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UC3842B
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HIGH PERFORMANCECURRENT MODECONTROLLERS
Order this document by UC3842B/D
D SUFFIXPLASTIC PACKAGE
CASE 751A(SO14)
N SUFFIXPLASTIC PACKAGE
CASE 626
D1 SUFFIXPLASTIC PACKAGE
CASE 751(SO8)
8
1
81
141
DeviceOperating
Temperature Range Package
ORDERING INFORMATION
UC384XBDUC384XBD1 TA = 0 to +70C
TA = 25 to +85C
SO14
UC384XBNUC284XBDUC284XBD1UC284XBNUC384XBVDUC384XBVD1UC384XBVN
TA = 40 to +105C
SO8PlasticSO14SO8PlasticSO14SO8Plastic
PIN CONNECTIONS
CompensationNC
Voltage FeedbackNC
Current SenseNC
RT/CT
CompensationVoltage Feedback
Current SenseRT/CT
Vref
VrefNCVCCVCOutputGndPower Ground
VCCOutputGnd
(Top View)
87
6
5
1
2
3
4
1
2
3
4
14
1312
11
5
67
1098
(Top View)
X indicates either a 2 or 3 to define specific device partnumbers.
1MOTOROLA ANALOG IC DEVICE DATA
The UC3842B, UC3843B series are high performance fixed frequencycurrent mode controllers. They are specifically designed for OffLine anddctodc converter applications offering the designer a costeffectivesolution with minimal external components. These integrated circuits featurea trimmed oscillator for precise duty cycle control, a temperaturecompensated reference, high gain error amplifier, current sensingcomparator, and a high current totem pole output ideally suited for driving apower MOSFET.
Also included are protective features consisting of input and referenceundervoltage lockouts each with hysteresis, cyclebycycle current limiting,programmable output deadtime, and a latch for single pulse metering.
These devices are available in an 8pin dualinline and surface mount(SO8) plastic package as well as the 14pin plastic surface mount (SO14).The SO14 package has separate power and ground pins for the totem poleoutput stage.
The UCX842B has UVLO thresholds of 16 V (on) and 10 V (off), ideallysuited for offline converters. The UCX843B is tailored for lower voltageapplications having UVLO thresholds of 8.5 V (on) and 7.6 V (off). Trimmed Oscillator for Precise Frequency Control Oscillator Frequency Guaranteed at 250 kHz Current Mode Operation to 500 kHz Automatic Feed Forward Compensation Latching PWM for CycleByCycle Current Limiting Internally Trimmed Reference with Undervoltage Lockout High Current Totem Pole Output Undervoltage Lockout with Hysteresis Low Startup and Operating Current
Simplified Block Diagram
5.0VReference
LatchingPWM
VCCUndervoltage
Lockout
Oscillator
ErrorAmplifier
7(12)
VC7(11)Output
6(10)PowerGround
5(8)
3(5)CurrentSenseInput
Vref
8(14)
4(7)
2(3)
1(1)Gnd 5(9)
RT/CT
VoltageFeedback
Input
R
R
+
VrefUndervoltage
Lockout
OutputCompensation
Pin numbers in parenthesis are for the D suffix SO14 package.
VCC
Motorola, Inc. 1996 Rev 1
UC3842B, 43B UC2842B, 43B
2 MOTOROLA ANALOG IC DEVICE DATA
MAXIMUM RATINGSRating Symbol Value Unit
Total Power Supply and Zener Current (ICC + IZ) 30 mAOutput Current, Source or Sink (Note 1) IO 1.0 AOutput Energy (Capacitive Load per Cycle) W 5.0 JCurrent Sense and Voltage Feedback Inputs Vin 0.3 to + 5.5 VError Amp Output Sink Current IO 10 mAPower Dissipation and Thermal Characteristics
D Suffix, Plastic Package, SO14 Case 751AMaximum Power Dissipation @ TA = 25CThermal Resistance, JunctiontoAir
D1 Suffix, Plastic Package, SO8 Case 751Maximum Power Dissipation @ TA = 25CThermal Resistance, JunctiontoAir
N Suffix, Plastic Package, Case 626Maximum Power Dissipation @ TA = 25CThermal Resistance, JunctiontoAir
PDRJA
PDRJA
PDRJA
862145
702178
1.25100
mWC/W
mWC/W
WC/W
Operating Junction Temperature TJ +150 COperating Ambient Temperature
UC3842B, UC3843BUC2842B, UC2843BUC3842BV, UC3843BV
TA0 to + 70
25 to + 8540 to +105
C
Storage Temperature Range Tstg 65 to +150 C
ELECTRICAL CHARACTERISTICS (VCC = 15 V [Note 2], RT = 10 k, CT = 3.3 nF. For typical values TA = 25C, for min/max values TA isthe operating ambient temperature range that applies [Note 3], unless otherwise noted.)
