•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
•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
•Latch Off or Continuous Retry After
Fault
•PWM Output Switch Usable to 1A
Peak Current
•1% Reference Accuracy
•500kHz Operation
•18 Pin DIL Package
BLOCK DIAGRA M
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 UC18 40 de vices. While still optimized for highly-efficient bootstrapped pri mary-side operati on in forward or flyback power converters,
the UC1841 is equall y adept in impl ementing b oth low a nd 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.
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-of f or automatic restart.
While pin compatib le with the UC1840 in all respects except that the polarity of the External Stop has bee n re versed, the UC1841 offers the following improvements:
1. Fault latch reset is accomplished with slow start discharge rather
than recycling the input voltage to the chip.
2. The External Stop input can be used for a fault delay to resist
shutdown from short durat ion transients.
3. The duty-cycle clamping function has been characterized an d
specified.
The UC1841 is characteri zed for -55°C to +125°C operation while the
UC2841 and UC3841 are desi gned for -25°C to +85°C and 0°to +70°C,
respectively.
UC1841
UC2841
UC3841
Note: Positive tru e lo gic, latch output s high wit h set, reset has prior ity.
Current Limit Off set05010mV
Current Shutdown O ffs et370400430360400440mV
Input Bias CurrentPin 7 = 0V-2-5-2-5µA
Common Mode Range*-0.43.0-0.43.0V
Current Limit Delay*T
J = 25°C, Pin 7 to 12, RL = 1k200400200400ns
* These paramet ers are guara nt eed by design bu t not 100% teste d in prod uct ion.
UC1841
UC2841
UC3841
3
FUNCTIONAL DES CRIP TIO N
PWM CONTROL
1. OscillatorGen era te s a fixed-frequency int er nal clock fro m an extern al R
K
Frequency =
C
where KC is a first order correction factor ≈ 0.3 log (CT X 1012).
RTCT
UC1841
UC2841
UC3841
T and CT.
2. Ramp Gener ator
Develops a linear ramp with a slope defined exter nally by
dv
dt
=
sense
R
voltage
RCR
CR is normally se le ct ed ≤ CT and its value will have some effect upon va lley volta ge.
Limiting the minimum value for I
C
R terminal can be used as an input port for curren t mode cont rol.
SENSE will establish a maximum duty cycle clamp .
3. Error AmplifierConventional operation al amp lifier f or closed- loop ga in and phase com pe nsat ion.
Low output impedance; unity-gain stable.
The output is held low by the slow star t voltag e at turn on in order to minimize over shoo t.
4. Reference Ge n era torPrecision 5.0V for internal and extern al usage to 50mA.
Tracking 3.0V ref er enc e for intern al usage only w ith nomina l accur acy of
± 2%.
40V clamp zener for chip OV prot ect ion, 100m A m axim um current .
5. PWM Comparat orGen era tes out put pulse which start s at term inat ion of clock pulse and ends whe n the ram p
input crosses the lowest of two posit iv e input s.
6. PWM LatchTerminat es the PW M output pulse whe n set by inputs fro m either th e PWM com pa rator , the
pulse-by-p ulse cur re nt limit compa rator , or the error latch. Reset s with each inter nal clock
pulse.
7. PWM Output SwitchTransisto r capable of sinking cu rrent to ground which is of f during the PWM on- tim e and turns
on to terminate the power pulse. Curr ent cap acit y is 400mA sat ur ated with peak
capacitanc e discharge in excess of one am p.
SEQUENCING FUNCTIONS
1. Start/UV SenseWith an increasing vo lta g e, it gene rates a turn-on signa l an d releases t he slow- star t clamp at
a start threshold.
With a decreasing volta ge, it genera tes a turn- of f com mand at a lower leve l separ at ed by a
200
µA hystere sis cur ren t.
2. Drive SwitchDisab le s most of the chip to hold internal curre nt consump tion low, and Driver Bias OF F, unt il
input voltage reaches start threshold.
3. Driver BiasSupplies drive current to exter nal power switch to prov ide tu rn-on bias .
4. Slow StartClamps low to hold PWM OFF. Upon release, rises with rate contro lled by R
SCS for slow
increase of output pulse widt h.
Can also be used as an alt ernate maximum duty cycle clamp wit h an ext er nal volt age divider.
PROTECTION FUNCTIO NS
1. Error LatchWhen set by mome nt ary input , this latch insur es im me diat e PWM shu tdown and hold of f until
reset. Inputs to Error Latch are :
a. OV > 3.2V (typically 3V)
b. Stop > 2.4V (typically 1.6V)
c. Current Sense 400mV ov er thresho ld (typical) .
Error Latch reset s whe n slow start voltage falls to 0.4V if Rese t Pin 5 < 2.8V. Wit h Pin 5 >
3.2V, Error Latch will remain set.
2. Current LimitingDifferential input comparator terminat es individual output pulses each t ime sense voltage
rises above thre shold.
When sense volta ge rise s to 400m V (typica l) abov e thres hold, a shutdo wn signal is sent t o
Error Latch.
3. External StopA voltage over 1.2 V will set the Erro r Latch and hold the ou tp ut off.
A voltage less than 0. 8V w ill defe a t the error latch and prevent shutd o wn.
A capacitor here will slow the action of the erro r latch for tra nsient prot ection by providing a
typical delay of 13ms/
µF.
