UC3842T UC3843T
UC3844T UC3845T
September 2001
HIGH PERFORMANCE CURRENTMODE PWM CONTROLLER
.TRIMMED OSCILLATORFOR PRECISE FRE-
QUENCY CONTROL
.OSCILLATOR FREQUENCY GUARANTEED
AT 250kHz
.CURRENT MODE OPERATION TO 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
The UC384XT family of control ICs provides the
necessary features to implement off-line or DC to
DC fixed frequencycurrent mode controlschemes
witha minimalexternalpartscount.Internallyimple-
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.
Differencesbetweenmembers of this family are the
under-voltagelockoutthresholdsandmaximumduty
cycle ranges. The UC3842T and UC3844T have
UVLO thresholds of 16V (on) and 10V (off), ideally
suited to off-line applications The corresponding
thresholdsfor theUC3843TandUC3845Tare 8.5V
and7.9V. TheUC3842Tand UC3843Tcanoperate
to dutycycles approaching100%.A rangeof zeroto
<50%isobtainedbytheUC3844TandUC3845Tby
theadditionofaninternaltoggleflipflopwhichblanks
theoutputoffeveryotherclockcycle.
BLOCK DIAGRAM (toggle flip flop used only in UC3844T and UC3845T)
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
1/15
*All voltages are with respectto pin5, all currents arepositive intothe 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
UC3842TD
UC3843TD
UC3844TD
UC3845TD
UC3842TN
UC3843TN
UC3844TN
UC3845TN
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.
UC3842T - UC3843T - UC3844T - UC3845T
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ELECTRICAL CHARACTERISTICS ( [note 1] Unless otherwise stated, these specifications apply for
0<T
amb < 105°C;Vi= 15V(note5); RT=10K;CT= 3.3nF)
Symbol Parameter Test Conditions Value Unit
Min. Typ. Max.
REFERENCE SECTION
∆VREF Line Regulation 12V ≤Vi≤25V 220mV
∆V
REF Load Regulation 1≤Io≤20mA 325mV
∆
V
REF/∆TTemperature Stability (Note 2) 0.2 mV/°C
Total Output Variation Line, Load, Temperature 4.85 5.15 V
eNOutput Noise Voltage 10Hz ≤f≤10KHz Tj=25
°
C
(note 2) 50 µV
Long Term Stability Tamb = 125°C, 1000Hrs (note 2) 525mV
I
SC Output Short Circuit -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
KHz
KHz
KHz
∆fOSC/∆VFrequency Change with Volt. VCC = 12V to 25V – 0.2 1 %
∆fOSC/∆TFrequency Change with Temp. TA=T
low to Thigh –1–%
V
OSC Oscillator Voltage Swing (peak to peak) – 1.6 – V
Idischg Discharge Current (VOSC =2V) TA=T
low to Thigh 7.3 – 8.8 mA
ERROR AMP SECTION
V2Input Voltage VPIN1 = 2.5V 2.42 2.50 2.58 V
IbInput Bias Current VFB = 5V -0.1 -2 µA
AVOL 2V ≤Vo≤4V 65 90 dB
BW Unity Gain Bandwidth TJ=25°C 0.7 1 MHz
PSRR Power Supply Rejec. Ratio 12V ≤Vi≤25V 60 70 dB
IoOutput Sink Current VPIN2 = 2.7V VPIN1 = 1.1V 2 12 mA
IoOutput Source Current VPIN2 = 2.3V VPIN1 = 5V -0.5 -1 mA
VOUT High VPIN2 = 2.3V;
RL= 15KΩto Ground 5 6.2 V
VOUT Low VPIN2 = 2.7V;
RL= 15KΩto Pin 8 0.8 1.1 V
CURRENT SENSE SECTION
GVGain (note 3 & 4) 2.85 3 3.15 V/V
V3Maximum Input Signal VPIN1 = 5V (note 3) 0.9 1 1.1 V
SVR Supply Voltage Rejection 12 ≤Vi≤25V (note 3) 70 dB
IbInput Bias Current -2 -10 µA
Delay to Output 100 300 ns
THERMAL DATA
Symbol Description Minidip SO8 Unit
Rth j-amb Thermal ResistanceJunction-ambient. max. 100 150 °C/W
U3842T - UC3843T - UC3844T - UC3845T
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Notes : 1. Max package power dissipation 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 trippointof latchwith VPIN2 =0.
