OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
DESCRIPTION
The FOD2742 Optically Isolated Amplifier consists of the popular KA431 precision
programmable shunt reference and an optocoupler. The optocoupler is a gallium arsenide
(GaAs) light emitting diode optically coupled to a silicon phototransistor. It comes in 3
grades of reference voltage tolerance = 2%, 1%, and 0.5%.
The Current Transfer Ratio (CTR) ranges from 100% to 200%. It also has an outstanding
temperature coefficient of 50 ppm/°C. 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.
FUNCTIONAL BLOCK DIAGRAM
When using the FOD2742, 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 8-pin small outline package.
FEATURES
• Optocoupler, precision reference and error amplifier in single package
NC
1
8 LED
C
2
7 FB
E
3
6 COMP
NC
4
5 GND
• 2.5V reference
• CTR 100% to 200%
• 2,500V RMS isolation
• UL approval E90700, Volume 2
• BSI approval 8661, 8662
• VDE approval 136616
• CSA approval 1113643
• Low temperature coefficient 50 ppm/°C max
• FOD2742A: tolerance 0.5%
FOD2742B: tolerance 1%
FOD2742C: tolerance 2%
APPLICATIONS
• Power supplies regulation
• DC to DC converters
PIN DEFINITIONS
Pin Number
Pin Name
Pin function description
1
NC
2
C
Phototransistor Collector
3
E
Phototransistor Emitter
4
NC
5
GND
6
COMP
7
FB
8
LED
Not connected
Not connected
Ground
Error Amplifier Compensation. This pin is the output of the error amplifier. *
Voltage Feedback. This pin is the inverting input to the error amplifier
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
Page 1 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
TYPICAL APPLICATION
V1
FAN4803
PWM
Control
VO
FOD2742
2
8
6
3
R1
7
R2
5
ABSOLUTE MAXIMUM RATINGS (TA = 25°C Unless otherwise specified.)
Parameter
Symbol
Value
Units
Storage Temperature
TSTG
-40 to +125
°C
Operating Temperature
TOPR
-25 to +85
°C
Input Voltage
VLED
37
V
Input DC Current
ILED
20
mA
Collector-Emitter Voltage
VCEO
70
V
Emitter-Collector Voltage
VECO
7
V
IC
50
mA
PD1
PD2
PD3
145
85
145
mW
mW
mW
Reflow Temperature Profile (refer to fig. 21)
Collector Current
Input Power Dissipation (note 1)
Transistor Power Dissipation (note 2)
Total Power Dissipation (note 3)
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.
© 2003 Fairchild Semiconductor Corporation
Page 2 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
ELECTRICAL CHARACTERISTICS (TA = 25°C Unless otherwise specified.)
INPUT CHARACTERISTICS
Parameter
Test Conditions
(ILED = 10 mA, VCOMP = VFB) (fig. 1)
LED Forward Voltage
ILED = 10 mA, VCOMP = VFB (fig. 1)
Reference Voltage
Deviation of VREF over
temperature
Ratio of VREF variation to the
output of the error amplifier
Symbol
Device
VF
ALL
VREF
TA = -25°C to +85°C (fig. 1) VREF (DEV)
ILED = 10 mA ∆VCOMP = 10V to VREF
(fig. 2) ∆VCOMP = 36V to 10V
Typ.
Max.
Unit
1.20
1.5
V
A
2.482
2.495
2.508
V
B
2.470
2.495
2.520
V
C
2.450
2.500
2.550
V
3.5
17
mV
-0.5
-2.7
-0.3
-2.0
mV/
V
ALL
∆VREF/
∆VCOMP
ALL
IREF
ALL
2.2
4
µA
TA = -25°C to +85°C (fig. 3)
IREF (DEV)
ALL
1.0
1.2
µA
VCOMP = VFB (fig. 1)
ILED (MIN)
ALL
0.45
1.0
mA
VLED = 37V, VFB = 0 (fig. 4)
I(OFF)
ALL
0.01
1.0
µA
VCOMP = VREF, ILED = 1mA to 20mA,
f ≥ 1.0 kHz
|ZOUT|
ALL
0.15
0.5
Ω
ILED = 10mA, R1 = 10KΩ (fig. 3)
Feedback Input Current
Deviation of IREF over
temperature
Minimum Drive Current
Off-state error amplifier current
Error amplifier output impedance
(see note 2)
Min.
