UC2842B/3B/4B/5B
UC3842B/3B/4B/5B
August1996
HIGH PERFORMANCE CURRENTMODE PWM CONTROLLER
.TRIMMED OSCILLATORFOR PRECISE FRE-
QUENCY CONTROL
.OSCILLATOR FREQUENCY GUARANTEED
AT 250kHz
.CURRENT MODE OPERATIONTO 500kHz
.AUTOMATIC FEED FORWARD COMPENSA-
TION
.LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
.INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGELOCKOUT
.HIGHCURRENT TOTEMPOLEOUTPUT
.UNDERVOLTAGE LOCKOUT WITH HYSTER-
ESIS
.LOWSTART-UPANDOPERATINGCURRENT
DESCRIPTION
TheUC384xBfamilyofcontrolICsprovidesthenec-
essaryfeatures to implement off-line or DC to DC
fixedfrequencycurrent modecontrolschemeswith
a minimal external parts count. Internally imple-
mentedcircuits includea trimmedoscillatorfor pre-
cise DUTY CYCLE CONTROL under voltagelock-
outfeaturingstart-upcurrentlessthan0.5mA,a pre-
cision reference trimmed for accuracy at the error
ampinput,logicto insurelatchedoperation,a PWM
comparatorwhichalsoprovidescurrentlimitcontrol,
and a totempole output stage designed to source
orsinkhighpeakcurrent.Theoutputstage,suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differencesbetween members of this family are the
under-voltagelockoutthresholdsandmaximumduty
cycle ranges. The UC3842B and UC3844B have
UVLO thresholds of 16V (on) and 10V (off), ideally
suitedoff-lineapplicationsThecorrespondingthresh-
oldsfortheUC3843BandUC3845Bare8.5Vand7.9
V. TheUC3842BandUC3843B canoperateto duty
cyclesapproaching100%. A range of the zero to <
50 % is obtainedbythe UC3844Band UC3845Bby
theadditionofaninternaltoggleflipflopwhichblanks
theoutputoffeveryotherclockcycle.
BLOCK DIAGRAM (toggle flip flop used only in UC3844B and UC3845B)
UVLO
S/R 5V
REF
34V
INTERNAL
BIAS
VREF GOOD
LOGIC
2.50V
T
S
R
OSC
R1V
CURRENT
SENSE
COMPARATOR
2R
+
- PWM
LATCH
7
5
4
2
1
3
8
6
ERROR AMP.
Vi
GROUND
RT/CT
VFB
COMP
CURRENT
SENSE
VREF
5V 50mA
OUTPUT
D95IN331
Minidip SO8
UC3842B
1/15
*All voltages are with respectto pin 5, all currents arepositive into the specified terminal.
PIN CONNECTION (top view)
COMP
VFB
ISENSE
RT/CTGROUND
OUTPUT
Vi
VREF
1
3
2
4
6
5
7
8
D95IN332
Minidip/SO8
ORDERINGNUMBERS
SO8 Minidip
UC2842BD1; UC3842BD1
UC2843BD1; UC3843BD1
UC2844BD1; UC3844BD1
UC2845BD1; UC3845BD1
UC2842BN; UC3842BN
UC2843BN; UC3843BN
UC2844BN; UC3844BN
UC2845BN; UC3845BN
ABSOLUTE MAXIMUM RATINGS
Symbol Parameter Value Unit
ViSupply Voltage (low impedance source) 30 V
ViSupplyVoltage (Ii < 30mA) Self Limiting
IOOutput Current ±1A
EOOutput Energy (capacitive load) 5 µJ
Analog Inputs (pins 2, 3) – 0.3 to 5.5 V
ErrorAmplifier Output SinkCurrent 10 mA
Ptot Power Dissipation at Tamb ≤25 °C (Minidip) 1.25 W
Ptot Power Dissipation at Tamb ≤25 °C (SO8) 800 mW
Tstg Storage Temperature Range – 65 to 150 °C
TLLead Temperature (soldering 10s) 300 °C
PIN FUNCTIONS
No Function Description
1 COMP This pin is the Error Amplifier output and is made available for loop compensation.
2V
FB This is the inverting input of the Error Amplifier. It is normally connected to the switching
power supply output through a resistor divider.
3I
SENSE A voltage proportional to inductor current is connected to this input. The PWM uses this
information to terminate the output switch conduction.