4
UC1841
UC2841
UC3841
Start/UV Hysteresis
PWM Output-Saturation Voltage
PWM Output Minimu m Pu lse W idthOscillator Frequency
Shutdown TimingError Amplifier Op en Loop Gain an d Phase
5
OPEN-LOOP TEST CIRCUIT
UC1841
UC2841
UC3841
R1
1
= 50 kHz
TCT
R
+ R2 + R3
R2 + R3
+0.2R1 = 12V
UV Fault V olt age = 3
OV Fault V olt age = 3
Nominal Frequency =
Start Voltage = 3
FLYBACK APPLICATION (A)
In this application (see Figure A, next page), complete
control is maintained on the primary side. Control power
is provide d by R
IN and CIN during start-up, and by a pri-
mary-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 regula-
tion.
An extension t o this application for more precise regulation would be the use of the UC1901 Isolated Feedback
Generator for direct closed-loop control to an output .
R1
R1
+ R2 + R3
R2 + R3
+ R2 + R3
R3
= 8V
= 32V
Curren t Lim it = 20 0mV
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.
REGULATOR APP LI C ATION (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 t o providin g 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
6
UC1841
UC2841
UC3841
Figure A. UC1841 Programma ble PW M Cont roller In A Simplified Flyback Regulator
Figure B. Overall Schematic For A 300 Watt, Off -line Power Converter Using The UC 3 841 For Control
7
UC1841
UC2841
UC3841
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
PROGRAMMABLE SOFT START AND
RESTART DELAY CIRCUIT
VOLTAGE FEED-FO RWARD COMBINED WITH
MAXIMUM DUTY-CYCLE CLAMP
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 R
R2 (assuming R2 > R
CS. Resistor R1 sets a 400mV offset across
CS) so that both the Error Amplifier and
Fault Shutdown can force the current complete ly to zero. R2 is
also used along with C
F as a small filter to attenuate leading-
edge spikes on the load current waveform. In this mode,
current limiting can be accomplished by divider R3/R4 which
forms a clamp o ver riding the output of the Erro r Amplifier.
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 d ut y
cycle is further reduc ed with increa sing VIN.
The minimum ramp current is:
R(MIN) =
l
REF− VINSENSE
R1
≈
4
R1
V
V
The threshold where VIN begins to add extra ramp curren t is:
R2 + R3R3
IN≈ 5.6V
V
Above the threshold, the ramp curr ent will be:
l
R (VARIAB ) ≈
4
R1
+
R2
−
5.6
R3
VIN− 5.6
8
PACKAGE OPTION ADDENDUM
www.ti.com
to Customer on an annual basis.
27-Sep-2005
Addendum-Page 2
PACKAGE OPTION ADDENDUM
www.ti.com
19-Jun-2007
PACKAGING INFORMATION
Orderable DeviceStatus
(1)
Package
Type
Package
Drawing
Pins Package
Qty
Eco Plan
5962-8992002VAOBSOLETECDIPJ18TBDCall TICall TI
UC1841JOBSOLETECDIPJ18TBDCall TICall TI
UC1841J883BOBSOLETECDIPJ18TBDCall TICall TI
UC1841LOBSOLETELCCCFK20TBDCall TICall TI
UC1841L883BOBSOLETELCCCFK20TBDCall TICall TI
UC2841DWACTIVESOICDW1840Green (RoHS &
no Sb/Br)
UC2841DWG4ACTIVESOICDW1840Green (RoHS &
no Sb/Br)
UC2841DWTRG4ACTIVESOICDW18TBDCall TICall TI
UC2841JOBSOLETECDIPJ18TBDCall TICall TI
UC2841NACTIVEPDIPN1820Green (RoHS &
no Sb/Br)
UC2841NG4ACTIVEPDIPN1820Green (RoHS &
no Sb/Br)
UC3841DWACTIVESOICDW1840Green (RoHS &
no Sb/Br)
UC3841DWG4ACTIVESOICDW1840Green (RoHS &
no Sb/Br)
UC3841DWTRACTIVESOICDW182000 Green (RoHS &
no Sb/Br)
UC3841DWTRG4ACTIVESOICDW182000 Green (RoHS &
no Sb/Br)
UC3841JOBSOLETECDIPJ18TBDCall TICall TI
UC3841NACTIVEPDIPN1820Green (RoHS &
no Sb/Br)
UC3841NG4ACTIVEPDIPN1820Green (RoHS &
no Sb/Br)
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in
a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
Lead/Ball Finish MSL Peak Temp
CU NIPDAULevel-2-260C-1 YEAR
CU NIPDAULevel-2-260C-1 YEAR
CU NIPDAUN / A for Pkg Type
CU NIPDAUN / A for Pkg Type
CU NIPDAULevel-2-260C-1 YEAR
CU NIPDAULevel-2-260C-1 YEAR
CU NIPDAULevel-2-260C-1 YEAR
CU NIPDAULevel-2-260C-1 YEAR
CU NIPDAUN / A for Pkg Type
CU NIPDAUN / A for Pkg Type
(3)
(2)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check
http://www.ti.com/productcontent for the latest availability information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements
for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered
at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and
package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS
compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame
retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material)
(3)
MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder
temperature.
Addendum-Page 1
PACKAGE OPTION ADDENDUM
www.ti.com
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is
provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the
accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take
reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on
incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited
information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI
to Customer on an annual basis.