AL9902

AL9902
LINEAR AND PWM DIMMING HIGH VOLTAGE LED DRIVER
Description
Pin Assignments
The AL9902 high-voltage PWM LED driver provides an efficient
solution for offline high-brightness LED lamps from rectified line
voltages ranging from 85VAC up to 277VAC. The AL9902 has an
SW
NC
SO
GND
CS
VIN
12
11
10
9
8
77
internal MOSFET that allows switching frequencies up to 300kHz,
with the switching frequency determined by an external single
resistor. The AL9902 topology creates a constant current through the
LEDs providing constant light output. The output current is
programmed by one external resistor.
EP1
The LED brightness can be varied by both linear and PWM dimming
using the AL9902’s LD and PWM pins respectively. The PWM input
operates with duty ratio of 0-100% and frequency of up to several
kHz.
11
2
NC
NC
EP2
3
4
PWM VDD
5
66
LD
Rosc
U-DFN6040-12
The AL9902 is available in the thermally enhanced U-DFN6040-12
and SO-16 packages.
NC
SO
NC
16
15
14
1
2
NC
NC
NC
GND
CS
13
12
11
10
NC
3
4
5
6
7
SW
NC
PWM VDD
VIN
9
Features


>90% Efficiency
Universal Rectified 85 to 277VAC Input Range










Internal MOSFET Up to 650V, 2A
High Switching Frequency Up to 300kHz
Internal Voltage Regulator Removes Start-Up Resistor
7.5V Regulated Output
Tighter Current Sense Tolerance Better Than ±5%
LED Brightness Control with Linear and PWM Dimming
Internal Over-Temperature Protection (OTP)
U-DFN6040-12 and SO-16 Packages
Totally Lead-Free & Fully RoHS compliant (Notes 1 & 2)
Halogen and Antimony Free. “Green” Device (Note 3)
Notes:
LD
8
Rosc
SO-16
Applications





LED Offline Lamps
High Voltage DC-DC LED Driver
Signage and Decorative LED Lighting
Back Lighting of Flat Panel Displays
General Purpose Constant Current Source
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
1
AC+
AC-
Z1
602V
2
HD06
3
C1
C3
4
D1
VIN
VDD
LD
AL9902
PWM
GND
C2
AL9902
Document number: DS37878 Rev. 1 - 2
Rosc
SW
SO
CS
L1
Rosc
Rsense
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AL9902
Pin Descriptions
Pin Name
NC
NC
PWM
U-DFN5040-10
1
2
3
SO-16
Functions
14
No connection
1, 2, 4, 10,16 No connection
5
Low Frequency PWM Dimming pin, also Enable input. Internal 200kΩ pull-down to GND
Internally regulated supply voltage.

7.5V nominal.
6
Can supply up to 1 mA for external circuitry. A sufficient storage capacitor is used to provide
storage when the rectified AC input is near the zero crossing.
Linear Dimming input. Changes the current limit threshold at current sense comparator and
changes the average LED current.
7
VDD
4
LD
5
ROSC
6
8
VIN
CS
GND
NC
SO
SW
EP1
7
9
8
9
11
10
12
EP1
EP2
EP2
11
12
13
15
3
NA
NA
Oscillator control.
A resistor connected between this pin and ground puts the AL9902 into fixed frequency mode and
sets the switching frequency.
Input voltage
Senses LED string and internal MOSFET switch current
Device ground
No connection
Source of the internal MOSFET Switch
Drain of the internal MOSFET switch.
Exposed Pad 1(bottom). Drain connection of internal power MOSFET.
Exposed Pad 2 (bottom). Substrate connection of control IC. Connect to GND directly underneath
the package and large PCB area to minimize junction to ambient thermal impedance.
Functional Block Diagram & Typical Application
VIN
VIN
VDD
LDO
OSC
7.5V
D1
Rosc
VDD
250mV
S
SW
Rosc
R
LD
SO
CS
OTP
PWM
Rsense
200K
AL9902
GND
AL9902
Document number: DS37878 Rev. 1 - 2
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AL9902
Absolute Maximum Ratings (Note 4) (@TA = +25°C, unless otherwise specified.)