4R
T
/CTThe oscillator frequency and maximum Output duty cycle are programmed by connecting
resistor RTto Vref and cpacitor CTto ground. Operation to 500kHz is possible.
5 GROUND This pin is the combined control circuitry and power ground.
6 OUTPUT This output directly drives the gate of a power MOSFET. Peak currents up to 1A are sourced
and sunk by this pin.
7V
CC This pin is the positive supply of the control IC.
8V
ref This is the reference output. It provides charging current for capacitor CTthrough resistor RT.
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
2/15
ELECTRICAL CHARACTERISTICS ( [note 1] Unless otherwise stated, these specifications apply for
-25 < Tamb <85°C forUC284XB;0 < Tamb <70°C forUC384XB;Vi=15V (note5); RT= 10K;CT= 3.3nF)
Symbol Parameter Test Conditions UC284XB UC384XB Unit
Min. Typ. Max. Min. Typ. Max.
REFERENCE SECTION
VREF Output Voltage Tj=25°CI
o= 1mA 4.95 5.00 5.05 4.90 5.00 5.10 V
∆VREF Line Regulation 12V ≤Vi≤25V 220 220mV
∆V
REF Load Regulation 1≤Io≤20mA 325 325mV
∆
V
REF/∆TTemperature Stability (Note 2) 0.2 0.2 mV/°C
Total Output Variation Line, Load, Temperature 4.9 5.1 4.82 5.18 V
eNOutput Noise Voltage 10Hz ≤f≤10KHz Tj=25
°
C
(note 2) 50 50 µV
Long Term Stability Tamb = 125°C, 1000Hrs
(note 2) 525 525mV
I
SC Output Short Circuit -30 -100 -180 -30 -100 -180 mA
OSCILLATOR SECTION
fOSC Frequency Tj=25
°
C
T
A=T
low to Thigh
TJ=25°C(R
T= 6.2k, CT=1nF)
49
48
225
52
–
250
55
56
275
49
48
225
52
–
250
55
56
275
KHz
KHz
KHz
∆fOSC/∆VFrequency Change with Volt. VCC = 12V to 25V – 0.2 1 – 0.2 1 %
∆fOSC/∆TFrequency Change with Temp. TA=T
low to Thigh –1––0.5–%
V
OSC Oscillator Voltage Swing (peak to peak) – 1.6 – – 1.6 – V
Idischg Discharge Current (VOSC =2V) TJ=25°C
T
A=T
low to Thigh
7.8
7.5 8.3
–8.8
8.8 7.8
7.6 8.3
–8.8
8.8 mA
mA
ERROR AMP SECTION
V2Input Voltage VPIN1 = 2.5V 2.45 2.50 2.55 2.42 2.50 2.58 V
IbInput Bias Current VFB = 5V -0.1 -1 -0.1 -2 µA
AVOL 2V ≤Vo≤4V 65 90 65 90 dB
BW Unity Gain Bandwidth TJ=25°C 0.7 1 0.7 1 MHz
PSRR Power Supply Rejec. Ratio 12V ≤Vi≤25V 60 70 60 70 dB
IoOutput Sink Current VPIN2 = 2.7V VPIN1 = 1.1V 2 12 2 12 mA
IoOutput Source Current VPIN2 = 2.3V VPIN1 = 5V -0.5 -1 -0.5 -1 mA
VOUT High VPIN2 = 2.3V;
RL= 15KΩto Ground 5 6.2 5 6.2 V
VOUT Low VPIN2 = 2.7V;
RL= 15KΩto Pin 8 0.8 1.1 0.8 1.1 V
CURRENT SENSE SECTION
GVGain (note 3 & 4) 2.85 3 3.15 2.85 3 3.15 V/V
V3Maximum Input Signal VPIN1 = 5V (note 3) 0.9 1 1.1 0.9 1 1.1 V
SVR Supply Voltage Rejection 12 ≤Vi≤25V (note 3) 70 70 dB
IbInput Bias Current -2 -10 -2 -10 µA
Delay to Output 150 300 150 300 ns
THERMAL DATA
Symbol Description Minidip SO8 Unit
Rth j-amb Thermal ResistanceJunction-ambient. max. 100 150 °C/W
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
3/15
Notes : 1. Max package powerdissipation limitsmustbe respected; low duty cyclepulse techniques are used during test maintain Tjas
close to Tamb aspossible.
