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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)： | 16/64頁 |
| 文件大?。?/td> | 667K |
| 代理商： | 88EM8081XX-SAG2C000-T |
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Functional Description
ISNS – Current Sensing / Over Current Protection
Copyright 2010 Marvell
Doc. No. MV-S106340-01 Rev. B
August 6, 2010, Preliminary Information
Document Classification: Proprietary
Page 23
It is important to note that the IC has internal compensation for the loop stability. There is no need for
external compensation.
3.6.2.4
Output Voltage Regulation - Non-isolated Output
When non-isolated, an opto-isolator is not necessary. The output voltage is sensed through the
output divider and the output of the divider is connected directly to the feedback pin FB. The IC then
controls the output voltage in the same way as for the isolated case. It is important to note that the IC
has internal compensation for the loop stability.
3.7
ISNS
– Current Sensing / Over Current Protection
3.7.1
ISNS – Peak Current Sensing
The 88EM8080/88EM8081 LED driver provides power factor correction (PFC) in addition to
providing regulation. Regarding the PFC function, the average input current is varied to be
proportional to the input voltage. This way the average input current will be sinusoidal. This input
current is the same as the primary MOSFET current in a flyback circuit. Therefore, the IC can sense
the MOSFET current and then use average current mode control to implement the PFC function.
The voltage across the resistor in series with MOSFET (connected between MOSFET source pin
and bridge diode negative DC terminal) is used as primary current sensing signal. The primary
MOSFET source pin is grounded to apply a PWM signal between the gate and the source. The
current sense signal in series MOSFET source therefore becomes negative with respect to ground.
The IC uses this negative signal for input current control to provide the PFC function.
3.7.2
ISNS – Average Current Mode Control
The voltage across the primary current sense resistor is proportional to the peak value of switching
current. An RC filter can be used to provide the average current. The output of the RC filter is then
applied to ISNS pin. The internal current loop adjusts the duty cycle so that average current is
sinusoidal. The amplitude of the sinusoidal current is adjusted by the external voltage/current loop to
achieve constant output load voltage/current.
3.7.3
ISNS – Adaptive Over Current Protection
The average current signal at the output of the RC filter is also used for over current protection. The
IC has an internal current comparator with four different internal thresholds for current protection on
ISNS pin as the input voltage signal varies. The input current varies inversely with the AC input
voltage. There are four different OCP thresholds at four different input voltage ranges. The different
Characteristics, on page 16. With these four adaptive over current protection steps the IC provides
almost constant power protection over the entire AC input voltage range.
3.7.4
OCP – Cycle by Cycle Over Current Protection
The voltage across the primary current sense resistor is proportional to the peak value of switching
current. This voltage can be used for cycle by cycle over current protection by the OCP pin. When
the OCP pin is pulled low the IC will shut down and there is no switched output signal at SW pin.
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