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參數(shù)資料
| 型號： | FOD2712 |
| 廠商： | FAIRCHILD SEMICONDUCTOR CORP |
| 元件分類： | 光電耦合器 |
| 英文描述： | OPTICALLY ISOLATED ERROR AMPLIFIER |
| 中文描述： | 1 CHANNEL TRANSISTOR OUTPUT OPTOCOUPLER |
| 封裝： | LEAD FREE PACKAGE-8 |
| 文件頁數(shù)： | 10/13頁 |
| 文件大?。?/td> | 581K |
| 代理商： | FOD2712 |
4/10/03
Page 10 of 13
2003 Fairchild Semiconductor Corporation
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
The FOD2712
The FOD2712 is an optically isolated error amplifier. It incor-
porates three of the most common elements necessary to
make an isolated power supply, a reference voltage, an error
amplifier, and an optocoupler. It is functionally equivalent to
the
popular RC431A shunt voltage regulator plus the CNY17F-3
optocoupler.
Powering the Secondary Side
The LED pin in the FOD2712 powers the secondary side, and
in particular provides the current to run the LED. The actual
structure of the FOD2712 dictates the minimum voltage that
can be applied to the LED pin: The error amplifier output has a
minimum of the reference voltage, and the LED is in series
with that. Minimum voltage applied to the LED pin is thus
1.24V + 1.5V = 2.74V. This voltage can be generated either
directly from the output of the converter, or else from a slaved
secondary winding. The secondary winding will not affect reg-
ulation, as the input to the FB pin may still be taken from the
output winding.
The LED pin needs to be fed through a current limiting resistor.
The value of the resistor sets the amount of current through
the LED, and thus must be carefully selected in conjunction
with the selection of the primary side resistor.
Feedback
Output voltage of a converter is determined by selecting a
resistor divider from the regulated output to the FB pin. The
FOD2712 attempts to regulate its FB pin to the reference
voltage, 1.24V. The ratio of the two resistors should thus be:
The absolute value of the top resistor is set by the input offset
current of 0.8μA. To achieve 1% accuracy, the resistance of
R
TOP
should be:
Compensation
The compensation pin of the FOD2712 provides the opportu-
nity for the designer to design the frequency response of the
converter. A compensation network may be placed between
the COMP pin and the FB pin. In typical low-bandwidth
systems, a 0.1μF capacitor may be used. For converters with
more stringent requirements, a network should be designed
based on measurements of the system’s loop. An excellent
reference for this process may be found in “Practical Design of
Power Supplies” by Ron Lenk, IEEE Press, 1998.
Secondary Ground
The GND pin should be connected to the secondary ground of
the converter.
No Connect Pins
The NC pins have no internal connection. They should not
have any connection to the secondary side, as this may
compromise the isolation structure.
Photo-Transistor
The Photo-transistor is the output of the FOD2712. In a normal
configuration the collector will be attached to a pull-up resistor
and the emitter grounded. There is no base connection neces-
sary.
The value of the pull-up resistor, and the current limiting resis-
tor feeding the LED, must be carefully selected to account for
voltage range accepted by the PWM IC, and for the variation in
current transfer ratio (CTR) of the opto-isolator itself.
Example:
The voltage feeding the LED pins is +12V, the volt-
age feeding the collector pull-up is +10V, and the PWM IC is
the Fairchild KA1H0680, which has a 5V reference. If we
select a 10K
resistor for the LED, the maximum current the
LED can see is (12V-2.74V) /10K
= 926μA. The CTR of the
opto-isolator is a minimum of 100%, and so the minimum
collector current of the photo-transistor when the diode is full
on is also 926μA. The collector resistor must thus be such that:
select 10K
to allow some margin.
R
BOTTOM
R
V
REF
V
1
–
=
V
-------R
1.24
–
TOP
80
μ
A
>
5V
–
COLLECTOR
R
926
μ
A or R
COLLECTOR
5.4K
;
>
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| FOD2712AR2 | 功能描述:光隔離放大器 Isolated Error Amp Optocoupler RoHS:否 制造商:Avago Technologies 絕緣電壓:5000 Vrms 參考電壓: 電流傳遞比: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Tube |
| FOD2712AR2V | 功能描述:光隔離放大器 OPTICALLY ISOLATED ERROR AMPLIFIER RoHS:否 制造商:Avago Technologies 絕緣電壓:5000 Vrms 參考電壓: 電流傳遞比: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Tube |
| FOD2712AV | 功能描述:光隔離放大器 OPTICALLY ISOLATED ERROR AMPLIFIER RoHS:否 制造商:Avago Technologies 絕緣電壓:5000 Vrms 參考電壓: 電流傳遞比: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Tube |
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