Symbol
Ratings
Unit
Maximum Input Voltage, VIN, to GND
-0.5 to +520
V
VCS
Maximum CS Input Pin voltage Relative to GND
-0.3 to +0.45
V
VLD
Maximum LD Input Pin Voltage Relative to GND
-0.3 to (VDD +0.3)
V
Maximum PWM input Pin Voltage Relative to GND
-0.3 to (VDD +0.3)
V
-0.5 to +650
V
VIN(MAX)
VPWM
Parameter
VSW
Maximum MOSFET Drain Pin Voltage Relative to GND
VSO
Maximum MOSFET Source Pin Voltage Relative to GND
-0.5 to (VDD +0.3)
V
VGate
Maximum MOSFET GATE pin Voltage Relative to GND
-0.5 to (VDD +0.3)
V
8.1
V
VDD(MAX)
Maximum VDD Pin Voltage Relative to GND
-
Continuous Power Dissipation (TA = +25C)
-
U-DFN6040-12 (derate 10mW/C above +25C)
-
-
1,000
mW
TJ
Junction Temperature Range
+150
°C
TST
ESD HBM
Storage Temperature Range
-65 to +150
°C
2,000
V
Notes:
Human Body Model ESD Protection (Note 5)
4. Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional
operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
All voltages are with respect to Ground. Currents are positive into, negative out of the specified terminal.
5. 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.
Maximum Ratings of Internal MOSFET (@TA = +25°C, unless otherwise specified.)
Characteristic
Drain-Source Voltage
Gate-Source Voltage
Continuous Drain Current (Note 5) VGS = 10V
Steady
State
TC = +25°C
TC = +100°C
Symbol
Value
Units
VDSS
650
V
VGSS
±30
V
ID
1.6
1
A
A
Pulsed Drain Current (Note 6)
IDM
3
Avalanche Current (Note 7) VDD = 100V, VGS = 10V, L = 60mH
IAR
0.8
A
Repetitive Avalanche Energy (Note 7) VDD = 100V, VGS = 10V, L = 60mH
EAR
22
mJ
Peak Diode Recovery
dv/dt
5
V/ns
Recommended Operating Conditions (@TA = +25°C, unless otherwise specified.)
Symbol
Min
Max
Input DC Supply Voltage Range
20
500
V
TA
Ambient Temperature Range
-40
+105
°C
ISW
Switch Pin Output Current
-
0.4
A
VDD
Maximum Recommended Voltage Applied to VDD Pin (Note 6)
-
8.1
V
VINDC
Parameter
VPWM(lo)
Pin PWM input Low Voltage
0
1
VPWM(hi)
Pin PWM input High Voltage
2.4
VDD
Note:
Unit
V
6. when using the AL9902 in isolated LED lamps an auxiliary winding might be used.
AL9902
Document number: DS37878 Rev. 1 - 2
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AL9902
Electrical Characteristics
(@TA = +25°C, unless otherwise specified.)
Specifications apply to AL9902 unless otherwise specified
Symbol
Parameter
Ishdn
Shut-Down Mode Supply Current
VDD
Internally Regulated Voltage
IDD(ext)
Conditions
Pin PWM to GND,
VIN = 20V
VIN = VIN(MIN) ~ 500V, (Note 8) lDD(ext) = 0,
Gate pin open
Min
Typ
Max
Unit
-
0.5
1
mA
7.2
7.5
8.1
V
-
-
1.0
mA
6.4
6.7
7.2
V
VDD Current Available for External Circuitry VIN = 20 to 100V (Note 7)
UVLO
VDD Under Voltage Lockout Threshold
VDD rising
∆UVLO
VDD Under Voltage Lockout Hysteresis
VDD falling
-
500
-
mV
RPWM
PWM Pull-Down Resistance
VPWM= 5V
150
200
250
kΩ
VT
MOSFET Threshold Voltage
ISW = 0.5A
-
4
-
V
VFD
MOSFET Diodes Forward Voltage
ID = 0.5A
-
0.85
-
V
Current Sense Threshold Voltage
TA = -40°C to +125°C
mV
VCS(hi)
Oscillator Frequency
fOSC
237.5
250
262.5
ROSC = 1MΩ
20
25
30
ROSC = 226kΩ
80
100
120
kHz
DMAXhf
Maximum Oscillator PWM Duty Cycle
fPWMhf = 25kHz, at GATE, CS to GND.