1. The deviation parameters VREF(DEV) and IREF(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, ∆VREF,
is defined as:
6
{ V REF ( DEV ) /V REF ( T A = 25°C ) } × 10
∆V REF ( ppm/°C ) = ----------------------------------------------------------------------------------------------------∆T A
where ∆TA is the rated operating free-air temperature range of the device.
2. The dynamic impedance is defined as |ZOUT| = ∆VCOMP/∆ILED. When the device is operating with two external resistors
(see Figure 2), the total dynamic impedance of the circuit is given by:
∆V
R1
Z OUT, TOT = -------- ≈ Z OUT × 1 + -------∆I
R2
© 2003 Fairchild Semiconductor Corporation
Page 3 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
OUTPUT CHARACTERISTICS (TA = 25°C Unless otherwise specified.)
Parameter
Test Conditions
(VCE = 10 V) (Fig. 5)
Collector dark current
Symbol
Min
ICEO
Typ
Max
Unit
1
50
nA
Emitter-collector voltage breakdown
(IE = 100 µA)
BVECO
7
10
V
Collector-emitter voltage breakdown
(IC = 1.0mA)
BVCEO
70
120
V
TRANSFER CHARACTERISTICS (TA = 25°C Unless otherwise specified.)
Parameter
Test Conditions
Symbol
Min
Typ
Max
Unit
CTR
100
140
200
%
0.16
0.4
V
Typ
Max
Unit
1.0
µA
Current transfer ratio
(ILED = 10 mA, VCOMP = VFB,
VCE = 5 V) (Fig. 6)
Collector-emitter
saturation voltage
(ILED = 10 mA, VCOMP = VFB,
VCE (SAT)
IC = 2.5 mA) (Fig. 6)
ISOLATION CHARACTERISTICS (TA = 25°C Unless otherwise specified.)
Parameter
Test Conditions
(RH = 45%, TA = 25°C, t = 5s,
VI-O = 3000 VDC) (note. 1)
Input-output insulation
leakage current
Withstand insulation
voltage
Symbol
II-O
(RH <= 50%, TA = 25°C, t = 1 min)
(note 1)
VISO
VI-O = 500 VDC (note 1)
RI-O
Resistance (input to output)
Min
2500
Vrms
1012
Ohm
SWITCHING CHARACTERISTICS (TA = 25°C Unless otherwise specified.)
Parameter
Test Conditions
Bandwidth
(Fig. 7)
Symbol
Min
Typ
Max
Unit
BW
50
kHZ
Common mode transient
immunity at output high
(ILED = 0 mA, Vcm = 10 VPP
RL = 2.2 kV (Fig. 8) (note 2)
CMH
1.0
kV/µs
Common mode transient
immunity at output low
(ILED = 10 mA, Vcm = 10 VPP
RL = 2.2 kV (Fig. 8) (note 2)
CML
1.0
kV/µs
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
Page 4 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
I(LED)
I(LED)
8
2
2
8
VF
6
6
V
7
V
R1
3
3
7
VCOMP
VREF
VREF
R2
5
5
FIG. 2. ∆VREF/∆VCOMP TEST CIRCUIT
FIG. 1. VREF, VF, ILED (min) TEST CIRCUIT
I(LED)
I(OFF)
8
2
8
2
IREF
6
6
3
7
V
3
V(LED)
7
V
R1
5
5
FIG. 4. I(OFF) TEST CIRCUIT
FIG. 3. IREF TEST CIRCUIT
8
I(LED)
ICEO
8
2
VCE
6
VCE
6
3
7
I(C)
2
V
3
7
VCOMP
VREF
5
5
FIG. 5. ICEO TEST CIRCUIT
© 2003 Fairchild Semiconductor Corporation
FIG. 6. CTR, VCE(sat) TEST CIRCUIT
Page 5 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
VCC = +5V DC
IF = 10 mA
RL
47Ω
8
1
1µf
VOUT
7
2
VIN
0.47V
0.1 VPP
3
6
4
5
Fig. 7 Frequency Response Test Circuit
VCC = +5V DC
IF = 0 mA (A)
IF = 10 mA (B)
R1
2.2kΩ
VOUT
1
8
2
7
3
6
4
5
_
A B
VCM
+
10VP-P
Fig. 8 CMH and CML Test Circuit