2. These parameters, although guaranteed, are not100%testedin production.
3. Parameter measured at trippoint of latchwithVPIN2 =0.
4. Gain definedas :
∆VPIN1
A= ;0≤V
PIN3 ≤0.8V
∆VPIN3
5. AdjustViabove thestart threshold beforesettingat 15 V.
ELECTRICAL CHARACTERISTICS (continued)
Symbol Parameter Test Conditions UC284XB UC384XB Unit
Min. Typ. Max. Min. Typ. Max.
OUTPUT SECTION
VOL Output Low Level ISINK = 20mA 0.1 0.4 0.1 0.4 V
ISINK = 200mA 1.6 2.2 1.6 2.2 V
VOH Output High Level ISOURCE = 20mA 13 13.5 13 13.5 V
ISOURCE = 200mA 12 13.5 12 13.5 V
VOLS UVLO Saturation VCC = 6V; ISINK = 1mA 0.1 1.1 0.1 1.1 V
trRise Time Tj=25
°
CC
L
= 1nF (2) 50 150 50 150 ns
tfFall Time Tj=25°CC
L
= 1nF (2) 50 150 50 150 ns
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold X842B/4B 15 16 17 14.5 16 17.5 V
X843B/5B 7.8 8.4 9.0 7.8 8.4 9.0 V
Min Operating Voltage
After Turn-on X842B/4B 9 10 11 8.5 10 11.5 V
X843B/5B 7.0 7.6 8.2 7.0 7.6 8.2 V
PWM SECTION
Maximum Duty Cycle X842B/3B 94 96 100 94 96 100 %
X844B/5B 47 48 50 47 48 50 %
Minimum Duty Cycle 0 0 %
TOTAL STANDBY CURRENT
Ist Start-up Current Vi= 6.5V for UCX843B/45B 0.3 0.5 0.3 0.5 mA
Vi= 14V for UCX842B/44B 0.3 0.5 0.3 0.5 mA
IiOperating Supply Current VPIN2 =V
PIN3 =0V 12 17 12 17 mA
V
iz Zener Voltage Ii= 25mA 30 36 30 36 V
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
4/15
Figure 1: OpenLoopTest Circuit.
RT
A2N2222
4.7KΩ
1KΩ
ERROR AMP.
ADJUST
4.7KΩ5KΩ
ISENSE
ADJUST
100KΩCOMP
VFB
ISENSE
RT/CT
1
2
3
4
CT
7
6
5
8
VREF
Vi
OUTPUT
GROUND
0.1µF
0.1µF
VREF
Vi
OUTPUT
GROUND
1W
1KΩ
D95IN343
Highpeakcurrentsassociatedwithcapacitiveloads
necessitate careful grounding techniques. Timing
and bypass capacitors should be connectedclose
to pin5 ina singlepointground.Thetransistorand
5KΩpotentiometerareusedtosampletheoscillator
waveformand applyan adjustableramp to pin 3.
10K 20K 30K 50K 100K 200K 300K 500K fOSC(KHz)
1
2
5
10
20
50
D95IN333
CT=10nF
CT=5nF
CT=2nF
C
T
=1nF
C
T
=500pF
C
T
=200pF
C
T
=100pF
Vi
=15V
TA=25°C
RT
(KΩ)
0.8
Figure 2: Timing Resistor vs. Oscillator Fre-
quency
10K 20K 30K 50K 100K 200K 300K 500K fOSC(KHz)
1
2
3
5
10
20
30
50
%
CT=10nF
CT=5nF
CT=2nF
CT=1nF
CT=500pF
CT=200pF
CT=100pF
D95IN334
Vi
=15V
TA=25°C
Figure 3: Output Dead-Time vs. Oscillator Fre-
quency
UC2842B
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
5/15
-55 -25 0 25 50 75 100 TA(°C)
7.0
7.5
8.0
8.5
Idischg
(mA)
D95IN335
Vi
=15V
VOSC=2V
Figure 4: Oscillator Discharge Current vs. Tem-
perature.
1235
40
50
60
70
80
90
Dmax
(%)
0.8 RT(KΩ)
D95IN336
Vi=15V
CT=3.3nF
TA=25°C
Idischg=7.5mA
Idischg=8.8mA
Figure 5: Maximum Output Duty Cycle vs. Tim-
ing Resistor.
