Page 1
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
DESCRIPTION
The FOD2712 Optically Isolated Amplifier consists of the
popular RC431A precision programmable shunt
reference and an optocoupler. The optocoupler is
a gallium arsenide (GaAs) light emitting diode
optically coupled to a silicon phototransistor. The
reference voltage tolerance is 1%. The current
transfer ratio (CTR) ranges from 100% to 200%.
It is primarily intended for use as the error amplifier/reference
voltage/optocoupler function in isolated ac to dc power supplies and
dc/dc converters.
When using the FOD2712, power supply designers can reduce the
component count and save space in tightly packaged designs. The
tight tolerance reference eliminates the need for adjustments in many
applications.
The device comes in a compact 8-pin small outline package.
FEATURES
• Optocoupler, precision reference and
error amplifier in single package
• 1.240V ± 1% reference
• CTR 100% to 200%
• 2,500V RMS isolation
• VDE approval 136616
• BSI approval 8661 and 8662
• UL approval E90700
• CSA approval 1113643
FUNCTIONAL BLOCK DIAGRAM
NC
1
C
2
E
3
NC
4 5
8
7
6
LED
FB
COMP
GND
APPLICATIONS
•Power system for workstations
•Telecom central office supply
•Telecom bricks
PACKAGE DIMENSIONS
0.164 (4.16)
0.144 (3.66)
1
0.202 (5.13)
0.182 (4.63)
SEATING PLANE
0.143 (3.63)
0.123 (3.13)
0.008 (0.20)
0.021 (0.53)
0.011 (0.28)
Lead Coplanarity : 0.004 (0.10) MAX
0.003 (0.08)
0.050 (1.27)
TYP
NOTE
All dimensions are in inches (millimeters)
0.244 (6.19)
0.224 (5.69)
0.010 (0.25)
0.006 (0.16)
PIN DEFINITIONS
Pin Number Pin Name Pin function description
1NCNot connected
2CPhototransistor Collector
3EPhototransistor Emitter
4NCNot connected
5 GND Ground
6 COMP Error Amplifier Compensation. This pin is the output of the error amplifier. *
7FBVoltage Feedback. This pin is the inverting input to the error amplifier
8 LED Anode LED. This pin is the input to the light emitting diode.
* The compensation network must be attached between pins 6 and 7.
© 2003 Fairchild Semiconductor Corporation
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TYPICAL APPLICATION
FAN4803
V
1
PWM
Control
FOD2712
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
V
O
2
3
ABSOLUTE MAXIMUM RATINGS
Parameter Symbol Value Units
Storage Temperature
Operating Temperature
Reflow Temperature Profile (refer to fig. 21)
Input Voltage
Input DC Current
Collector-Emitter Voltage
Emitter-Collector Voltage
Collector Current
Input Power Dissipation (note 1) PD1 145 mW
Tr ansistor Power Dissipation (note 2) PD2 85 mW
Total Power Dissipation (note 3) PD3 145 mW
Notes
1. Derate linearly from 25°C at a rate of 2.42 mW/ °C
2. Derate linearly from 25°C at a rate of 1.42 mW/ °C.
3. Derate linearly from 25°C at a rate of 2.42 mW/ °C.
4. Functional operation under these conditions is not implied. Permanent damage may occur if the device is subjected to conditions
outside these ratings.
(T
= 25°C Unless otherwise specified.)
A
T
STG
T
OPR
V
LED
I
LED
V
CEO
V
ECO
I
C
8
6
7
5
-55 to +125 °C
-40 to +85 °C
13.2 V
20 mA
30 V
7V
50 mA
R1
R2
© 2003 Fairchild Semiconductor Corporation
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4/10/03
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/ ∆
∆
/ ∆
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
ELECTRICAL CHARACTERISTICS
(V
CC
= 12V, T
= 25°C Unless otherwise specified.)
A
INPUT CHARACTERISTICS
Parameter Test Conditions Symbol Min Typ** Max Unit
(I
LED forward voltage
= 10 mA, V
LED
Reference voltage
(-40 to +85°C)
(V
COMP
= V
FB
, I
(25°C) 1.228 1.240 1.252
Deviation of V
over temperature - See Note 1 (T
REF
Ratio of Vref variation
to the output of the error amplifier
Feedback input current
Deviation of I
over temperature - See Note 1 (T
REF
Minimum drive current
Off-state error amplifier current
Error amplifier
(V
(I
COMP
LED
= V
= V
V
COMP
= 10 mA, R1 = 10 k Ω ) (Fig.3) I
(V
(V
= 6 V, V
LED
, I
FB
LED
output impedance - See Note 2
1. The deviation parameters V
REF(DEV)
and I
REF(DEV)
are defined as the differences between the maximum and minimum values
obtained over the rated temperature range. The average full-range temperature coefficient of the reference input voltage, ∆ V
is defined as:
A
OUT
25°C=(){}106×
| = ∆ V
COMP
I
LED
V
∆V
REF
where ∆ T
ppm/°C()
is the rated operating free-air temperature range of the device.
