TI UC2841

UC1841
UC2841
UC3841
Programmable, Off-Line, PWM Controller
FEATURES
DESCRIPTION
•
All Control, Driving, Monitoring, and
Protection Functions Included
•
Low-current, Off-line Start Circuit
•
Voltage Feed Forward or Current
Mode Control
•
Guaranteed Duty Cycle Clamp
•
The UC1841 family of PWM controllers has been designed to increase
the level of versatility while retaining all of the performance features of
the earlier UC1840 devices. While still optimized for highly-efficient bootstrapped primary-side operation in forward or flyback power converters,
the UC1841 is equally adept in implementing both low and high voltage
input DC to DC converters. Important performance features include a
low-current starting circuit, linear feed-forward for constant volt-second
operation, and compatibility with either voltage or current mode topologies.
PWM Latch for Single Pulse per Period
•
Pulse-by-Pulse Current Limiting Plus
Shutdown for Over-Current Fault
•
No Start-up or Shutdown Transients
•
Slow Turn-on Both Initially and After
Fault Shutdown
•
Shutdown Upon Over- or
Under-Voltage Sensing
1. Fault latch reset is accomplished with slow start discharge rather
than recycling the input voltage to the chip.
•
Latch Off or Continuous Retry After
Fault
2. The External Stop input can be used for a fault delay to resist
shutdown from short duration transients.
•
PWM Output Switch Usable to 1A
Peak Current
3. The duty-cycle clamping function has been characterized and
specified.
•
1% Reference Accuracy
•
500kHz Operation
•
The UC1841 is characterized for -55°C to +125°C operation while the
UC2841 and UC3841 are designed for -25°C to +85°C and 0°to +70°C,
respectively.
18 Pin DIL Package
In addition to start-up and normal regulating PWM functions, these devices include built in protection from over-voltage, under-voltage, and
over-current fault conditions with the option for either latch-off or automatic restart.
While pin compatible with the UC1840 in all respects except that the polarity of the External Stop has been reversed, the UC1841 offers the following improvements:
BLOCK DIAGRAM
Note: Positive true logic, latch outputs high with set, reset has priority.
6/93
UC1841
UC2841
UC3841
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, +VIN (Pin 15) (Note 2)
Voltage Driven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +32V
Current Driven, 100mA maximum . . . . . . . . . . . . Self-limiting
PWM Output Voltage (Pin 12) . . . . . . . . . . . . . . . . . . . . . . . 40V
PWM Output Current, Steady-State (Pin 12) . . . . . . . . . 400mA
PWM Output Peak Energy Discharge . . . . . . . . . . . . 20µJoules
Driver Bias Current (Pin 14) . . . . . . . . . . . . . . . . . . . . . -200mA
Reference Output Current (Pin 16) . . . . . . . . . . . . . . . . -50mA
Slow-Start Sink Current (Pin 8) . . . . . . . . . . . . . . . . . . . . 20mA
VIN Sense Current (Pin 11). . . . . . . . . . . . . . . . . . . . . . . . 10mA
Current Limit Inputs (Pins 6 & 7) . . . . . . . . . . . . . -0.5 to +5.5V
Stop Input (Pin 4) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +5.5V
Comparator Inputs
(Pins 1, 7, 9-11, 16) . . . . . . . . . . . . Internally clamped at 12V
Power Dissipation at TA = 25°C (Note 3) . . . . . . . . . . . 1000mW
Power Dissipation at TC = 25°C (Note 3) . . . . . . . . . . . 2000mW
Operating Junction Temperature . . . . . . . . . . -55°C to +150°C
Storage Temperature Range. . . . . . . . . . . . . . -65°C to +150°C
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . . +300°C
Note 1: All voltages are with respect to ground, Pin 13.
Currents are positive-into, negative-out of the specified
terminal.
Note 2: All pin numbers are referenced to DIL-18 package.
Note 3: Consult Packaging Section of Databook for thermal
limitations and considerations of package.