-
-
100
%
VLD
Linear Dimming Pin Voltage Range
TA = <125°C, VIN = 20V
0
-
250
mV
TSD
Thermal Shut-Down (Junction)
Use DFN JA when ISW=0.4A, VDS=1V
-
141
-
TSDH
Thermal Shut-Down Hysteresis
-
-
25
-
-
65
-
C/W
-
5
-
C/W
-
100
-
C/W
-
15
-
C/W
JA
Thermal Resistance Junction-to-Ambient
JC
Thermal Resistance Junction-to-Case
JA
Thermal Resistance Junction-to-Ambient
JC
Thermal Resistance Junction-to-Case
Notes:
U-DFN6040-12 (Note 8)
SO-16
°C
7. Also limited by package power dissipation limit, whichever is lower.
8. Device mounted on FR-4 PCB (25mm x 25mm 1oz copper, minimum recommended pad layout on top. For better thermal performance, larger
copper pad for heat-sink is needed.
Internal MOSFET Characteristic
OFF CHARACTERISTICS (Note 9)
Symbol
Min
Typ
Max
Unit
Test Condition
Drain-Source Breakdown Voltage
BVDSS
650
—
—
V
VGS = 0V, ID = 250μA
Zero Gate Voltage Drain Current
IDSS
—
—
1
µA
VDS = 650V, VGS = 0V
Gate-Source Leakage
IGSS
—
—
±100
nA
VGS = ±30V, VDS = 0V
VGS(th)
3
—
5
V
VDS = VGS, ID = 250μA
RDS (ON)
—
4
5
Ω
VGS = 10V, ID = 1A
VSD
—
0.7
1
V
VGS = 0V, IS = 1A
Ciss
—
479
—
pF
Output Capacitance
Coss
—
29
—
pF
Reverse Transfer Capacitance
Crss
—
pF
Rg
—
1.9
2
—
Gate Resistance
—
Ω
Total Gate Charge
Qg
—
14
—
nC
Gate-Source Charge
Qgs
—
2.5
—
nC
Gate-Drain Charge
Qgd
—
7.3
—
nC
Turn-On Delay Time
tD(on)
—
17
—
ns
Turn-On Rise Time
tr
—
33
—
ns
Turn-Off Delay Time
tD(off)
—
31
—
ns
Turn-Off Fall Time
tf
—
25
—
ns
Body Diode Reverse Recovery Time
trr
—
174
—
ns
Qrr
—
884
—
nC
ON CHARACTERISTICS (Note 9)
Gate Threshold Voltage
Static Drain-Source On-Resistance
Diode Forward Voltage
DYNAMIC CHARACTERISTICS (Note 10)
Input Capacitance
Body Diode Reverse Recovery Charge
Notes:
