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UC1965
UC2965
UC3965
application
INFO
available
Precision Reference with Low Offset Error Amplifier
FEATURES
DESCRIPTION
• Accessible 2.5V Precision Reference
The UC3965 is suitable for applications needing greater precision and
more functionality than the TL431 type shunt regulators. The wide range
VCC input capability enables the device to be biased from the secondary
side output voltage rail, resulting in closed loop soft start.
• 0.4% Initial Reference Accuracy
• 1% Reference Accuracy over Line,
Load, and Full Temperature Range
• Low 1mV Offset Error Amplifier
• Supports Closed Loop Soft Start
• 2X Inverting Amplifier / Buffer Output
• 4.1V Undervoltage Lockout
• ICC 2mA at 5V
• 8-Pin SOIC or DIL Package
The UC3965 includes an accessible 2.5V precision reference which offers
0.4% initial and 1% reference accuracy over line, load, and full temperature range with a low offset error amplifier, a 2X inverting amplifier/buffer,
and an undervoltage lockout circuit. The IC is ideally suited for applications where high precision PWM power supply regulation is required.
Typically, the error amplifier is configured to compare a fraction of the to
be regulated power supply voltage to the on-chip 2.5V reference. The 2X
amplifier/buffer output is then used to drive a PWM controller or regulator.
The UC3965 is also capable of driving an optocoupler diode for isolated
applications.
BLOCK DIAGRAM
OFFSET
VCC
1
2
+
OB
–
GND
4
2k
7
VFB
6
VREF
5
NI
VCC
3
15k
COMP
VREF
2.5V
UVLO
5k
VOUT
8
–
EA
+
7.5k
5k
UDG-98055
UC1965
UC2965
UC3965
CONNECTION DIAGRAM
ABSOLUTE MAXIMUM RATINGS
VCC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to 20V
VREF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to 6V
VFB, COMP, NI, VOUT. . . . . . . . . . . . . . . . . . . . . –0.3V TO 6V
Storage Temperature . . . . . . . . . . . . . . . . . . . −65°C to +150°C
Junction Temperature . . . . . . . . . . . . . . . . . . . –55°C to +150°C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300°C
DIL-8, SOIC-8 (Top View)
J or N Package, D Package
Currents are positive into, negative out of the specified terminal.
All voltages are with respect to ground. Consult Packaging Section of Databook for thermal limitations and considerations of
packages.
OFFSET
1
8
COMP
VCC
2
7
VFB
VOUT
3
6
VREF
GND
4
5
NI
ORDERING INFORMATION
TEMPERATURE RANGE
–55°C to +125°C
–40°C to +85°C
UC1965J
UC2965D
UC2965N
UC3965D
UC3965N
0°C to +70°C
PACKAGE
CDIP
SOIC
PDIP
SOIC
PDIP
ELECTRICAL CHARACTERISTICS Unless otherwise specified, VCC = 5V, TA = TJ.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX UNITS
General
VCC
Operating Current
4.3
VCC = 5V
20
V
4
mA
200
µA
4.3
V
300
400
mV
1.5
2
Minimum Voltage to Start
3.9
4.1
Hysteresis
200
Undervoltage Current
VREF
VREF Initial Accuracy
+25°C
2.49
2.5
2.51
V
VREF Over Temperature
–55°C to +125°C
2.48
2.5
2.52
V
Total Output Variation
Line, Load, Temperature
2.475
2.5
2.525
V
Line Regulation
VCC = 4.3V to 20V
2
10
mV
Load Regulation
0µA to 500µA
2
10
mV
Short Circuit Current
VREF = 0V
2
Input Bias
VCM = 2.5V
200
Input Offset Voltage
VCM = 2.5V
Input Offset Current
VCM = 2.5V
mA
Error Amplifier
Gain Bandwidth Product
VIN = 50mV P-P (Note 1)
Open Loop Gain
VOUT = 1V to 3.75V
400
nA
1
2
mV
–100
0
100
nA
6
MHz
80
100
dB
UC1965
UC2965
UC3965
ELECTRICAL CHARACTERISTICS Unless otherwise specified, VCC = 5V, TA = TJ.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX UNITS
Error Amplifier Section (cont.)
