參數(shù)資料
型號(hào)： 88EM8081XX-SAG2C000-T
廠商： MARVELL SEMICONDUCTOR INC
元件分類(lèi)： 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(yè)數(shù)： 33/64頁(yè)
文件大?。?/td> 667K
代理商： 88EM8081XX-SAG2C000-T
Design and Applications Information
Output LED Current Control
Copyright 2010 Marvell
Doc. No. MV-S106340-01 Rev. B
August 6, 2010, Preliminary Information
Document Classification: Proprietary
Page 39
generate an error voltage of VC at the output after comparing the output voltage of U2 to the internal
reference voltage of U1.
An optocoupler is connected between the output (LED+) and the U1 error voltage output through a
resistor R23. The current through the optocoupler is a function of error voltage at the output of
TL431and is varied until the output voltage of U2 is equal to U1 reference voltage of 2.5V.
The current through the opto-transistor and resistor R11 is proportional to the opto-diode current by
a factor of current transfer ratio of the optocoupler. The voltage across R11 is applied as input to FB
pin. Therefore, the voltage at the FB pin is a function of the error voltage output of U1.The voltage at
the FB pin controls the duty cycle of the drive signal to the external MOSFET Q1. The switching
current of Q1 is sensed by the voltage across resistor R5. At steady state conditions the FB pin
voltage will be 2.5V which is equal to the nominal value of internal reference for
88EM8080/88EM8081 IC.
The voltage across R5 is filtered and the filtered voltage is proportional to the average current. The
duty cycle is varied so that the average of the input current through the ISNS pin follows the AC
Input voltage. The amplitude of the AC sinusoidal input current is varied to adjust the output voltage,
therefore adjusting the LED current until the output voltage of U2 equals the reference voltage of U1
(2.5V, typical).
During startup, the FB pin voltage is zero and the duty cycle at the SW pin is 6% (typical). Soft start
is provided until feedback voltage reaches 2.1V which is 87.5% of the reference voltage of U3 (2.5V,
typical). During the time the voltage at FB pin rises to 2.1V, the internal current reference increases
linearly. The internal current reference determines how fast the power is delivered to the secondary
side in addition to other circuit parameters. Therefore the soft start time is the duration for internal
current reference to raise linearly. When the FB pin reaches 2.1V, the internal feedback loop starts
closed loop operation to eventually reach steady state. It is to be noted that LEDs will not conduct
until the voltage across them reaches a minimal value. This means the LED load is open circuited
during startup. The voltage across the output capacitors C11 and C13 is zero initially at starting. The
output capacitors C11 and C13 will get charged rather quickly and the output voltage could
overshoot the steady state value. During starting condition when LEDs are not conducting or if the
LED string is open circuited, the output voltage across the LED string may go higher than the normal
steady state value and the zener D6 will be conducting. The output voltage of U1 is equal to the
zener voltage. Once the LEDs starts conducting the output voltage of U1 starts decreasing and will
come to a steady state value at which point the voltage across D6 will be much lower than the zener
conduction voltage.
If the ambient temperature is increased, the CTR of the optocoupler will become less, then more
current through the optodiode becomes necessary for LED current regulation. This means the
output voltage of TL431 will be lower than the steady state voltage at the lower ambient. If the
ambient is decreased, TL431 voltage will be higher than the steady state voltage at the higher
ambient. The design of the TL431 should be such that the zener diode D6 should not conduct during
normal operation at any temperature. In addition the output voltage of TL431 cannot go below 2.5V
for any reason during steady state operation.
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