VDS = 25V, VGS = 0V,
f = 1MHz
VDS = 0V, VGS = 0V, f = 1MHz
VDS = 520V, VGS = 10V,
ID = 2A
VDS = 325V, VGS = 10V,
RG = 25Ω, ID = 2.5A
VDS = 100V, IF = 2A,
di/dt = 100A/μs
9. Short duration pulse test used to minimize self-heating effect.
10. Guaranteed by design. Not subject to production testing.
AL9902
Document number: DS37878 Rev. 1 - 2
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AL9902
3.0
460
2.5
440
2.0
420
INPUT CURRENT (µA)
CURRENT SENSE THRESHOLD (mV)
Typical Characteristics
1.5
1.0
0.5
0.0
V IN = 15V
380
360
340
300
-1.0
280
-40
-1.5
-40
-15
10
35
60
85
AMBIENT TEMPERATURE (°C)
Change in Current Sense Threshold vs. Ambient Temperature
-15
10
35
60
AMBIENT TEMPERATURE (°C)
85
Input Current vs. Ambient Temperature
450
1.0

0.5
ROSC = 226k
-0.5
R OSC = 1M 
-1.0
-1.5
-2.0
-40
-15
10
35
60
85
AMBIENT TEMPERATURE (°C)
Change in Oscillation Frequency vs. Ambient Temperature
SHORT CIRCUIT OUTPUT CURRENT (mA)
1.5
CHANGE IN FREQUENCY (%)
400
320
-0.5
0.0
V IN = 400V
ILED(NOM) = 180mA
400
350
300
250
200
150
85 105 125 145 165 185 205 225 245 265
INPUT VOLTAGE (VRMS)
180mA LED Driver Short Circuit Output Current vs. Input Voltage
IOUT vs. VLD Dimming Control
AL9902
Document number: DS37878 Rev. 1 - 2
IOUT vs. PWM Dimming Control at 1KHz
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AL9902
Typical Characteristics (continued)
180mA LED Driver Output Current vs. Input Voltage
180mA LED Driver Power Factor vs. Input Voltage
AL9902
Document number: DS37878 Rev. 1 - 2
180mA LED Driver Efficiency vs. Input Voltage
180mA LED Driver Input Power Dissipation vs. Input Voltage
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AL9902
Typical Characteristics (cont.)
2.0
1.8
VDS = 20V
VGS = 10V
VGS = 6.0V
ID, DRAIN CURRENT (A)
VGS = 8.0V
1.6
ID, DRAIN CURRENT (A)
10
VGS = 20V
1.4
1.2
1.0
VGS = 5.5V
0.8
0.6
1
0.1
TA = 150°C
0.01
0.4
TA = 85°C
0.2
TA = -55°C
VGS = 5.0V
0
1
2
3
4
5
6
7
8
9
VDS, DRAIN-SOURCE VOLTAGE (V)
Figure
1 Typical
Characteristics
Typical
Output Output
Characteristics
10
0.001
4.8
4.6
4.4
4.2
VGS = 10V
4
3.8
3.6
3.4
3.2
3
0
1
2
3
4
5
6
7
VGS, GATE-SOURCE VOLTAGE (V)
Typical
Transfer
Characteristics
Figure
2 Typical
Transfer
Characteristics
8
20
5
R DS(ON), DRAIN-SOURCE ON-RESISTANCE ()
RDS(ON), DRAIN-SOURCE ON-RESISTANCE ()
0.0
TA = 25°C
TA = 125°C
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
ID, DRAIN-SOURCE CURRENT (A)
Figure 3 Typical On-Resistance vs.
Drain vs.
Current
Gateand
Voltage
Typical On-Resistance
Drainand
Current
Gate Voltage
AL9902
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18
16
14
ID = 1.0A
12
10
8
6
4
2
0
4
6
8
10
12
14
16
18
20
VGS, GATE-SOURCE VOLTAGE (V)
Figure 4 Typical Transfer Characteristics
Typical Transfer Characteristics
July 2015
© Diodes Incorporated
AL9902
3
15
VGS = 10V
12
RDS(ON), DRAIN-SOURCE
ON-RESISTANCE (NORMALIZED)
RDS(ON), DRAIN-SOURCE ON-RESISTANCE ()
Typical Characteristics (cont.) Internal MOSFET
TA = 150°C
TA = 125°C
9
TA = 85°C
6
TA = 25°C
3
TA = -55°C
0
2
VGS = 10 V
ID = 1A
1.5
1
0.5
0
-50
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
ID, DRAIN CURRENT (A)
Typical On-Resistance
DrainOn-Resistance
Current and Temperature
Figure 5vs.
Typical
vs.
