LEDs
2
1 LED
0
-2
-4
L = 100µH
RS = 100m
TA = 25°C
V CTRL = Open
-6
-8
300
250
200
150
50
-10
0
6
9
1 LED
100
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 25 LED Current Deviation vs. Input Voltage
10
4 LEDs 5 LEDs 6 LEDs
2 LEDs3 LEDs
6
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 26 Switching Frequency vs. Input Voltage
L = 68µH
RS = 100m
TA = 25°C
V CTRL = Open
SWITCHING FREQUENCY (kHz)
300
LED CURRENT ERROR (%)
12
350
8
6
4
2
0
-2
-4
-6
250
200
150
1 LED
100
50
-8
-10
9
7 LEDs
8 LEDs
6
9
0
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 27 LED Current Deviation vs. Input Voltage
10
7 LEDs 8 LEDs
5 LEDs
6 LEDs
3 LEDs
4
LEDs
2 LEDs
6
9
12
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 28 Switching Frequency vs. Input Voltage
9
12
600
8
SWITCHING FREQUENCY (kHz)
LED CURRENT ERROR (%)
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 29 LED Current Deviation vs. Input Voltage
AL8807A
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0
6
15 18 21 24 27 30 33 36
INPUT VOLTAGE (V)
Figure 30. Switching Frequency vs. Input Voltage
March 2013
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AL8807A
Application Information
AL8807A Operation
The AL8807A is a hysteretic LED current switching regulator sometimes known as an equal ripple switching regulator. In normal operation, when
voltage is applied at +VIN (See Figure 31), the AL8807A 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 (See Figure 32).
This rising current produces a voltage ramp across R1. The internal circuit of the AL8807A 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 AL8807A. 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, with a switching current determined by
the input voltage and LED chain voltage.
In normal operation the off time is relatively constant (determined mainly by the LED chain voltage) with only the on-time varying as the input
voltage changes. At duty cycles up to around 80% the ramp of the LED/switch current is very linear; however, as the duty cycle approaches 95%
the LED current ramp starts to become more exponential. This has two effects:
1.
The overall on time starts to increase lowering the overall switching frequency.
2.
The average LED current starts to increase – which may impact accuracy.
Ch4: LED Current
VIN = 12V
TA =25ºC
2 LEDs
20ns/div
No C2
Ch2: 2V/div
Ch4: 100mA/div
Ch2: SW Pin
Figure 31 Typical Application Circuit
Figure 32 Typical Operating Waveform (C2 not fitted)
LED Current Control
With the CTRL pin open circuit, the LED current is determined by the resistor, R1, (see Figure 31), connected between VIN and SET. The
nominal average output current in the LED(s) is defined as:
ILED
VTH
R1
where VTH is nominally 100mV
For example for a desired LED current of 660mA the resulting resistor is:
R1
AL8807A
Document number: DS35990 Rev. 2 - 2
VTH
0 .1
150m
ILED 0.66
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AL8807A
Application Information (cont.)
Analog Dimming
Further control of the LED current can be achieved by driving the CTRL pin with an external voltage lower than 2.5V; the average LED current
becomes:
ILED
VCTRL VTH
VREF R SET
Where VREF is nominally 2.5V
1.10
The LED current decreases linearly with the CTRL voltage when
VCTRL ≤ 2.5V, as in Figure 9 for 2 different current levels.
1.00
0.90
Note that 100% brightness setting corresponds to VCTRL = VREF,
If a voltage greater than 2.6V is applied to the CTRL pin an internal
clamp is activated which results in the internal reference voltage being
applied to the hysteresis control circuitry. This prevents the LED
current from being overdriven and will still set the LED current to
RS = 100m
0.80
LED CURRENT (A)
nominally 2.5V.
VIN = 12V
2 LEDs
L = 68µH
TA = 25°C
0.70
RS = 150m
0.60
0.50
0.40
0.30
approximately.
ILED
0.20
V
TH
R SET
0.10
0.00
0
1
2
3
4
5
CTRL VOLTAGE (V)
Figure 33 LED Current vs. CTRL Pin Voltage
As the CTRL pin is reduced below 2.5V the sense voltage will proportionally decrease. This means that the time taken for the LED/Switch current
to ramp up to the upper threshold will decrease. The AL8807A, being a hysteretic converter, automatically compensates for the reduction in LED
current by reducing its lower threshold voltage and therefore its off-time. It therefore remains in continuous conduction mode maintaining a better
dimming accuracy than other peak-switch current control topologies.
A result of the reduced on- and off-times results in an increase of the switching frequency. This phenomenon can be seen in Figure 34.
Figure 34 Switching Frequency vs. VCTRL
Ultimately at very small CTRL pin voltages the AL8807A will switch much faster than its nominal switching frequency which due to propagation
delays leads to a non-linear degrading of accuracy.
The degradation in linear dimming accuracy at small CTRL pin voltages can be improved by using larger value inductors which cause the
AL8807A to oscillate at lower frequencies.
A further cause of loss of linearity as small CTRL pin voltages is the internal offsets of the control loop; at a CTRL pin voltage of 0.25V the
nominal LED current sense voltage has been reduced to 10mV.