Output Low Level
Output High Level
IOUT = 0µA
0.8
V
IOUT = 100µA
1.2
V
IOUT = 0µA
4
V
IOUT = –500µA
4
V
Short Circuit Circuit
VCOMP = 0V
8
mA
CMRR
VCM = 1.25V to 3.75V
70
100
dB
PSRR
VCC = 4.3V to 20V
70
100
dB
Rising Slew Rate
2
V/µs
Falling Slew Rate
0.4
V/µs
Inverting Buffer Amplifier
VCM = 2.5V
Output Offset Voltage
VCM = 2.5V
Gain Bandwidth Product
VIN = 50mV P-P (Note 1)
Closed Loop Gain
Inverting Gain
Output Low Level
IOUT = 0µA
0.3
V
IOUT = 100µA
0.5
V
4
V
Output High Level
Short Circuit Circuit
–20
1
2
0
20
µA
Input Bias
1.5
–2.04
IOUT = 0mA
–2
mV
MHz
–1.96
V/V
IOUT = –4mA
4
V
VOUT = 0V
18
mA
CMRR
VCM = 1.25V to 3.75V
70
100
dB
PSRR
VCC = 4.3V to 20V
70
100
dB
Rising Slew Rate
0.9
V/µs
Falling Slew Rate
0.9
V/µs
Note 1: Guaranteed by design. Not 100% tested in production.
PIN DESCRIPTIONS
COMP: The output of the error amplifier and the input to
the inverting terminal of the internal output buffer. This
pin is available to compensate the high frequency gain of
the error amplifier.
GND: The reference and power ground for the device.
NI: The non-inverting input to the error amplifier.
OFFSET: The non-inverting input to the internal output
buffer.
VCC: The power input to the device. The minimum to
maximum operating voltage is 4.3V to 20V.
VFB: The inverting terminal of the error amplifier used as
both the voltage sense input to the error amplifier and its
other compensation point.
VOUT: The emitter of the output transistor. This pin is
the output of the inverting buffer. This pin has the capability to drive an optocoupler or a PWM controller directly.
VREF: The output of the trimmed precision reference.
This reference maintains within 1% of its initial value over
its entire line, load, and temperature range.
UC1965
UC2965
UC3965
APPLICATION INFORMATION
For designs requiring input-output isolation, the UC3965
is used in secondary side output voltage sensing. As
shown in Fig. 1, the precision reference and low offset
error amplifier can be used in converters, such as the
isolated flyback, where the primary side error amplifier is
not used or simply not present. In this case, the
UCC3809 is used as the primary side controller.
The precision reference of the UC3965 is tied to the
non-inverting input of the device’s internal error amplifier.
The output voltage of the converter is resistively divided
and compared to this reference at the inverting input.
This error amplifier has a low 1mV input offset voltage
that insures accurate regulation of the output. The internal error amplifier drives the inverting input of the output
buffer (OB) which drives an optocoupler diode. The wide
range VCC voltage enables the device to be biased from
the secondary side output voltage rail, resulting in closed
loop soft start.
As the output voltage increases beyond its desired value,
the voltage difference at the error amplifier increases.
This results in less drive at the inverting input of the internal buffer, increasing its output drive to the optocoupler.
If the application does not require input-output isolation,
this buffer could be used to drive the PWM directly.
+V OUT
1
2
3
OFFSET
COMP
VCC
VREF
2.5V
UVLO
VOUT
5k
+
OB
–
4
+V IN
15k
GND
2k
1 FB
2 SS
3 RT1
REF 8
VFB
VCC
–
EA
+
7.5k
5k
UCC3809
8
VREF
NI
7
6
5
–V OUT
VDD 7
OUT 6
4 RT2 GND 5
–V IN
UDG-99080
Figure 1. Typical application diagram.
ADDITIONAL INFORMATION
For additional application information biasing
UC3965, please refer to the following publication:
the
[1] Application Note U-165, Design Review: Isolated 50W
Flyback with the UCC3809 Primary Side Controller and
the UC3965 Precision Reference and Error Amplifier, by
Lisa Dinwoodie.
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
IMPORTANT NOTICE
Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue
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pertaining to warranty, patent infringement, and limitation of liability.
TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent
TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily
performed, except those mandated by government requirements.
Customers are responsible for their applications using TI components.
In order to minimize risks associated with the customer’s applications, adequate design and operating
safeguards must be provided by the customer to minimize inherent or procedural hazards.
TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent
that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other
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Copyright  2000, Texas Instruments Incorporated
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