Drain Current and Temperature
0
-25
0
25
50
75 100 125 150
TJ, JUNCTION TEMPERATURE ( C)
On-Resistance
Variation
withwith
Temperature
Figure
6 On-Resistance
Variation
Temperature
15
5
VGS(th), GATE THRESHOLD VOLTAGE (V)
RDS(ON), DRAIN-SOURCE ON-RESISTANCE ()
VGS = 20V
ID = 2A
2.5
12
VGS = 20V
ID = 2A
9
VGS = 10V
ID = 1A
6
3
0
-50
AL9902
Document number: DS37878 Rev. 1 - 2
ID = 250µA
ID = 1mA
4
3.5
3
2.5
2
-25
0
25
50
75 100 125 150
TJ, JUNCTION TEMPERATURE (C)
Figure 7 On-Resistance Variation with Temperature
On-Resistance Variation with Temperature
4.5
-50
-25
0
25
50
75 100 125 150
TJ, JUNCTION TEMPERATURE ( C)
Figure 8 Gate Threshold Variation vs. Ambient Temperature
Gate Threshold Variation vs. Ambient Temperature
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AL9902
Typical Characteristics (cont.) Internal MOSFET
2
1000
Ciss
CT, JUNCTION CAPACITANCE (pF)
1.8
IS, SOURCE CURRENT (A)
1.6
1.4
TA = 150°C
1.2
T A = 25°C
1
TA = 125°C
0.8
TA = -55°C
0.6
TA = 85°C
0.4
100
Coss
10
Crss
0.2
f = 1MHz
0
0
1
0.3
0.6
0.9
1.2
1.5
V SD, SOURCE-DRAIN VOLTAGE (V)
Figure
Diode Forward
vs. Current
Diode9Forward
VoltageVoltage
vs. Current
0
5
10
15
20
25
30
35
VDS, DRAIN-SOURCE VOLTAGE (V)
Typical10Typical
Junction Junction
Capacitance
Figure
Capacitance
40
10
10
8
ID, DRAIN CURRENT (A)
VGS GATE THRESHOLD VOLTAGE (V)
RDS(on)
Limited
6
VDS = 520V
ID = 2A
4
1
DC
PW = 1s
PW = 100ms
PW = 10ms
0.01
2
0
0
2
4
6
8
10
12
14
Qg, TOTAL GATE CHARGE (nC)
Figure 11 Gate Charge
Gate Charge
AL9902
Document number: DS37878 Rev. 1 - 2
16
PW = 10s
0.1
0.001
1
TJ(max) = 150°C
TA = 25°C
VGS = 10V
Single Pulse
DUT on 1 * MRP Board
PW = 1ms
PW = 100µs
10
100
VDS, DRAIN-SOURCE VOLTAGE (V)
Figure 12 SOA, Safe Operation Area
SOA, Safe Operation Area
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AL9902
Applications Information
DC-DC and Offline LED Driver
The AL9902 is a cost-effective offline buck LED driver-converter specifically designed for driving LED strings. It is suitable for being used with
either rectified AC line or any DC voltage between 5-500V. See Figure 1 for typical circuit.
1
AC+
2
Z1
602V
AC-
HD06
3
C1
C3
4
D1
VIN
VDD
LD
AL9902
PWM
GND
C2
Rosc
SW
SO
CS
L1
Rosc
Rsense
Figure 1 Typical Application Circuit (without PFC)
Buck Design Equations:
D
VLEDs
VIN
t ON 
L
D
f osc
( VIN  VLEDs )  t ON
0.3  ILED
RSENSE 
0.25
Where ILED x 0.3 = IRIPPLE
ILED  (0.5  (ILED  0.3))
Design Example
For an AC line voltage of 120V, the nominal rectified input voltage VIN = 120V x 1.41 = 169V. From this and the LED chain voltage the duty cycle
can be determined:
D = VLEDs /VIN = 30/169 = 0.177
From the switching frequency, for example fOSC = 50 kHz, the required on-time of the internal MOSFET can be calculated:
tON = D/fOSC = 3.5 µs
The value of the inductor is determined as follows:
L = (VIN - VLEDs) * tON / (0.3 * ILED) = 4.6mH
AL9902
Document number: DS37878 Rev. 1 - 2
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AL9902
Applications Information (continued)
Input Bulk Capacitor
For offline lamps, an input bulk capacitor is required to ensure that the rectified AC voltage is held above twice the LED string voltage throughout
the AC line cycle. The value can be calculated from:
CIN 
Pin  (1  Dch )
2  VLine _ min  2fL  VDC _ max
Where
Dch : Capacity charge work period, generally about 0.2~0.25
fL : Input frequency for full range (85~265VRMS)
VDC _ max Should be set 10~15% of
2 VLine _ min
If the capacitor has a 15% voltage ripple then a simplified formula for the minimum value of the bulk input capacitor approximates to:
I
 VLEDs  0.06
CMIN = LED
VIN2
Power Factor Correction
If power factor improvement is required then for the input power less than 25W, a simple method for improving the power factor can be
implemented by potential dividing down the rectified mains voltage (resistors R1 and R2 in Figure 2) and feeding it into the LD pin. The current
drawn from the supply voltage will follow an approximate half sine wave. A filter across the LEDs reduces the potential for flicker. This circuit also
significantly reduces the size of input capacitors.