PLCC-20, LCC-20
(TOP VIEW)
Q or L Package
PACKAGE PIN FUNCTIONS
FUNCTION
PIN
Comp
Start/UV
OV Sense
Stop
Reset
CUR Thresh
CUR Sense
Slow Start
RT/CT
Ramp
VIN Sense
PWM Out
Ground
Drive Bias
+VIN Supply
5.0V REF
Inv. Input
N.I. Input
CONNECTION DIAGRAMS
DIL-18, SOIC-18 (TOP VIEW)
J or N, DW Package
1
2
3
4
5
7
8
9
10
11
12
13
14
15
17
18
19
20
ELECTRICAL CHARACTERISTICS: Unless otherwise stated, these specifications apply for TA = -55°C to +125°C for the
UC1841, -25°C to +85°C for the UC2841, and 0°C to +70°C for the UC3841; VIN = 20V, RT = 20kΩ, CT = .001mfd, RR = 10kΩ,
CR = .001mfd, Current Limit Threshold = 200mV, TA = TJ.
UC1841 / UC2841
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
4.5
6
UC3841
MIN
UNITS
TYP
MAX
4.5
6
mA
Power Inputs
Start-Up Current
VIN = 30V, Pin 2 = 2.5V
Operating Current
VIN = 30V, Pin 2 = 3.5V
10
14
10
14
mA
Supply OV Clamp
IIN = 20mA
33
40
45
33
40
45
V
Reference Voltage
TJ = 25°C
4.95
5.0
5.05
4.9
5.0
5.1
V
Line Regulation
VIN = 8 to 30V
10
15
10
20
mV
Load Regulation
IL = 0 to 10mA
10
20
10
30
mV
5.15
V
-80
-100
-80
-100
mA
50
53
50
55
kHz
Reference Section
Temperature Stability
Over Operating Temperature Range
Short Circuit Current
VREF = 0, TJ = 25°C
4.9
5.1
4.85
Oscillator
Nominal Frequency
TJ = 25°C
47
Voltage Stability
VIN = 8 to 30V
Temperature Stability
Over Operating Temperature Range
Maximum Frequency
RT = 2kΩ, CT = 330pF
0.5
45
500
2
45
1
55
0.5
43
500
1
%
57
kHz
kHz
UC1841
UC2841
UC3841
ELECTRICAL CHARACTERISTICS: Unless otherwise stated, these specifications apply for TA = -55°C to +125°C for the
UC1841, -25°C to +85°C for the UC2841, and 0°C to +70°C for the UC3841; VIN = 20V, RT = 20kΩ, CT = .001mfd, RR = 10kΩ,
CR = .001mfd, Current Limit Threshold = 200mV, TA = TJ.
PARAMETER
TEST CONDITIONS
UC1841 / UC2841
MIN
TYP
MAX
-11
-14
-0.9
-.95
0.3
0.4
3.9
4.2
0.5
UC3841
UNITS
MIN
TYP
MAX
-11
-14
-0.9
-.95
0.6
0.3
0.4
0.6
V
4.5
3.9
4.2
4.5
V
5
2
10
mV
2
1
5
µA
0.5
µA
Ramp Generator
Ramp Current, Minimum
ISENSE = -10µA
Ramp Current, Maximum
ISENSE = 1.0mA
Ramp Valley
Ramp Peak
Clamping Level
µA
mA
Error Amplifier
Input Offset Voltage
VCM = 5.0V
Input Bias Current
0.5
Input Offset Current
0.5
Open Loop Gain
∆VO= 1 to 3V
60
Output Swing (Max. Output ≤
Ramp Peak - 100mV)
Minimum Total Range
0.3
CMRR
VCM = 1.5 to 5.5V
70
80
PSRR
VIN = 8 to 30V
70
80
Short Circuit Current
VCOMP = 0V
Gain Bandwidth*
TJ = 25°C, AVOL = 0dB
Slew Rate*
TJ = 25°C, AVCL = 0dB
66
-4
1
60
3.5
80
70
80
-4
1
0.8
dB
3.5
70
-10
2
66
0.3
V
dB
dB
-10
mA
2
MHz
0.8
V/µs
PWM Section
Continuous Duty Cycle
Range* (other than zero)
Minimum Total Continuous Range,
Ramp Peak < 4.2V
4
50% Duty Cycle Clamp
RSENSE to VREF = 10k
42
Output Saturation
95
4
47
52
42
IOUT = 20mA
0.2
0.4
0.2
0.4
V
IOUT = 200mA
1.7
2.2
1.7
2.2
V
47
95
%
52
%
Output Leakage
VOUT = 40V
0.1
10
0.1
10
µA
Comparator Delay*
Pin 8 to Pin 12, TJ = 25°C, RL = 1kΩ
300
500
300
500
ns
3.0
3.2
3.0
3.2
V
Sequencing Functions
Comparator Thresholds
Pins 2, 3, 5
2.8
2.8
Input Bias Current
Pins 3, 5 = 0V
-1.0
-4.0
-1.0
-4.0
µA
Input Leakage
Pins 3, 5 = 10V
0.1
2.0
0.1
2.0
µA
Start/UV Hysteresis Current
Pin 2 = 2.5V
170
200
220
170
200
230
µA
Ext. Stop Threshold
Pin 4
0.8
1.6
2.4
0.8
1.6
2.4
V
Error Latch Activate Current
Pin 4 = 0V, Pin 3 > 3V
-120
-200
-120
-200
µA
2
3
2
3
V
µA
Driver Bias Saturation Voltage, IB = -50mA
VIN - VOH
Driver Bias Leakage
VB = 0V
-0.1
-10
-0.1
-10
Slow-Start Saturation
IS = 10mA
0.2
0.5
0.2
0.5
V
Slow-Start Leakage
VS = 4.5V
0.1
2.0
0.1
2.0
µA
Current Control
Current Limit Offset
Current Shutdown Offset
Input Bias Current
370
Pin 7 = 0V
Common Mode Range*
0
5
400
430
-2
-5
200
400
-0.4
Current Limit Delay*
TJ = 25°C, Pin 7 to 12, RL = 1k
* These parameters are guaranteed by design but not 100% tested in production.