UC284XB UC384XB, XBV
Characteristics Symbol Min Typ Max Min Typ Max Unit
REFERENCE SECTIONReference Output Voltage (IO = 1.0 mA, TJ = 25C) Vref 4.95 5.0 5.05 4.9 5.0 5.1 VLine Regulation (VCC = 12 V to 25 V) Regline 2.0 20 2.0 20 mVLoad Regulation (IO = 1.0 mA to 20 mA) Regload 3.0 25 3.0 25 mVTemperature Stability TS 0.2 0.2 mV/CTotal Output Variation over Line, Load, and Temperature Vref 4.9 5.1 4.82 5.18 VOutput Noise Voltage (f = 10 Hz to 10 kHz, TJ = 25C) Vn 50 50 VLong Term Stability (TA = 125C for 1000 Hours) S 5.0 5.0 mVOutput Short Circuit Current ISC 30 85 180 30 85 180 mA
OSCILLATOR SECTIONFrequency
TJ = 25CTA = Tlow to ThighTJ = 25C (RT = 6.2 k, CT = 1.0 nF)
fOSC4948225
52
250
5556275
4948225
52
250
5556275
kHz
Frequency Change with Voltage (VCC = 12 V to 25 V) fOSC/V 0.2 1.0 0.2 1.0 %Frequency Change with Temperature
TA = Tlow to ThighfOSC/T 1.0 0.5 %
Oscillator Voltage Swing (PeaktoPeak) VOSC 1.6 1.6 VDischarge Current (VOSC = 2.0 V)
TJ = 25CTA = Tlow to Thigh (UC284XB, UC384XB)TA = Tlow to Thigh (UC384XBV)
Idischg7.87.5
8.3
8.88.8
7.87.67.2
8.3
8.88.88.8
mA
NOTES: 1. Maximum Package power dissipation limits must be observed.2. Adjust VCC above the Startup threshold before setting to 15 V.3. Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible.
Tlow = 0C for UC3842B, UC3843B Thigh = +70C for UC3842B, UC3843BTlow = 25C for UC2842B, UC2843B Thigh = +85C for UC2842B, UC2843BTlow = 40C for UC3842BV, UC3843BV Thigh = +105C for UC3842BV, UC3843BV
UC3842B, 43B UC2842B, 43B
3MOTOROLA ANALOG IC DEVICE DATA
ELECTRICAL CHARACTERISTICS (VCC = 15 V [Note 2], RT = 10 k, CT = 3.3 nF. For typical values TA = 25C, for min/max values TA isthe operating ambient temperature range that applies [Note 3], unless otherwise noted.)
UC284XB UC384XB, XBV
Characteristics Symbol Min Typ Max Min Typ Max Unit
ERROR AMPLIFIER SECTIONVoltage Feedback Input (VO = 2.5 V) VFB 2.45 2.5 2.55 2.42 2.5 2.58 VInput Bias Current (VFB = 5.0 V) IIB 0.1 1.0 0.1 2.0 AOpen Loop Voltage Gain (VO = 2.0 V to 4.0 V) AVOL 65 90 65 90 dBUnity Gain Bandwidth (TJ = 25C) BW 0.7 1.0 0.7 1.0 MHzPower Supply Rejection Ratio (VCC = 12 V to 25 V) PSRR 60 70 60 70 dBOutput Current
Sink (VO = 1.1 V, VFB = 2.7 V)Source (VO = 5.0 V, VFB = 2.3 V)
ISinkISource
2.0 0.5
121.0
2.0 0.5
121.0
mA
Output Voltage SwingHigh State (RL = 15 k to ground, VFB = 2.3 V)Low State (RL = 15 k to Vref, VFB = 2.7 V)
(UC284XB, UC384XB) (UC384XBV)
VOHVOL
5.0