L1
1
AC+
Z1
602V
2
LED+
HD06
AC-
C1
4
R2
C4
C2
D4
LD
VDD
R1
VIN
AL9902
PWM
GND
C3
Rosc
LED-
SW
SO
CS
L2
Rosc
Rsense
Figure 2 Typical Application Circuit with Simple PFC
Passive power factor correction using 3 high voltage diodes and 2 identical capacitors can be implemented. For further design information please
see AN75 from the Diodes website.
DC-DC Buck LED Driver
The design procedure for an AC input buck LED driver outlined in the previous chapters equally applies to DC input LED drivers.
AL9902
Document number: DS37878 Rev. 1 - 2
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AL9902
Applications Information (cont.)
DC-DC Boost LED Driver
Due to the topology of the AL9902 LED driver-converter, it is capable of being used in boost configurations as shown in Figure 3 – at reduced
accuracy. The accuracy can be improved by measuring the LED current with an op amp and use the op amp’s output to drive the LD pin.
A Boost LED driver is used when the forward voltage drop of the LED string is higher than the input supply voltage. For example, the Boost
topology can be appropriate when input voltage is supplied by a 48V power supply and the LED string consists of twenty HB LEDs, as the case
may be for a street light.
L1
VIN
VDD
VIN
PWM
C1
AL9902
LD
GND
C2
Rosc
SW
SO
CS
D1
Rosc
C3
Rsense
Figure 3 Boost LED driver
In a Boost converter, when the internal MOSFET is ON, the energy is stored in the inductor which is then delivered to the output when the internal
MOSFET switches OFF. If the energy stored in the inductor is not fully depleted by the next switching cycle (continuous conduction mode) the DC
conversion between input and output voltage is given by:
VOUT 
VIN
V
 VIN
, D  OUT
1 D
VOUT
From the switching frequency, fOSC, the on-time of the MOSFET can be calculated:
t ON 
D
fOSC
From this the required inductor value can be determined by:
L
VIN  t ON
0.3  ILED
The Boost topology LED driver requires an output capacitor to deliver current to the LED string during the time that the internal MOSFET is on.
In boost LED driver topologies, if the LEDs should become open circuit damage may occur to the power switch and so some form of detection
should be present to provide overvoltage detection/protection.
AL9902
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AL9902
Applications Information (cont.)
General Application
The AL9902 is capable of operating in isolated or non-isolated topologies. It can also be made to operate in continuous as well as discontinuous
conduction mode.
VIN
VIN
VDD
LDO
OSC
7.5V
D1
Rosc
VDD
250mV
S
SW
Rosc
R
LD
SO
CS
OTP
Rsense
PWM
200K
AL9902
GND
Figure 4 Typical Application Circuit
The AL9902 contains a high-voltage LDO (see figure 4), the output of the LDO provides a power rail to the internal circuitry including the gate
driver. A UVLO on the output of the LDO prevents incorrect operation at low input voltage to the VIN pin.
In a non-isolated Buck LED driver when the gate pin goes high, the internal power MOSFET (Q1) is turned on causing current to flow through the
LEDs, inductor (L1) and current sense resistor (RSENSE). When the voltage across RSENSE exceeds the current sense pin threshold, the internal
MOSFET Q1 is turned off. The energy stored in the inductor causes the current to continue to flow through the LEDs via diode D1.