3
3.0
360
0
10
mV
400
440
mV
-2
-5
µA
3.0
V
200
400
ns
-0.4
UC1841
UC2841
UC3841
FUNCTIONAL DESCRIPTION
PWM CONTROL
1. Oscillator
Generates a fixed-frequency internal clock from an external RT and CT.
Frequency =
KC
RTCT
where KC is a first order correction factor ≈ 0.3 log (CT X 1012).
2. Ramp Generator
Develops a linear ramp with a slope defined externally by
3. Error Amplifier
4. Reference Generator
5. PWM Comparator
6. PWM Latch
7. PWM Output Switch
SEQUENCING FUNCTIONS
1. Start/UV Sense
2. Drive Switch
3. Driver Bias
4. Slow Start
PROTECTION FUNCTIONS
1. Error Latch
2. Current Limiting
3. External Stop
dv sense voltage
=
RRCR
dt
CR is normally selected ≤ CT and its value will have some effect upon valley voltage.
Limiting the minimum value for ISENSE will establish a maximum duty cycle clamp.
CR terminal can be used as an input port for current mode control.
Conventional operational amplifier for closed-loop gain and phase compensation.
Low output impedance; unity-gain stable.
The output is held low by the slow start voltage at turn on in order to minimize overshoot.
Precision 5.0V for internal and external usage to 50mA.
Tracking 3.0V reference for internal usage only with nominal accuracy of ± 2%.
40V clamp zener for chip OV protection, 100mA maximum current.
Generates output pulse which starts at termination of clock pulse and ends when the ramp
input crosses the lowest of two positive inputs.
Terminates the PWM output pulse when set by inputs from either the PWM comparator, the
pulse-by-pulse current limit comparator, or the error latch. Resets with each internal clock
pulse.
Transistor capable of sinking current to ground which is off during the PWM on-time and turns
on to terminate the power pulse. Current capacity is 400mA saturated with peak
capacitance discharge in excess of one amp.
With an increasing voltage, it generates a turn-on signal and releases the slow-start clamp at
a start threshold.
With a decreasing voltage, it generates a turn-off command at a lower level separated by a
200µA hysteresis current.
Disables most of the chip to hold internal current consumption low, and Driver Bias OFF, until
input voltage reaches start threshold.
Supplies drive current to external power switch to provide turn-on bias.
Clamps low to hold PWM OFF. Upon release, rises with rate controlled by RSCS for slow
increase of output pulse width.
Can also be used as an alternate maximum duty cycle clamp with an external voltage divider.
When set by momentary input, this latch insures immediate PWM shutdown and hold off until
reset. Inputs to Error Latch are:
a. OV > 3.2V (typically 3V)
b. Stop > 2.4V (typically 1.6V)
c. Current Sense 400mV over threshold (typical).
Error Latch resets when slow start voltage falls to 0.4V if Reset Pin 5 < 2.8V. With Pin 5 >
3.2V, Error Latch will remain set.
Differential input comparator terminates individual output pulses each time sense voltage
rises above threshold.
When sense voltage rises to 400mV (typical) above threshold, a shutdown signal is sent to
Error Latch.
