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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)： | 30/64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 36
Document Classification: Proprietary
August 6, 2010, Preliminary Information
5.3.1.1
R1 and R2 Resistor Divider Design
The following equation can be used for the design of R1 and R2.
Equation (6)
Where VOUT_MAX is the output over voltage regulation point during starting or under a no load
condition, R1 and R2 are in k, VD1 is the voltage drop of the diode D1 and VREF is the reference
voltage inside the 88EM8080/88EM8081 IC. The nominal value of VREF is 2.5V.
This equation takes into account of the internal leakage at FB pin. The leakage at the FB pin is
equivalent to a 200k Resistor. Because the forward current of diode D1 is less (less than 25mA) the
voltage drop VD1 is about 0.1V to 0.3V depending on the diode and the temperature of the diode. It
should be noted that under steady state conditions the current loop should be active and not the
voltage loop. Therefore care must be taken in selection of R2 to make sure that the voltage at the
cathode of D1 is less than the voltage at the cathode of D2 under steady state conditions.
5.3.1.2
RC Filter Design
In single stage PFC LED applications, the output has twice the line frequency ripple which may
trigger the OVP function through the voltage at the FB pin. An RC filter defined by R3 and Ccp, is
designed to filter the twice the line frequency ripple embedded in the feedback signal. The cutoff
frequency of the RC filter should be much lower than twice the line frequency. The following equation
(7) is used for the design of R3 and CCP.
Equation (7)
In some situations the output current will be very low and the output capacitance could be selected
to be a high value. In this case, twice the line frequency ripple at the output will be very low and this
RC filter may not be necessary.
5.3.1.3
Design of Operational Amplifier Circuit
The DC Gain of the non-inverting amplifier circuit is based on the following equation:
Equation (8)
However, the average current through the LED is controlled by the following equation:
Equation (9)
Where, IAVG is the average LED current, VD2 is the voltage drop on the diode D2 and KU2 is the DC
gain of the operational amplifier circuit. From equation (9) the required KU2 can be determined.
R5 and R4 are designed from equation (8).
V
OUT_MAX
R
1
--------------------------
V
D1
1
R
1
------
1
R
2
------
+
–
1
R
1
------
1
R
2
------
1
200
---------
++
-------------------------------------------------------------------
V
REF
=
1
2
πR
3Ccp
----------------------
100Hz
K
U2
1
R
5
R
4
------
+
=
I
AVG
R
sen2
×
K
U2
() V
D2
–
×
V
REF
=
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