A
REF DEV()/VREFTA
-----------------------------------------------------------------------------------------------------=
∆T
2. The dynamic impedance is defined as |Z
Figure 2), the total dynamic impedance of the circuit is given by:
= V
COMP
= 10 mA (Fig.1)
LED
= -40 to +85°C) V
A
to 12 V) (Fig.2)
REF
= -40 to +85°C) I
A
COMP
)(Fig.1) V
FB
(I
= 10 mA,
LED
= V
) (Fig.1) I
FB
= 0) (Fig.4) I
FB
= 0.1 mA to 15 mA,
f<1 kHZ)
F
V
REF
REF (DEV)
V
REF
V
COMP
REF
REF (DEV)
LED (MIN)
(OFF)
|
|Z
OUT
1.221 1.259
-1.5 -2.7 mV/V
0.15 0.5 µA
0.15 0.3 µA
55 80 µA
0.001 0.1 µA
0.25 Ohm
1.5 V
4 12 mV
. When the device is operating with two external resistors (see
V
REF
,
Z
OUT, TOT
© 2003 Fairchild Semiconductor Corporation
∆V
=
--------Z
∆I
OUT
R1
1
--------+×≈
R2
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4/10/03
Page 4
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
OUTPUT CHARACTERISTICS
(T
= 25°C Unless otherwise specified.)
A
Parameter Test Conditions Symbol Min Typ Max Unit
(V
Collector dark current
Collector-emitter voltage breakdown
Emitter-collector voltage breakdown
TRANSFER CHARACTERISTICS
(T
= 10 V) (Fig. 5) I
CE
(I
= 1.0mA) BV
C
(I
= 100 µA) BV
E
= 25°C Unless otherwise specified.)
A
CEO
CEO
ECO
50 nA
70 V
7V
Parameter Test Conditions Symbol Min Typ Max Unit
(I
(I
LED
LED
Current transfer ratio
Collector-emitter
saturation voltage
ISOLATION CHARACTERISTICS
= 10 mA, V
V
CE
= 10 mA, V
I
= 2.5 mA) (Fig. 6)
C
(T
= 25°C Unless otherwise specified.)
A
= V
COMP
FB
= 5 V) (Fig. 6)
= V
COMP
FB
,
CTR 100 200 %
,
V
CE
(SAT)
0.4 V
Parameter Test Conditions Symbol Min Typ Max Unit
Input-output insulation
leakage current
Withstand insulation
voltage
Resistance (input to output)
(RH = 45%, T
V
= 3000 VDC) (note. 1)
I-O
(RH <= 50%, T
V
I-O
= 25°C, t = 5s,
A
= 25°C, t = 1 min)
A
(notes. 1)
= 500 VDC (note. 1) R
I
I-O
V
ISO
I-O
2500 Vrms
12
10
1.0 µA
Ohm
SWITCHING CHARACTERISTICS (T
= 25°C Unless otherwise specified.)
A
Parameter Test Conditions Symbol Min Typ Max Unit
Bandwidth (Fig. 7)
(I
Common mode transient
immunity at output high
Common mode transient
immunity at output low
= 0 mA, Vcm = 10 V
LED
RL = 2.2 kΩ (Fig. 8) (note. 2)
(I
= 10 mA, Vcm = 10 V
LED
RL = 2.2 kΩ (Fig. 8) (note. 2)
PP
PP
B
W
CMH 1.0 kV/µs
CML 1.0 kV/µs
10 kHZ
Notes
1. Device is considered as a two terminal device: Pins 1, 2, 3 and 4 are shorted together and Pins 5, 6, 7 and 8 are shorted together.
2. Common mode transient immunity at output high is the maximum tolerable (positive) dVcm/dt on the leading edge of the
common mode impulse signal, Vcm, to assure that the output will remain high. Common mode transient immunity at output low
is the maximum tolerable (negative) dVcm/dt on the trailing edge of the common pulse signal,Vcm, to assure that the output will
remain low.