AL8807A
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AL8807A
Application Information (cont.)
Soft Start
The AL8807A does not have in-built soft-start action; this can be seen in Figure 35.
Figure 35 Start up without any capacitor on CTRL Pin (VIN = 12V, ILED = 667mA, 2 LEDs)
At power–up VIN rises exponentially, due to the bulk capacitor, the internal reference will reach 2.5V before VIN reaches the Under-Voltage LockOut turn-on threshold at around 5.6V. This causes the CTRL pin voltage to rise and reaches 2.5V – 100% LED current - before the AL8807A
fully turns on. When the AL8807A turns on, its output switch turns causing the inductor current to increase until it reaches the upper threshold of
the sense current level and the switching process begins.
Adding an external capacitor from the CTRL pin to ground will provide a soft-start delay (see Figures 36 and 37).
Figure 37 Soft Start with 100nF Capacitor on CTRL Pin
Figure 36 Soft Start
Adding a capacitor to the CTRL pin provides a soft-start by increasing the time taken for the CTRL voltage to rise to 2.5V and by slowing down
the rate of rise of the control voltage at the input of the comparator in the hysteresis control block (refer to Figure 36). This capacitor has 2
effects:
1.
It reduces the minimum start-up current. The bigger the capacitor the lower the CTRL pin voltage will be when UVLO level is
exceeded and the output switch turns on..
2.
The rate at which the inductor/LED current is ramped up is dependent on the size of the capacitor.
As can been seen in Figure 37 adding a capacitor increases the time taken for the output to reach 90% of its final value.
There are many factors which set the initial current and ramp rate. Some practical examples are shown below with conditions
Vin 12V L=68uH 2 LEDs at 667mA,
CDIM
0nF
10nF
22nF
47nF
100nF
470nF
AL8807A
Document number: DS35990 Rev. 2 - 2
Initial Current 90%
80mA
80mA
80mA
80mA
80mA
40mA
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Rise Time
0.45ms
0.55ms
0.8ms
1.8ms
4.2ms
42ms
March 2013
© Diodes Incorporated
AL8807A
Application Information (cont.)
Input Bulk Capacitor Selection
The small size of ceramic capacitors makes them ideal for AL8807A 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 AL8807A; however a 4.7μF input capacitor is suggested for input voltages
approaching 36V.
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 AL8807A applications.
Inductor Selection
Recommended inductor values for the AL8807A are in the range 33μH to 100μH.
Higher values of inductance are recommended at higher supply voltages as they result in lower switching frequencies which in turn reduce the
errors due to switching delays. Higher values of inductance also result in a smaller change in output current over the supply voltage range. (See
graphs).
Figure 38 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 AL8807A are listed in the table below:
L
DCR
ISAT
(µH)
(V)
(A)
MSS1038-333
33
0.093
2.3
MSS1038-683
68
0.213
1.5
NPIS64D330MTRF
33
0.124
1.1
Part No.
AL8807A
Document number: DS35990 Rev. 2 - 2
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March 2013
© Diodes Incorporated
AL8807A
Application Information (cont.)
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times over the supply voltage and load current range.
The following equations can be used as a guide, with reference to Figure 39 – typical switching waveform.
Switch ‘On’ time
tON
LI
VIN VLED IAVG x RS rL RSW
VIN = 12V
TA =25ºC
2 LEDs
20ns/div
SW Pin: 2V/div
Off
Switch ‘Off’ time
tOFF
LI
VLED VD IAVG x RS rL
Where:
On
L is the coil inductance (H)
rL is the coil resistance (Ω)
RS is the current sense resistance (Ω)
Figure 39 Typical Switching Waveform
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) {= 0.25Ω nominal (SOT25)}
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.
1.6
The maximum power dissipation can be calculated using the following
formula:
where
TJ(MAX) is the maximum operating junction temperature,
TA is the ambient temperature,
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 AL8807A’s
junction to ambient thermal resistance,JA and device power dissipation.
JA, is layout dependent and package dependent; the AL8807AW5’s JA on a
POWER DISSIPATION (W)
1.4
PD(MAX) = (TJ(MAX) − TA) / JA
1.2
1.0
SOT25
51mm x 51mm
0.8
0.6
0.4
SOT25
25mm x 25mm
25 x 25mm single layer PCB with 1oz copper standing in still air is
approximately +250°C/W and around 130°C/W on a 51mm x 51mm dual layer
board with maximum coverage top and bottom and 3 vias.
The maximum power dissipation at TA = +25°C can be calculated by the
following formulas:
MSOP-8EP
51mm x 51mm
0.2
0
-40 -25 -10 5 20 35 50 65 80 95 110 125
AMBIENT TEMPERATURE (°C)
Figure 40 Derating Curve for Different PCB
PD(MAX) = (+125°C − +25°C) / (250°C/W) = 0.4W for single-layer 25mm x25mm PCB
PD(MAX) = (+125°C − +25°C) / (130°C/W) =0.77W for dual layer 51mm x 51mm PCB
Figure 40, shows the power derating of the AL8807AW5 on two different PCBs and the AL8807AMP on one PCB.