6.2
0.8
1.1
5.0
6.2
0.80.8
1.11.2
V
CURRENT SENSE SECTIONCurrent Sense Input Voltage Gain (Notes 4 & 5)
(UC284XB, UC384XB)(UC384XBV)
AV2.85
3.0
3.15
2.852.85
3.03.0
3.153.25
V/V
Maximum Current Sense Input Threshold (Note 4)(UC284XB, UC384XB)(UC384XBV)
Vth0.9
1.0
1.1
0.90.85
1.01.0
1.11.1
V
Power Supply Rejection RatioVCC = 12 V to 25 V, Note 4
PSRR 70 70 dB
Input Bias Current IIB 2.0 10 2.0 10 A
Propagation Delay (Current Sense Input to Output) tPLH(In/Out) 150 300 150 300 nsOUTPUT SECTIONOutput Voltage
Low State (ISink = 20 mA)(ISink = 200 mA) (UC284XB, UC384XB)
(UC384XBV)High State (ISource = 20 mA) (UC284XB, UC384XB)
(UC384XBV)(ISource = 200 mA)
VOL
VOH
13
12
0.11.6
13.5
13.4
0.42.2
1312.912
0.11.61.613.513.513.4
0.42.22.3
V
Output Voltage with UVLO ActivatedVCC = 6.0 V, ISink = 1.0 mA
VOL(UVLO) 0.1 1.1 0.1 1.1 V
Output Voltage Rise Time (CL = 1.0 nF, TJ = 25C) tr 50 150 50 150 nsOutput Voltage Fall Time (CL = 1.0 nF, TJ = 25C) tf 50 150 50 150 ns
UNDERVOLTAGE LOCKOUT SECTIONStartup Threshold (VCC)
UCX842B, BVUCX843B, BV
Vth157.8
168.4
179.0
14.57.8
168.4
17.59.0
V
Minimum Operating Voltage After TurnOn (VCC)UCX842B, BVUCX843B, BV
VCC(min)9.07.0
107.6
118.2
8.57.0
107.6
11.58.2
V
NOTES: 2. Adjust VCC above the Startup threshold before setting to 15 V.3. Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible.
Tlow = 0C for UC3842B, UC3843B Thigh = +70C for UC3842B, UC3843BTlow = 25C for UC2842B, UC2843B Thigh = +85C for UC2842B, UC2843BTlow = 40C for UC3842BV, UC3843BV Thigh = +105C for UC3842BV, UC3843BV
4. This parameter is measured at the latch trip point with VFB = 0 V.5. Comparator gain is defined as: AV
V Output CompensationV Current Sense Input
UC3842B, 43B UC2842B, 43B
4 MOTOROLA ANALOG IC DEVICE DATA
ELECTRICAL CHARACTERISTICS (VCC = 15 V [Note 2], RT = 10 k, CT = 3.3 nF, for typical values TA = 25C, for min/max values TAis the operating ambient temperature range that applies [Note 3], unless otherwise noted.)
UC284XB UC384XB, BV
Characteristics Symbol Min Typ Max Min Typ Max Unit
PWM SECTIONDuty Cycle
Maximum (UC284XB, UC384XB)Maximum (UC384XBV)Minimum
DC(max)DC(min)
94
96
0
9493
9696
0
%
TOTAL DEVICEPower Supply Current
Startup (VCC = 6.5 V for UCX843B, Startup (VCC 14 V for UCX842B, BV)Operating (Note 2)
ICC + IC
0.3
12
0.5
17
0.3
12
0.5
17
mA
Power Supply Zener Voltage (ICC = 25 mA) VZ 30 36 30 36 VNOTES: 2. Adjust VCC above the Startup threshold before setting to 15 V.
3. Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible.Tlow = 0C for UC3842B, UC3843B Thigh = +70C for UC3842B, UC3843BTlow = 25C for UC2842B, UC2843B Thigh = +85C for UC2842B, UC2843BTlow = 40C for UC3842BV, UC3843BV Thigh = +105C for UC3842BV, UC3843BV
0.8
2.0
5.08.0
20
5080
R T, TI
MING
RES
ISTO
R (k
)
1.0 M500 k200 k100 k50 k20 k10 kfOSC, OSCILLATOR FREQUENCY (kHz)
VCC = 15 VTA = 25C
Figure 1. Timing Resistorversus Oscillator Frequency
Figure 2. Output Deadtimeversus Oscillator Frequency
1.0 M500 k200 k100 k50 k20 k10 kfOSC, OSCILLATOR FREQUENCY (kHz)
1.0
2.0
5.0
10
20
50
100
% D
T, PE
RCEN
T OUT
PUT D
EADT
IME
1
2
Figure 3. Oscillator Discharge Currentversus Temperature
Figure 4. Maximum Output Duty Cycleversus Timing Resistor
, DI
SCHA
RGE C
URRE
NT (m
A)
7.0 55
TA, AMBIENT TEMPERATURE (C) 25 0 25 50 75 100 125
disch
gI
7.5
8.0
8.5
9.0VCC = 15 VVOSC = 2.0 V
, MA
XIMU
M OU
TPUT
DU
TY CY
CLE
(%)
max
D 400.8
RT, TIMING RESISTOR (k)1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
50
60
70
80
90
100
Idischg = 7.5 mA
VCC = 15 VCT = 3.3 nFTA = 25C
1. CT = 10 nF2. CT = 5.0 nF3. CT = 2.0 nF4. CT = 1.0 nF5. CT = 500 pF6. CT = 200 pF7. CT = 100 pF
5
Idischg = 8.8 mA
7
3
6
4
VCC = 15 VTA = 25C
UC3842B, 43B UC2842B, 43B
5MOTOROLA ANALOG IC DEVICE DATA
Figure 5. Error Amp Small SignalTransient Response
Figure 6. Error Amp Large SignalTransient Response
Figure 7. Error Amp Open Loop Gain andPhase versus Frequency
Figure 8. Current Sense Input Thresholdversus Error Amp Output Voltage
Figure 9. Reference Voltage Changeversus Source Current
Figure 10. Reference Short Circuit Currentversus Temperature
20AVO
L, O
PEN
LOOP
VO
LTAG
E GA
IN (d
B)
10 M10f, FREQUENCY (Hz)
Gain
Phase
VCC = 15 VVO = 2.0 V to 4.0 VRL = 100 KTA = 25C
0
30
60
90
120
150
180100 1.0 k 10 k 100 k 1.0 M
0
20
40
60
80
100, EX
CESS
PHA
SE (D
EGRE
ES)
0VO, ERROR AMP OUTPUT VOLTAGE (V)
0
, CU
RREN
T SEN
SE IN
PUT T
HRES
HOLD
(V)
V th
0.2
0.4
0.6
0.8
1.0
1.2
2.0 4.0 6.0 8.0
VCC = 15 V
TA = 25C
TA = 55CTA = 125C
VCC = 15 V
TA = 55C
TA = 25C, REF
EREN
CE V
OLTA
GE C
HANG
E (m
V)
16
0Iref, REFERENCE SOURCE CURRENT (mA)
20 40 60 80 100 120
ref
V
12
8.0
4.0
0
20
24
TA = 125C
VCC = 15 VRL 0.1
, RE
FERE
NCE
SHOR
T CIR
CUIT
CURR
ENT (
mA)
SCI
50 55
TA, AMBIENT TEMPERATURE (C) 25 0 25 50 75 100 125
70
90
110
1.0 s/DIV0.5 s/DIV
20 m
V/DI
V
/
2.55 V
2.50 V
2.45 V
3.0 V
2.5 V
2.0 V
VCC = 15 VAV = 1.0TA = 25C
VCC = 15 VAV = 1.0TA = 25C
UC3842B, 43B UC2842B, 43B
6 MOTOROLA ANALOG IC DEVICE DATA
Sink Saturation(Load to VCC)
TA = 55C
VCC
Source Saturation(Load to Ground)
0Vsa
t, OUT
PUT S
ATUR
ATIO
N VO
LTAG
E (V)
8000IO, OUTPUT LOAD CURRENT (mA)
200 400 600
1.0
2.0
3.0
2.0
1.0
0
TA = 55C
Figure 11. Reference Load Regulation Figure 12. Reference Line Regulation
Figure 13. Output Saturation Voltageversus Load Current Figure 14. Output Waveform
Figure 15. Output Cross Conduction Figure 16. Supply Current versus Supply Voltage
TA = 25C
RT = 10 kCT = 3.3 nFVFB = 0 VISense = 0 VTA = 25C
, SU
PPLY
CU
RREN
T (mA
)CCI
00
VCC, SUPPLY VOLTAGE (V)10 20 30 40
5
10
15
20
25
UCX8
43B
UCX8
42B
TA = 25C
Gnd
VCC = 15 V80 s Pulsed Load
120 Hz Rate
2.0 ms/DIV 2.0 ms/DIV
VCC = 15 VIO = 1.0 mA to 20 mATA = 25C
VCC = 12 V to 25 TA = 25C
, OU
TPUT
VO
LTAG
E CH
ANGE
(2.0 m
V/DIV)
V O
, OU
TPUT
VO
LTAG
E CH
ANGE
(2.0 m
V/DIV)
V O
VCC = 30 VCL = 15 pFTA = 25C
VCC = 15 VCL = 1.0 nFTA = 25C
50 ns/DIV
100 ns/DIV
100 m
A/DI
V20
V/D
IV
90%
10%
, OU
TPUT
VO
LTAG
EOV
, SU
PPLY
CU
RREN
TCCI
UC3842B, 43B UC2842B, 43B
7MOTOROLA ANALOG IC DEVICE DATA
PIN FUNCTION DESCRIPTION Pin
F i D i i8Pin 14Pin Function Description
1 1 Compensation This pin is the Error Amplifier output and is made available for loop compensation.