© 2003 Fairchild Semiconductor Corporation
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OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
I
I
(LED)
V
82
V
F
6
7
V
REF
5
3
V
R2
(LED)
8
6R1
7
V
COMP
V
REF
5
2
3
FIG. 1. V
V
, VF, I
REF
FIG. 3. I
8
(min) TEST CIRCUIT
LED
I
(LED)
8
I
REF
6
7
R1
5
TEST CIRCUIT
REF
2
I
CEO
FIG. 2. ∆V
2
3
V
REF/∆VCOMP
I
(OFF)
V
(LED)
FIG. 4. I
I
(LED)
TEST CIRCUIT
8
6
7
5
TEST CIRCUIT
(OFF)
8
2
3
I
(C)
2
6
7
5
FIG. 5. I
TEST CIRCUIT FIG. 6. CTR, V
CEO
© 2003 Fairchild Semiconductor Corporation
V
CE
3
V
6
7
V
V
REF
5
COMP
TEST CIRCUIT
CE(sat)
Page 5 of 13
V
CE
3
4/10/03
Page 6
VCC = +5V DC
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
= 10 mA
I
F
R
L
V
OUT
1
2
3
4
8
7
0.1 V
PP
6
5
1µf
47Ω
V
IN
0.47V
Fig. 7 Frequency Response Test Circuit
= +5V DC
V
CC
I
= 0 mA (A)
F
IF = 10 mA (B)
R1
2.2kΩ
1
8
V
OUT
© 2003 Fairchild Semiconductor Corporation
2
3
4
VCM
_
10V
+
P-P
7
6
5
Fig. 8 CMH and CML Test Circuit
Page 6 of 13
A
B
4/10/03
Page 7
TYPICAL PERFORMANCE CURVES
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
Fig. 9a LED Current vs Cathode Voltage
15
TA = 25°C
= V
V
COMP
10
5
0
-5
- SUPPLY CURRENT (mA)
LED
-10
I
-15
-1.0 -0.5 0.0 0.5 1.0 1.5
FB
V
- CATHODE VOLTAGE (V) V
COMP
Fig. 10 Reference Voltage vs Ambient Temperature
1.254
I
= 10 mA
LED
1.248
1.242
REFERENCE VOLTAGE (V)
1.236
-
REF
V
1.230
-40 -20 0 20 40 60 80
TA - AMBIENT TEMPERATURE (°C) TA - AMBIENT TEMPERATURE (°C)
Fig. 9b LED Current vs Cathode Voltage
150
TA = 25°C
120
= V
V
COMP
90
µA)
60
30
0
-30
-60
- SUPPLY CURRENT (
-90
LED
I
-120
-150
-1.0 -0.5 0.0 0.5 1.0 1.5
FB
- CATHODE VOLTAGE (V)
COMP
Fig. 11 Reference Current vs Ambient Temperature
350
I
= 10 mA
LED
R
= 10 kΩ
1
300
250
200
150
REFERENCE CURRENT (nA)
-
100
REF
I
50
-40 -20 0 20 40 60 80 100
© 2003 Fairchild Semiconductor Corporation
Fig. 12 Off Current vs Ambient Temperature
100
V
= 13.2 V
LED
V
= 0
FB
10
- OFF CURRENT (nA)
1
(OFF)
I
0.1
-40 -20 0 20 40 60 80 100
- AMBIENT TEMPERATURE (°C)
T
A
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Page 8
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
Fig.13 LED Forward Current vs Forward Voltage
20
15
10
FORWARD CURRENT (mA)
5
-
LED
I
0
0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35
70˚C
25˚C
0˚C
VF - FORWARD VOLTAGE (V)
Fig. 15 Collector Current vs Ambient Temperature
30
VCE = 5 V
25
20
15
10
5
- COLLECTOR CURRENT (mA)
C
0
I
-40 -20 0 20 40 60 80 100
TA - AMBIENT TEMPERATURE (°C)
I
I
I
I
LED
LED
LED
LED
= 20 mA
= 10 mA
= 5 mA
1 mA
=
Fig.14 Dark Current vs Temperature
VCE = 10V
1000
100
10
- DARK CURRENT (nA)
1
CEO
I
0.1
-40 -20 0 20 40 60 80 100
TA - AMBIENT TEMPERATURE (°C)
Fig. 16 Current Transfer Ratio vs LED Current
160
V
= 5 V
CE
140
120
100
80
60
40
20
) - CURRENT TRANSFER RATIO (%)
F
0
/I
C
(I
01020304050
I
- FORWARD CURRENT (mA)
LED
70˚C
0˚C
25˚C
© 2003 Fairchild Semiconductor Corporation
Fig. 17 Saturation Voltage vs Ambient Temperature
0.22
0.20
0.18
0.16
0.14
- SATURATION VOLTAGE (V)
0.12
CE (sat)
V
0.10
-40 -20 0 20 40 60 80 100
TA - AMBIENT TEMPERATURE (°C)
Page 8 of 13
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Page 9
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
Fig. 18 Collector Current vs Collector Voltage
45
TA = 25°C
40
35
= 20 mA
I
I
I
I
LED
LED
LED
LED
= 10 mA
= 5 mA
= 1 mA
30
25
20
15
10
- COLLECTOR CURRENT (mA)
C
I
5
0
012345678910
VCE - COLLECTOR-EMITTER VOLTAGE (V)