SOT25 – 25mm x 25mm: AL8807AW5’s JA on a 25 x 25mm single layer PCB with 1oz copper
SOT25 – 25mm x 25mm: AL8807AW5’s JA on a 51mm x 51mm dual layer board with maximum coverage top and bottom and 3 vias
MSOP-8EP - 51mm x 51mm: AL8807AMP’s JA on a 51mm x 51mm dual layer board with maximum coverage top and bottom and 4 vias
Figure 40 shows that the MSOP-8EP version of the AL8807A can handle more power than its SOT25 version. So the AL8807AMP is the
preferred variant when operating at larger supply voltage rails (>24V) and/or driving larger LED currents. This is especially true in high power
density/space constraint applications such as high power 24VAC MR16 applications.
AL8807A
Document number: DS35990 Rev. 2 - 2
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AL8807A
Application Information (cont.)
EMI and Layout Considerations
The AL8807A 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 AL8807A 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 turnon 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 41.
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 AL8807A 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.
Figure 41 PCB Loop Resonance
An example of good layout is shown in Figure 42 - the stray track inductance should be less than 5nH.
VIN
SW
SET
GND
CTRL
Place D1 anode , SW pin and
Inductor as close as possible to
minimize ringing
Figure 42 Recommended PCB Layout
Recommendations for minimising radiated EMI and other transients and thermal considerations are:
1.
2.
3.
4.
5.
6.
The decoupling capacitor (C1) has to be placed as close as possible to the VIN pin and D1 Cathode.
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.
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.
The majority of the conducted heat from the AL8807A 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.
AL8807A
Document number: DS35990 Rev. 2 - 2
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AL8807A
Application Information (cont.)
Fault Condition Operation
Open circuit LEDs
The AL8807A has by default open LED protection. If the LEDs should become open circuit the AL8807A will stop oscillating; the SET pin will rise
to VIN and the SW pin will then fall to GND. No excessive voltages will be seen by the AL8807A.
LED Chain Shorted Together
If the LED chain should become shorted together (the anode of the top LED becomes shorted to the cathode of the bottom LED) the AL8807A will
continue to switch and the current through the AL8807A’s internal switch will still be at the expected current - so no excessive heat will be
generated within the AL8807A. However, the duty cycle at which it operates will change dramatically and the switching frequency will most likely
decrease. See Figure 43 for an example of this behavior at 24V input voltage driving 3 LEDs.
The on-time of the internal power MOSFET switch is significantly reduced because almost all of the input voltage is now developed across the
inductor. The off-time is significantly increased because the reverse voltage across the inductor is now just the Schottky diode voltage (See Figure
43) causing a much slower decay in inductor current.
Figure 43 Switching Characteristics (normal operation to LED chain shorted out)
High Temperature Operation and Protection
The AL8807A is a high efficiency switching LED driver capable of operating junction temperatures up to +125°C. This allows it operate with
ambient temperature in excess of 100°C given the correct thermal impedance to free air. If a fault should occur that leads to increased ambient
temperatures and hence junction temperature then the Over-Temperature Protection (OTP) of the AL8807A will cut in turning the output of the
AL8807A off. This will allow the junction temperature of the AL8807A to cool down and potentially giving an opportunity for the fault to clear itself.
The OTP shutdown junction temperature of the AL8807A is approximately +150°C with a hysteresis of +25°C. This means that the AL8807A will
never switch-off with a junction temperature below +125°C allowing the designer to design the system thermally to fully utilize the wide operating
junction temperature of the AL8807A.
AL8807A
Document number: DS35990 Rev. 2 - 2
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AL8807A
Ordering Information
7” Tape and Reel
Part Number Suffix
Part Number
Status
Package Code
Packaging
AL8807AW5-7
Preview (Note 11)
W5
SOT25
3000/Tape & Reel
-7
AL8807AMP-13
New Product
MP
MSOP-8EP
2500/Tape & Reel
-13
Note:
Quantity
11. Expected release in 4Q 2012.
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
AL8807AW5-7
Package
SOT25
Identification Code
C6
(2) MSOP-8EP
Part Number
AL8807AMP-13
AL8807A
Document number: DS35990 Rev. 2 - 2
Package
MSOP-8EP
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AL8807A
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
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
AL8807A
Document number: DS35990 Rev. 2 - 2
18 of 20
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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
AL8807A
Suggested Pad Layout
Please see AP02001 at http://www.diodes.com/datasheets/ap02001.pdf for latest version.
(1) SOT25
C2
Dimensions Value (in mm)
Z
3.20
G
1.60
X
0.55
Y
0.80
C2
C1
Z
C2
C1
G
2.40
0.95
Y
X
(2) MSOP-8EP
X
C
Y
G
Dimensions
Y2
C
G
X
X1
Y
Y1
Y2
Y1
X1
AL8807A
Document number: DS35990 Rev. 2 - 2
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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
AL8807A
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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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final and determinative format released by Diodes Incorporated.
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written approval of the Chief Executive Officer of Diodes Incorporated. As used herein:
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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
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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
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Copyright © 2013, Diodes Incorporated
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