4. Gain definedas :
∆VPIN1
A= ;0≤V
PIN3 ≤0.8V
∆VPIN3
5. AdjustViabovethestart threshold beforesettingat 15 V.
ELECTRICAL CHARACTERISTICS (continued)
Symbol Parameter Test Conditions Value Unit
Min. Typ. Max.
OUTPUT SECTION
VOL Output Low Level ISINK = 20mA 0.1 0.4 V
ISINK = 200mA 1.6 2.2 V
VOH Output High Level ISOURCE = 20mA 13 13.5 V
ISOURCE = 200mA 12 13.5 V
VOLS UVLO Saturation VCC = 6V; ISINK = 1mA 0.1 1.1 V
trRise Time Tj=25
°
CC
L
= 1nF (2) 50 150 ns
tfFall Time Tj=25°CC
L
= 1nF (2) 50 150 ns
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold UC3842T/4T 15 16 17 V
UC3843T/5T 7.8 8.4 9.0 V
Min Operating Voltage
After Turn-on UC3842T/4T 9 10 11 V
UC3843T/5T 7.0 7.6 8.2 V
PWM SECTION
Maximum Duty Cycle UC3842T/3T 94 96 100 %
UC3844T/5T 47 48 50 %
Minimum Duty Cycle 0%
TOTAL STANDBY CURRENT
Ist Start-up Current Vi= 6.5V for UC3843T/45T 0.3 0.5 mA
Vi= 14V for UC3842T/44T 0.3 0.5 mA
IiOperating Supply Current VPIN2 =V
PIN3 =0V 12 17 mA
V
iz Zener Voltage Ii= 25mA 30 36 V
UC3842T - UC3843T - UC3844T - UC3845T
4/15
Figure1: 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
D.U.T.
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
U3842T - UC3843T - UC3844T - UC3845T
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.
UC3842T - UC3843T - UC3844T - UC3845T
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
U3842T - UC3843T - UC3844T - UC3845T
7/15
Figure15 : ErrorAmp Configuration.
Zi
Zf
1mA
2
1
VFB
COMP
2.5V
D95IN345
+
-
Figure16 : UnderVoltageLockout.
UC3842T
UC3844T
UC3843T
UC3845T
16V 8.4V
10V 7.6V
VON
VOFF
ViON/OFF
COMMAND
TO REST OF IC
7
<0.5mA
<17mA
ICC
VCC
VOFF VON
D99IN1058
Figure17 : 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
UC3842T - UC3843T - UC3844T - UC3845T
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Figure18 : 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
D99IN1059
UC3842T UC3842T
Figure19 : IsolatedMOSFETDrive and 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
()
U3842T - UC3843T - UC3844T - UC3845T
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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 resistorsare 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
UC3842T - UC3843T - UC3844T - UC3845T
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D99IN1060
+
-
+
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
UC384XT
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.
U3842T - UC3843T - UC3844T - UC3845T
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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+R
2where 0 <VCLAMP <1V Ipk(max) =VCLAMP
RS
Figure 25: Soft-Startand Error Amplifier Output Duty Cycle Clamp.
UC3842T - UC3843T - UC3844T - UC3845T
12/15
DIM. mm inch
MIN. TYP. MAX. MIN. TYP. MAX.
A 1.75 0.069
a1 0.1 0.25 0.004 0.010
a2 1.65
0.065
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 (1) 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 (1) 3.8 4.0 0.15 0.157
L 0.4 1.27 0.016 0.050
M 0.6 0.024
S8°(max.)
(1) D andF do not includemold flash orprotrusions. Mold flash or
potrusions shall not exceed0.15mm (.006inch).
SO8
OUTLINE AND
MECHANICAL DATA
U3842T - UC3843T - UC3844T - UC3845T
13/15
Minidip
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
Z 1.52 0.060
OUTLINE AND
MECHANICAL DATA
UC3842T - UC3843T - UC3844T - UC3845T
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Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the conse-
quences 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 STMicroelectronics. Specification mentioned in this
publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMi-
croelectronics products are not authorized for use as critical components in life support devices or systems without express written
approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics
2001 STMicroelectronics – Printed in Italy – All Rights Reserved
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