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| 型號： | 88EM8081XX-SAG2C000-T |
| 廠商： | MARVELL SEMICONDUCTOR INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 2 A POWER FACTOR CONTROLLER, 120 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | GREEN, SOIC-8 |
| 文件頁數(shù)： | 35/64頁 |
| 文件大小： | 667K |
| 代理商： | 88EM8081XX-SAG2C000-T |
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88EM8080/88EM8081
Datasheet
Doc. No. MV-S106340-01 Rev. B
Copyright 2010 Marvell
Page 40
Document Classification: Proprietary
August 6, 2010, Preliminary Information
5.3.4
Isolated LED Current Control - Circuit Design
The relation between output current signal and the reference voltage of U1 is provided by the
following equation:
Equation (12)
Where VREFU1 is the TL431 reference voltage and is equal to 2.5V nominal, ILEDavg is the average
current through the LED.
R8 is selected to be a low value so that the power dissipation is minimum. For the reference design it
is 0.1
.
One can select R2 and R3 from the above equation after selecting R8 since VREFU1 is equal to 2.5V.
C11 and C13 are selected so that the second harmonic ripple at the output is reasonable.
The Zener diode D1 is selected to be 4.7V so that the voltage at the FB pin cannot exceed the rated
maximum voltage for the FB pin.
The output voltage of U1 error amplifier (VC), and the secondary side optocoupler diode current are
related by the following equation:
Equation (13)
Where VFD is the forward voltage drop of opto-coupler diode and is about 1V.
VC is also related to the FB pin voltage or the reference voltage of U3 by the following equation:
Equation (14)
Where VFB_REG is the reference voltage of U3 and CTR is the current transfer ratio of optocoupler.
At steady state no load condition, the secondary side error amplifier enters into positive saturation
status because of no current feedback signal. Therefore, the output voltage VC increases to the
maximum clamp voltage VZD6 as pointed out in the previous section. VZD6 should be higher than
normal operation voltage of VC to keep enough margin. Then the no load output voltage will increase
until the FB pin voltage reaches 2.5V. The following equation can be used to calculate the no load
output voltage.
Equation (15)
VO_NOLOAD is also equal to the maximum voltage when the LED string is broken or when the sense
line is open.
The following equation is provided to calculate the output over voltage protection point.
Equation (16)
V
REFU1
R
2
R
3
+
R
3
------------------
I
LEDavg
×
R
8
×
=
I
sf
V
O
V
FD
–
V
C
–
R
23
------------------------------------
=
V
O
V
FD
–
V
C
–
R
23
------------------------------------
CTR
R
11
V
FB_REG
=
×
V
O_NOLOAD
V
FD
–
V
ZD6
–
R
23
---------------------------------------------------------------
CTR
R
11
V
FB_REG
=
×
V
O_OVP
V
FD
–
V
ZD6
–
R
23
-----------------------------------------------------
CTR
R
11
V
FB_OVP
=
×
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