Fig. 20 Voltage Gain Vs Frequency
0
) dB
in
/V
o
-5
Fig. 19 Delta V
1
(mV/V)
0
COMP
/DELTA V
REF
-1
DELTA V
-2
-40 -20 0 20 40 60 80 100
100 Ω
500 Ω
/Delta V
REF
TA - AMBIENT TEMPERATURE (°C)
vs Ambient Temperature
COMP
VOLTA GE GAIN, A(V
© 2003 Fairchild Semiconductor Corporation
-10
-15
10 100 1000
RL=1 kΩ
FREQUENCY kHz
Page 9 of 13
4/10/03
Page 10
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
The FOD2712
The FOD2712 is an optically isolated error amplifier. It incorporates 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 regulation, 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:
R
TOP
-------------------------R
BOTTOM
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
should be:
TOP
V
OUT
-------------------------------- 8 0 µA>
R
TOP
V
OUT
-------------- 1–=
V
REF
1.24–
Compensation
The compensation pin of the FOD2712 provides the opportunity 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 necessary.
The value of the pull-up resistor, and the current limiting resistor 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 voltage feeding the collector pull-up is +10V, and the PWM IC is
the Fairchild KA1H0680, which has a 5V reference. If we
select a 10KΩ resistor for the LED, the maximum current the
LED can see is (12V-2.74V) /10KΩ = 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:
10V 5V–
------------------------------------926µA or R
R
COLLECTOR
COLLECTOR
5.4KΩ;><
© 2003 Fairchild Semiconductor Corporation
select 10KΩ to allow some margin.
Page 10 of 13
4/10/03
Page 11
ORDERING INFORMATION
Example: FOD2712 X Y
X Y
Packaging Option
R1: Tape and Reel (500 per reel) V:VDE tested
R2: Tape and Reel (2,500 per reel)
MARKING INFORMATION
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
1
2712
43
Definitions
1Fairchild logo
2Device number
VDE mark (Note: Only appears on parts ordered with VDE
3
option – See order entry table)
4 One digit year code, e.g., ‘3’
5Two digit work week ranging from ‘01’ to ‘53’
6 Assembly package code
2
SYYXV
5
6
© 2003 Fairchild Semiconductor Corporation
Page 11 of 13
4/10/03
Page 12
Carrier Tape Specifications
3.5 ± 0.2
8.3 ± 0.1
0.3 MAX
4.0 ± 0.1
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
8.0 ± 0.1
2 ± 0.05
Ø1.5 MIN
1.75 ± 0.10
5.5 ± 0.05
12.0 ± 0.3
5.2 ± 0.2
Reflow Profile
300
250
200
150
100
Temperature (°C)
50
0
0
0.1 MAX
6.4 ± 0.2
User Direction of Feed
230°C, 10–30 s
245°C peak
Time above 183°C, 120–180 sec
Ramp up = 2–10°C/sec
0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (Minute)
Ø1.5 + 0.1/-0
• Peak reflow temperature: 245°C (package surface temperature)
• Time of temperature higher than 183°C for 120–180 seconds
• One time soldering reflow is recommended
© 2003 Fairchild Semiconductor Corporation
Page 12 of 13
4/10/03
Page 13
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2712
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO
ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME
ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN;
NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and (c) whose failure to perform
when properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to
result in a significant injury of the user.
2. A critical component in any component of a life support
device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
© 2003 Fairchild Semiconductor Corporation
Page 13 of 13
4/10/03
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