© 2003 Fairchild Semiconductor Corporation
Page 6 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
TYPICAL PERFORMANCE CURVES
Fig. 9a – LED Current vs. Cathode Voltage
15
Fig. 9b – LED Current vs. Cathode Voltage
1.0
TA = 25°C
VCOMP = VFB
TA = 25°C
VCOMP = VFB
ILED - Supply Current (mA)
ILED - Supply Current (mA)
10
5
0
-5
0.5
0.0
-0.5
-10
-15
-1.0
-1
0
1
3
2
-1
0
VCOMP - Cathode Voltage (V)
1
2
3
VCOMP - Cathode Voltage (V)
Fig. 10 – Reference Voltage vs. Ambient Temperature
Fig. 11 – Reference Current vs Ambient Temperature
2.510
ILED = 10mA
R1 = 10kΩ
ILED = 10mA
2.506
IREF - Reference Current (µA)
VREF - Reference Voltage (V)
2.508
2.504
2.502
2.500
2.498
2.496
2.494
3
2
2.492
2.490
-40
-20
0
20
40
60
80
-40
100
-20
0
20
40
60
80
100
TA - Ambient Temperature (°C)
TA - Ambient Temperature (°C)
Fig. 12 – Off-State Current vs. Ambient Temperature
Fig. 13 – Forward Current vs. Forward Voltage
20
VLED = 37V
IF - Forward Current (mA)
IOFF - Off-state Current (nA)
100
10
1
-40
-20
0
20
40
60
80
100
25°C
10
0°C
70°C
5
0.9
1.0
1.1
1.2
1.3
1.4
VF - Forward Voltage (V)
TA - Ambient Temperature (°C)
© 2003 Fairchild Semiconductor Corporation
15
Page 7 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
TYPICAL PERFORMANCE CURVES
Fig. 15 – Collector Current vs. Ambient Temperature
Fig. 14 – Dark Current vs. Ambient Temperature
30
VCE = 10V
1000
VCE = 5V
IC - Collector Current (mA)
ICEO - Dark Current (nA)
25
100
10
1
ILED = 20mA
20
ILED = 10mA
15
10
ILED = 5mA
5
ILED = 1mA
0
0.1
-40
-20
0
20
40
60
80
0
100
10
20
Fig. 16 – Current Transfer Ratio vs. LED Current
50
60
70
80
90
100
Fig. 17 – Saturation Voltage vs. Ambient Temperature
0.26
160
ILED = 10mA
IC = 2.5mA
VCE = 5V
0.24
0°C
140
VCE(sat) - Saturation Voltage (V)
(IC/IF) - Current Transfer Ratio (%)
40
TA - Ambient Temperature (°C)
TA - Ambient Temperature (°C)
25°C
120
70°C
100
80
60
0.22
0.20
0.18
0.16
0.14
0.12
0.10
-40
40
1
10
ILED - Forward Current (mA)
100
-20
0
20
40
60
80
100
TA - Ambient Temperature (°C)
Fig. 19 – Rate of Change Vref to Vout vs. Temperature
Fig. 18 – Collector Current vs. Collector Voltage
35
-0.22
TA = 25°C
-0.24
30
ILED = 20mA
Delta Vref / Delta Vout ( mV/V)
IC - Collector Current (mA)
30
25
20
ILED = 10mA
15
10
ILED = 5mA
-0.26
-0.28
-0.30
-0.32
-0.34
-0.36
-0.38
-0.40
5
-0.42
ILED = 1mA
0
0
1
2
3
4
5
6
7
VCE - Collector-Emitter Voltage (V)
© 2003 Fairchild Semiconductor Corporation
8
9
10
-0.44
-40
-20
0
20
40
60
80
100
Temperature - °C
Page 8 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
TYPICAL PERFORMANCE CURVES
Fig. 20 – Voltage Gain vs. Frequency
5
VCC=10V
IF=10mA
Voltage Gain - dB
0
RL = 100Ω
RL = 500Ω
-5
RL = 1kΩ
-10
-15
1
© 2003 Fairchild Semiconductor Corporation
10
100
Frequency - kHz
Page 9 of 13
1000
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
The FOD2742
Compensation
The FOD2742 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 KA431 shunt voltage regulator plus the CNY17F-X
optocoupler.