10 100 1K 10K 100K 1M f(Hz)
-20
0
20
40
60
80
(dB)
180
150
120
90
60
30
φ
D95IN337
Vi
=15V
VO=2V to
4V
RL
=100K
TA=25°C
Gain
Phase
Figure 6: Error Amp Open-LoopGain and
Phase vs. Frequency.
0246V
O
(V)
0.0
0.2
0.4
0.6
0.8
1.0
Vth
(V)
D95IN338
Vi=15V
TA=-40°C
TA=125°C
TA=25°C
Figure 7: Current Sense Input Threshold vs. Er-
ror Amp Output Voltage.
0 20406080100I
ref(mA)
D95IN339
0
10
20
30
40
50
60 Vi=15V
TA=-40°C
TA=125°C
TA=25°C
Figure 8: ReferenceVoltage Change vs.
Source Current.
-55 -25 0 25 50 75 100 TA(°C)
D95IN340
50
60
70
80
90
100
I
SC
(mA) Vi=15V
RL≤0.1Ω
Figure 9: Reference Short Circuit Current vs.
Temperature.
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
6/15
0 200 400 600 IO(mA)
0
1
2
3
-2
-1
Vsat
(V)
D95IN341
Vi
=15V
80µs Pulsed Load 120Hz Rate
TA=-40°C
TA=25°C
Vi
TA=-40°CTA=25°C
GND
Sink
Saturation
(Load to Vi)
Source
Saturation
(Load to Ground)
Figure 10: Output Saturation Voltagevs. Load
Current.
0 102030V
i
(V)
0
5
10
15
20
Ii
(mA)
UCX843/45
UCX842/44
RT
=10K
CT
=3.3nF
VFB
=0V
ISense
=0V
TA=25°C
D95IN342
Figure 11: SupplyCurrent vs. Supply Voltage.
Figure 12: Output Waveform. Figure 13: Output Cross Conduction
5V REG
OSCILLATOR
PWM
CLOCK
8
4
5
6
RT
CT
GND
OUTPUT
7
Vi
ID
CT
OUTPUT
LARGE RT/SMALL CT
CT
OUTPUT
SMALL RT/LARGE CT
D95IN344
Figure 14: Oscillator and Output Waveforms.
Vi=15V
CL= 1.0nF
TA=25°C
90%
10%
50ns/DIV
Vi=30V
CL= 15pF
TA=25°C
V
O
I
CC
100ns/DIV
100mA/DIV
20V/DIV
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
7/15
Figure 15 : ErrorAmp Configuration.
Zi
Zf
1mA
2
1
VFB
COMP
2.5V
D95IN345
+
-
Figure16 : UnderVoltageLockout.
UC3842B
UC3844B
UC3843B
UC3845B
16V 8.4V
10V 7.6V
VON
VOFF
ViON/OFF
COMMAND
TO REST OF IC
7
<0.5mA
<17mA
ICC
VCC
VOFF VON
D95IN346
Figure 17 : Current SenseCircuit .
ERROR
AMPL. 2R
R1V
CURRENT
SENSE
COMPARATOR
1
CURRENT
SENSE
COMP
CRS
R3
5
GND
IS
D95IN347
Peakcurrent (is) isdeterminedby the formula
1.0 V
IS max ≈RS
A small RC filtermay be required to suppressswitchtransients.
During UVLO, the Output is low
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
8/15
Figure 18 : SlopeCompensationTechniques.
RS
R1
ISRSLOPE CT
RT
VREG 8
RT/CT
ISENSE
4
35
GND RS
R1
IS
RSLOPE CT
RT
VREG 8
RT/CT
ISENSE
4
35
GND
D95IN348
Figure19 : IsolatedMOSFETDriveand Current TransformerSensing.
7
6
COMP/LATCH
ISOLATION
BOUNDARY
D95IN349
5.0Vref
VCC
+
-
+
-
Q
S
R
+
-
3R
RSNS
C
Vin
Q1
NP
VGS Waveforms
+
0+
0
-- 50% DC 25% DC
Ipk =V(pin 1) -1.4
3RS
NS
NP
()
UC3842B UC3842B
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
9/15
Figure 20 : LatchedShutdown.