The AL9902’s LDO provides all power to the rest of the IC including gate drive and this removes the need for large high power start-up resistors.
This means that to during normal operation the AL9902 requires around 0.5mA from the high voltage power rail. The LDO can also be used to
supply up to 1mA to external circuits.
The AL9902 operates and regulates by limiting the peak current of the internal MOSFET; the peak current sense threshold is nominally set at
250mV. The AL9902 is capable of operating in a fixed frequency (PWM) mode and also variable frequency (fixed off-time) mode to regulate the
LED current.
The same basic operation is true for isolated topologies; however in these the energy stored in the transformer delivers energy to LEDs during the
off-cycle of the internal MOSFET.
The on-resistance of the AL9902’s internal power MOSFET means that it can drive up to 2A.
Design Parameters
Setting the LED Current
In the non-isolated buck converter topology, figure 4, the average LED current is not the peak current divided by 2 - however, there is a certain
error due to the difference between the peak and the average current in the inductor. The following equation accounts for this error:
R SENSE 
250mV
ILED  (0.5 * IRIPPLE ))
AL9902
Document number: DS37878 Rev. 1 - 2
.
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AL9902
Applications Information (cont.)
Setting Operating Frequency
The AL9902 is capable of operating between 25 and 450kHz switching frequency range. The switching frequency is programmed by connecting
an external resistor between ROSC pin and ground. The corresponding oscillator period is:
tOSC =
R osc  22
µs
25
with ROSC in kΩ
The switching frequency is the reciprocal of the oscillator period. Typical values for R OSC vary from 75kΩ to 1MΩ.
In buck mode the duty cycle, D, is
VLEDs
; so when driving small numbers of LEDs from high-input voltages, the duty cycle will be reduced and
VIN
care should be taken to ensure that tON > tBLANK. The simplest way to do this is to reduce/limit the switching frequency by increasing the R OSC
value. Reducing the switching frequency will also improve the efficiency.
When operating in buck mode, the designer must keep in mind that the input voltage must be maintained higher than two times the forward
voltage drop across the LEDs. This limitation is related to the output current instability that may develop when the AL9902 operates at a duty cycle
greater than 0.5. This instability reveals itself as an oscillation of the output current at a sub-harmonic (SBO) of the switching frequency.
Inductor Selection
The non-isolated buck circuit, Figure 4, is usually selected and has two operation modes: continuous and discontinuous conduction modes. A
buck power stage can be designed to operate in continuous mode for load current above a certain level usually 15% to 30% of full load. Usually
the input voltage range, the output voltage and load current are defined by the power stage specification. This leaves the inductor value as the
only design parameter to maintain continuous conduction mode. The minimum value of inductor to maintain continuous conduction mode can be
determined by the following example.
The required inductor value is determined from the desired peak-to-peak LED ripple current in the inductor; typically around 30% of the nominal
LED current.
L=
VIN  VLEDs   D
0.3  ILED   fOSC
, where D is duty cycle
The next step is determining the total voltage drop across the LED string. For example, when the string consists of 10 High-Brightness LEDs and
each diode has a forward voltage drop of 3.0V at its nominal current; the total LED voltage V LEDS is 30V.
Dimming
The LED brightness can be dimmed either linearly (using the LD pin) or via pulse width modulation (using the PWM-D pin); or a combination of
both - depending on the application. Pulling the PWM pin to ground will turn off the AL9902. When disabled, the AL9902’s quiescent current is
typically 0.5mA. Reducing the LD voltage will reduce the LED current but it will not entirely turn off the external power transistor and hence the
LED current – this is due to the finite blanking period. Only the PWM pin will turn off the power transistor.
Linear dimming is accomplished by applying a 45 to 250mV analog signal to the LD pin. This overrides the default 250mV threshold level of the
CS pin and reduces the output current. If an input voltage greater than 250mV is applied to the LD then the output current will not change.
The LD pin also provides a simple cost-effective solution to soft start; by connecting a capacitor to the LD pin down to ground at initial power up,
the LD pin will be held low causing the sense threshold to be low. As the capacitor charges up the current sense threshold will increase thereby
causing the average LED current to increase.