The compensation pin of the FOD2742 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.
Powering the Secondary Side
The LED pin in the FOD2742 powers the secondary side, and
in particular provides the current to run the LED. The actual
structure of the FOD2742 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 2.5V
+ 1.5V = 4.0V. 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.
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
Feedback
The Photo-transistor is the output of the FOD2742. In a normal
configuration the collector will be attached to a pull-up resistor
and the emitter grounded. There is no base connection necessary.
Output voltage of a converter is determined by selecting a
resistor divider from the regulated output to the FB pin. The
FOD2742 attempts to regulate its FB pin to the reference
voltage, 2.5V. The ratio of the two resistors should thus be:
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.
R TOP
V OUT
-------------------------- = -------------- – 1
R BOTTOM
V REF
The absolute value of the top resistor is set by the input offset
current of 5.2µA. To achieve 0.5% accuracy, the resistance of
RTOP should be:
V OUT – 2.5
----------------------------- > 1040µA
R TOP
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 10KV resistor for the LED, the maximum current the
LED can see is (12V-4V) /10KΩ = 800µA. The CTR of the
opto-isolator is a minimum of 100%, so the minimum collector
current of the photo-transistor when the diode is full on is also
800µA. The collector resistor must thus be such that:
10V – 5V
------------------------------------ < 800µA or R COLLECTOR > 6.25KΩ;
R COLLECTOR
select 12KΩ to allow some margin.
© 2003 Fairchild Semiconductor Corporation
Page 10 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
Package Dimensions
Footprint Drawing
0.164 (4.16)
0.144 (3.66)
0.024 (0.61)
1
SEATING PLANE
0.202 (5.13)
0.182 (4.63)
0.060 (1.52)
0.275 (6.99)
0.155 (3.94)
0.010 (0.25)
0.006 (0.16)
0.143 (3.63)
0.123 (3.13)
0.008 (0.20)
0.003 (0.08)
0.021 (0.53)
0.011 (0.28)
0.244 (6.19)
0.224 (5.69)
0.050 (1.27)
0.050 (1.27)
TYP
Lead Coplanarity : 0.004 (0.10) MAX
ORDERING INFORMATION
Option
Order Entry Identifier
R1
R1
R1V
R1V
R2
R2
R2V
R2V
Description
Tape and reel (500 units per reel)
VDE0884, Tape and reel (500 units per reel)
Tape and reel (2,500 units per reel)
VDE0884, Tape and reel (2,500 units per reel)
MARKING INFORMATION
Definitions
1
2742A
V
3
X YY S
4
5
© 2003 Fairchild Semiconductor Corporation
2
6
1
Fairchild logo
2
Device number
3
VDE mark (Note: Only appears on parts ordered with VDE
option – See order entry table)
4
One digit year code
5
Two digit work week ranging from ‘01’ to ‘53’
6
Assembly package code
Page 11 of 13
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OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
Carrier Tape Specifications
8.0 ± 0.10
3.50 ± 0.20
2.0 ± 0.05
Ø1.5 MIN
4.0 ± 0.10
0.30 MAX
1.75 ± 0.10
5.5 ± 0.05
12.0 ± 0.3
8.3 ± 0.10
5.20 ± 0.20
6.40 ± 0.20
0.1 MAX
Ø1.5 ± 0.1/-0
User Direction of Feed
Reflow Profile
Temperature (°C)
300
230°C, 10–30 s
250
245°C peak
200
150
Time above 183°C, 120–180 sec
100
Ramp up = 2–10°C/sec
50
• 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
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Time (Minute)
© 2003 Fairchild Semiconductor Corporation
Page 12 of 13
12/9/04
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2742A
FOD2742B
FOD2742C
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.
© 2003 Fairchild Semiconductor Corporation
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.
Page 13 of 13
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