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PDF iML8681 Data sheet ( Hoja de datos )
| Número de pieza | iML8681 | |
| Descripción | Application Notes | |
| Fabricantes | IML | |
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iML8681 Application Note
iML8681 Application Notes
Table of Content
1. IC Description.................................................................................................................................................2
2. Features .........................................................................................................................................................2
3. Package and Pin Diagrams ...........................................................................................................................2
4. Typical Application Circuit and Waveforms ....................................................................................................3
5. Evaluation Modules........................................................................................................................................3
6. Key Parameters by Order Number ................................................................................................................4
7. Operation Theory ...........................................................................................................................................5
8. Application Information ..................................................................................................................................9
8.1 Selection of Driving Scheme.............................................................................................................9
8.2 Total VF of the LED string............................................................................................................... 13
8.3 Selection for Option ....................................................................................................................... 16
8.4 VF Variation .................................................................................................................................... 17
8.5 Thermal Protection......................................................................................................................... 18
8.6 Layout Suggestion ......................................................................................................................... 19
© 2013 IML Inc. All rights reserved.
1
Rev. 0.1 - Jun. 2013
Free Datasheet http://www.Datasheet4U.com
1 page  
iML8681 Application Note
7. Operation Theory
The iML8681 is a Two Terminal Current Controller (TTCC) that regulates the current flowing through an
LED string according to the voltage established across the string. The typical application for the device is to
power an LED string from an AC voltage source as in an LED light bulb or light tube.
The iML8681 has 4 functional pins -- Anode (A), Cathode (K), MS1, and MS2. In typical applications,
the LED string is divided into several segments. The Anode pin is connected to the most positive terminal of
the LED segment and the Cathode pin is connected to the most negative terminal. The MS1 and MS2 pins
are used for device mode selection. One of four device modes (Mode 0 through Mode 3) is selected by
connecting MS1 and/or MS2 to the Cathode (logic 0) or by leaving both pins open (internally pulled up to
logic 1). The regulating current (IPEAK), turn on voltage (VON,TH), and turn off voltage (VOFF,TH) are different
for each mode. Please refer to the diagram and the table in below. In Mode 0, the chip is connected to the
LED string in series and acts as a constant current sink, regulating the current through the string. In Modes 1
through 3, the device, connected in parallel with a string segment, acts as a current shunt or bypass when
the voltage across the segment, vAK, is below the mode threshold.
The following explains the operation theory in detail. The MS1 and MS2 pins will not be shown in the
applications drawings to simplify the drawings.
Mode
3
2
1
0
MS2
1
1
0
0
MS1
1
0
1
0
IPEAK
33%
55%
80%
100%
VON,TH / VOFF,TH
89%/76%
94%/80%
98%/85%
N/A
MS1 and MS2 Pins Connection for Each Mode
Current and On/Off Voltage Thresholds for Each Mode
The i-v curve of the iML8681 operating in Mode 0 is shown in below. There are two operating regions –
the linear region and the constant current (saturation) region. When the voltage across the Anode pin (A)
and the Cathode pin (K), vAK, is lower than the dropout voltage (VDROP), the conduction current, iAK, increases
as vAK rises. When vAK rises beyond VDROP, the chip enters the constant current region and iAK will be kept at
predetermined level, IPEAK, as vAK increases. Power dissipation is most important when operating in Mode 0.
Use the exposed thermal pad to increase power dissipation. Soldering the thermal pad of the part to a
copper foil on the PCB will improve thermal conductivity.
Anode (A)
iAK
Constant Current Region
iAK=IPEAK
iML8681
vAK
IPEAK
iAK
Cathode (K)
Linear Region
iAK inceases as vAK rises
0
VDROP
i-v Curve of the iML8681 Operating in Mode 0
vAK
© 2013 IML Inc. All rights reserved.
5
Rev. 0.1 - Jun. 2013
Free Datasheet http://www.Datasheet4U.com
5 Page 
iML8681 Application Note
D. 4-Stage Driving Scheme
VAC ~
LED1
U1
iML8681
U4
iML8681
iLED
LED2
LED3
LED4
U2
iML8681
U3
iML8681
IPEAK4
IPEAK1
IPEAK2
IPEAK3
VAC
iLED
4-stage Driving Scheme and Corresponding Waveform
In the 4-stage driving scheme, four iML8681 chips, U1, U2, U3, and U4 are used. U1, U2, and
U3 are connected in parallel with LED1, LED2, and LED3, respectively. U4 is connected in series
with the entire string. Assuming IPEAK4 > IPEAK1 > IPEAK2 > IPEAK3. U1, U2, U3, and U4 are turned on at
the beginning of the cycle. Because the setting current of U3, IPEAK3, is lowest, U3 will clamp the LED
current at IPEAK3. When U3 is turned off, U2 will then clamp the LED current at IPEAK2. When U2 is
turned off, U1 will then clamp the LED current at IPEAK1. When U1, U2, and U3 are turned off, U4 will
clamp the LED current at IPEAK4.
Beware that IPEAK4 must be higher than IPEAK1, IPEAK2, and IPEAK3. The relationship between IPEAK1,
IPEAK2, and IPEAK3 determines the sequence that segments LED1, LED2 and LED3 light up. If LED3 is
planned to light up earlier and longer than LED2 and LED1, IPEAK3 should be lower than IPEAK2 and
IPEAK1. Usually, U4 works in Mode 0, and it is recommended that U1, U2, and U3 work in Mode 1,
Mode 2, and Mode 3, respectively.
For the 4-stage driving scheme, it is not required that U1 works in Mode 1, U2 works in Mode 2,
or U3 works in Mode 3. Only U4 is restricted to operate in Mode 0 and with the highest current level.
The system designer can arrange the sequence arbitrarily depending on system needs. Flexibility is
an important advantage of iML8681 driving system.
The LED segment lighting sequence for a 4-stage driving scheme is demonstrated in the figure
below. In the example, the current path over the rising part of the VAC period is shown step by step.
© 2013 IML Inc. All rights reserved.
11
Rev. 0.1 - Jun. 2013
Free Datasheet http://www.Datasheet4U.com
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