A voltage over 1.2V will set the Error Latch and hold the output off.
A voltage less than 0.8V will defeat the error latch and prevent shutdown.
A capacitor here will slow the action of the error latch for transient protection by providing a
typical delay of 13ms/µF.
4
UC1841
UC2841
UC3841
Start/UV Hysteresis
PWM Output-Saturation Voltage
Oscillator Frequency
PWM Output Minimum Pulse Width
Error Amplifier Open Loop Gain and Phase
Shutdown Timing
5
UC1841
UC2841
UC3841
OPEN-LOOP TEST CIRCUIT
Nominal Frequency =
1
= 50 kHz
RTCT
 R1 + R2 + R3 
Start Voltage = 3 
 +0.2R1 = 12V
 R2 + R3 
 R1 + R2 + R3 
UV Fault Voltage = 3 
 = 8V
 R2 + R3 
 R1 + R2 + R3 
OV Fault Voltage = 3 
 = 32V
R3


FLYBACK APPLICATION (A)
Current Limit = 200mV
Current Fault Voltage = 600mV
Duty Cycle Clamp = 50%
Not shown, are protective snubbers or additional interface
circuitry which may be required by the choice of the highvoltage switch, Qs, or the application; however, one example of power transistor interfacing is provided on the
following page.
In this application (see Figure A, next page), complete
control is maintained on the primary side. Control power
is provided by RIN and CIN during start-up, and by a primary-referenced low voltage winding, N2, for efficient operation after start. The error amplifier loop is closed to
regulate the DC voltage from N2 with other outputs following through their magnetic coupling − a task made
even easier with the UC1841’s feed−forward line regulation.
REGULATOR APPLICATION (B)
With the addition of a level shifting transistor, Q1, the
UC1841 is an ideal control circuit for DC to DC converters
such as the buck regulator shown in Figure B opposite. In
addition to providing constant current drive pulses to the
PIC661 power switch, this circuit has full fault protection
and high speed dynamic line regulation due to its feedforward capability. An additional feature is the ability to
An extension to this application for more precise regulation would be the use of the UC1901 Isolated Feedback
Generator for direct closed-loop control to an output.
6
UC1841
UC2841
UC3841
Figure A. UC1841 Programmable PWM Controller In A Simplified Flyback Regulator
Figure B. Overall Schematic For A 300 Watt, Off-line Power Converter Using The UC3841 For Control
7
UC1841
UC2841
UC3841
PROGRAMMABLE SOFT START AND
RESTART DELAY CIRCUIT
ERROR LATCH INTERNAL CIRCUITRY
The Error Latch consists of Q5 and Q6 which, when both on,
turns off the PWM Output and pulls the Slow-Start pin low. This
latch is set by either the Over-Voltage or Current Shutdown
comparators, or by a high signal on Pin 4. Reset is accomplished by either the Reset comparator or a low signal on Pin
4. An activation time delay can be provided with an external
capacitor on Pin 4 in conjunction with the ≈ 100µA collector
current from Q4.
CURRENT MODE CONTROL
VOLTAGE FEED-FORWARD COMBINED WITH
MAXIMUM DUTY-CYCLE CLAMP
In this circuit, R1 is used in conjunction with CR (not shown) to
establish a minimum ramp charging current such that the ramp
voltage reaches 4.2V at the required maximum output pulse
width.
The purpose of Q1 is to provide an increasing ramp current
above a threshold established by R2 and R3 such that the duty
cycle is further reduced with increasing VIN.
Since Pin 10 is a direct input to the PWM comparator, this
point can also serve as a current sense port for current mode
control. In this application, current sensing is ground referenced through RCS. Resistor R1 sets a 400mV offset across
R2 (assuming R2 > RCS) so that both the Error Amplifier and
Fault Shutdown can force the current completely to zero. R2 is
also used along with CF as a small filter to attenuate leadingedge spikes on the load current waveform. In this mode,
current limiting can be accomplished by divider R3/R4 which
forms a clamp overriding the output of the Error Amplifier.
The minimum ramp current is:
lR(MIN) =
VREF − VIN SENSE 4V
≈
R1
R1
The threshold where VIN begins to add extra ramp current is:
 R2 + R3 
VIN ≈ 5.6V 

 R3 
Above the threshold, the ramp current will be:
VIN − 5.6 5.6
4
l R (VARIAB ) ≈
+
−
R1
R2
R3
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8
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