PWM dimming is achieved by applying an external PWM signal to the PWM pin. The LED current is proportional to the PWM duty cycle and the
light output can be adjusted between 0 and 100%. The PWM signal enables and disables the AL9902 - modulating the LED current. The ultimate
accuracy of the PWM dimming method is limited only by the minimum gate pulse width, which is a fraction of a percentage of the low frequency
duty cycle. PWM dimming of the LED light can be achieved by turning on and off the converter with low frequency 50Hz to 1000Hz TTL logic level
signal.
With both modes of dimming it is not possible to achieve average brightness levels higher than the one set by the current sense threshold level of
the AL9902. If a greater LED current is required then a smaller sense resistor should be used.
Output Open Circuit Protection
The non-isolated buck LED driver topology provides inherent protection against an open circuit condition in the LED string due to the LEDs being
connected in series with the inductor. Should the LED string become open circuit then no switching occurs and the circuit can be permanently left
in this state with damage to the rest of the circuit.
AL9902
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AL9902
Ordering Information
AL9902 XXX - 13
Package
Packing
FDF : U-DFN6040-12
S16 : SO-16
13 : Tape & Reel
Part Number
VCS Tolerance
Package Code
Packaging
AL9902FDF-13
AL9902S16-13
±5%
±5%
FDF
S16
U-DFN6040-12
SO-16
13” Tape and Reel
Quantity
Part Number Suffix
3,000/Tape & Reel
-13
3,000/Tape & Reel
-13
Marking Information
( Top View )
Logo
Part Number
AL9902
Document number: DS37878 Rev. 1 - 2
AL9902
YY WW X X
YY : Year : 15, 16,17~
WW : Week : 01~52; 52
represents 52 and 53 week
X X : Internal Code
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AL9902
Package Outline Dimensions (All dimensions in mm.)
Please see AP02002 at http://www.diodes.com/datasheets/ap02002.pdf for latest version.
U-DFN6040-12
A1
A3
A
U-DFN6040-12
Dim Min Max Typ
A 0.55 0.65 0.60
A1
0
0.05 0.02
A3
0.15
b 0.35 0.45 0.40
D 5.95 6.05 6.00
D1 1.95 2.15 2.05
D2 2.35 2.55 2.45
e
1.00
E 3.95 4.05 4.00
E1 2.10 2.30 2.20
E2 1.80 2.00 1.90
L 0.35 0.45 0.40
Z
0.30
All Dimensions in mm
Seating Plane
D
e
D2
D1
E
E2
E1
L
b
Z
SO-16
H
E
Gauge Plane
L

Detail ‘A’
D
A
A2
B
AL9902
Document number: DS37878 Rev. 1 - 2
e
A1
C
SO-16
Dim
Min
Max
A
1.40
1.75
A1
0.10
0.25
A2
1.30
1.50
B
0.33
0.51
C
0.19
0.25
D
9.80
10.00
E
3.80
4.00
e
1.27 Typ
H
5.80
6.20
L
0.38
1.27

0
8
All Dimensions in mm
Detail ‘A’
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AL9902
Suggested Pad Layout
Please see AP02001 at http://www.diodes.com/datasheets/ap02001.pdf for the latest version.
U-DFN6040-12
X3
Dimensions
Y
C
G
Y1
X1
G1
X2
Y2
Y3
Pin1
C
G
G1
X
X1
X2
X3
Y
Y1
Y2
Y3
Value
(in mm)
0.500
0.650
0.350
0.250
1.075
1.275
2.750
0.400
1.150
1.000
2.300
X
SO-16
X1
Dimensions
C
X
X1
Y
Y1
Y1
Y
Value
(in mm)
1.270
0.670
9.560
1.450
6.400
Pin 1
X
AL9902
Document number: DS37878 Rev. 1 - 2
C
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AL9902
IMPORTANT NOTICE
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INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
(AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION).
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This document is written in English but may be translated into multiple languages for reference. Only the English version of this document is the
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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:
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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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representatives against any damages arising out of the use of Diodes Incorporated products in such safety-critical, life support devices or systems.
Copyright © 2015, Diodes Incorporated
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