D95IN350
BIAS
+
-EA
R
+
OSC
2N
3905
2N
3903
1mA
R
R
2R
1
2
8
4
SCR must be selected for a holding current of less than 0.5mA at TA(min).
The simple two transistor circuit can be used in place of the SCR as shown. All resistors are 10K.
5
D95IN351
+
-EA
Ri
+
1mA
RdR
2R
5
CfRf
1
2
From VO2.5V
+
-EA
RP
+
1mA
RdR
2R
5
CfRf
1
2
From VO2.5V
Error Amp compensation circuit for stabilizing any current-mode topology
except
for boost and flyback converters operating with continuous inductor current.
CP
Ri
Error Amp compensation circuit for stabilizing current-mode boost and flyback
topologies operating with continuous inductor current.
Figure 21: Error Amplifier Compensation
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
10/15
D95IN353
+
-
+
RA
1
7
f=
R
BIAS
OSC
C
6
VREF
RRB
+
-
+
-EA R
2R
R
S
Q
84
5
2
3
5K
5K
5K NE555
8
4
2
1
5
TO ADDITIONAL
UCX84XAs
1.44
(RA+2R
B
)C Dmax =RB
RA+2R
B
Figure 23: ExternalDuty Cycle Clamp and Multi Unit Synchronization.
D95IN352
+
-EA
+
R
2R
5
RT
1
2
EXTERNAL
SYNC INPUT
The diode clamp is required if the Sync amplitude is large enough to cause
the bottom side of CTto go more than 300mV below ground
R
BIAS
OSC
CT
0.01µF
47Ω
4
8
VREF
R
Figure 22: ExternalClock Synchronization.
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
11/15
D95IN354
+
R
BIAS
OSC
C
R
+
-
+
-EA R
2R
R
S
Q
8
4
2
1
5
1mA
1V
+
-
5Vref
1MΩ
Figure 24: Soft-StartCircuit
D95IN355
+
R
BIAS
OSC
C
R
+
-
+
-EA R
2R
R
S
Q
8
4
2
1
5
1mA
1V +
-
5Vref
R2
R1
VClamp
+
-
Comp/Latch
7
RS
VCC
Q1
Vin
7
6
5
BC109
VCLAMP =·R1
R1+R2where 0 <VCLAMP <1V Ipk(max) =VCLAMP
RS
Figure 25: Soft-Startand Error Amplifier Output Duty Cycle Clamp.
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
12/15
SO8 PACKAGE MECHANICAL DATA
DIM. mm inch
MIN. TYP. MAX. MIN. TYP. MAX.
A 1.75 0.069
a1 0.1 0.25 0.004 0.010
a2 1.6 0.063
a3 0.65 0.85 0.026 0.033
b 0.35 0.48 0.014 0.019
b1 0.19 0.25 0.007 0.010
C 0.25 0.5 0.010 0.020
c1 45 (typ.)
D 4.8 5.0 0.189 0.197
E 5.8 6.2 0.228 0.244
e 1.27 0.050
e3 3.81 0.150
F 3.8 4.0 0.15 0.157
L 0.4 1.27 0.016 0.050
M 0.6 0.024
S 8 (max.)
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
13/15
MINIDIP PACKAGE MECHANICAL DATA
DIM. mm inch
MIN. TYP. MAX. MIN. TYP. MAX.
A 3.32 0.131
a1 0.51 0.020
B 1.15 1.65 0.045 0.065
b 0.356 0.55 0.014 0.022
b1 0.204 0.304 0.008 0.012
D 10.92 0.430
E 7.95 9.75 0.313 0.384
e 2.54 0.100
e3 7.62 0.300
e4 7.62 0.300
F 6.6 0.260
I 5.08 0.200
L 3.18 3.81 0.125 0.150
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
14/15
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for
the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its
use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifica-
tions mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information pre-
viously supplied. SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or
systems without express written approval of SGS-THOMSON Microelectronics.
1996 SGS-THOMSON Microelectronics All Rights Reserved
SGS-THOMSON Microelectronics GROUP OF COMPANIES
Australia - Brazil - France - Germany- Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco - The Netherlands - Singapore -
Spain - Sweden - Switzerland - Taiwan - Thaliand - United Kingdom - U.S.A.
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
15/15