2 3 VoltageFeedback
This is the inverting input of the Error Amplifier. It is normally connected to the switching powersupply output through a resistor divider.
3 5 CurrentSense
A voltage proportional to inductor current is connected to this input. The PWM uses thisinformation to terminate the output switch conduction.
4 7 RT/CT The Oscillator frequency and maximum Output duty cycle are programmed byconnecting resistor RT to Vref and capacitor CT to ground. Operation to 500 kHzis possible.
5 Gnd This pin is the combined control circuitry and power ground.
6 10 Output This output directly drives the gate of a power MOSFET. Peak currents up to 1.0 A are sourcedand sunk by this pin.
7 12 VCC This pin is the positive supply of the control IC.
8 14 Vref This is the reference output. It provides charging current for capacitor CT through resistor RT.
8 PowerGround
This pin is a separate power ground return that is connected back to the power source. It is usedto reduce the effects of switching transient noise on thecontrol circuitry.
11 VC The Output high state (VOH) is set by the voltage applied to this pin. With a separate powersource connection, it can reduce the effects of switching transient noise on the control circuitry.
9 Gnd This pin is the control circuitry ground return and is connected back to the power source ground.
2,4,6,13 NC No connection. These pins are not internally connected.
UC3842B, 43B UC2842B, 43B
8 MOTOROLA ANALOG IC DEVICE DATA
OPERATING DESCRIPTIONThe UC3842B, UC3843B series are high performance,
fixed frequency, current mode controllers. They arespecifically designed for OffLine and dctodc converterapplications offering the designer a costeffective solutionwith minimal external components. A representative blockdiagram is shown in Figure 17.Oscillator
The oscillator frequency is programmed by the valuesselected for the timing components RT and CT. Capacitor CTis charged from the 5.0 V reference through resistor RT toapproximately 2.8 V and discharged to 1.2 V by an internalcurrent sink. During the discharge of CT, the oscillatorgenerates an internal blanking pulse that holds the centerinput of the NOR gate high. This causes the Output to be in alow state, thus producing a controlled amount of outputdeadtime. Figure 1 shows RT versus Oscillator Frequencyand Figure 2, Output Deadtime versus Frequency, both forgiven values of CT. Note that many values of RT and CT willgive the same oscillator frequency but only one combinationwill yield a specific output deadtime at a given frequency. Theoscillator thresholds are temperature compensated to within6% at 50 kHz. Also because of industry trends moving theUC384X into higher and higher frequency applications, theUC384XB is guaranteed to within 10% at 250 kHz. Theseinternal circuit refinements minimize variations of oscillatorfrequency and maximum output duty cycle. The results areshown in Figures 3 and 4.
In many noisesensitive applications it may be desirableto frequencylock the converter to an external system clock.This can be accomplished by applying a clock signal to thecircuit shown in Figure 20. For reliable locking, thefreerunning oscillator frequency should be set about 10%less than the clock frequency. A method for multiunitsynchronization is shown in Figure 21. By tailoring the clockwaveform, accurate Output duty cycle clamping can beachieved.
Error AmplifierA fully compensated Error Amplifier with access to the
inverting input and output is provided. It features a typical dcvoltage gain of 90 dB, and a unity gain bandwidth of 1.0 MHzwith 57 degrees of phase margin (Figure 7). Thenoninverting input is internally biased at 2.5 V and is notpinned out. The converter output voltage is typically divideddown and monitored by the inverting input. The maximuminput bias current is 2.0 A which can cause an outputvoltage error that is equal to the product of the input biascurrent and the equivalent input divider source resistance.
The Error Amp Output (Pin 1) is provided for external loopcompensation (Figure 31). The output voltage is offset by twodiode drops (1.4 V) and divided by three before it connectsto the noninverting input of the Current Sense Comparator.This guarantees that no drive pulses appear at the Output(Pin 6) when pin 1 is at its lowest state (VOL). This occurswhen the power supply is operating and the load is removed,
or at the beginning of a softstart interval (Figures 23, 24).The Error Amp minimum feedback resistance is limited by theamplifiers source current (0.5 mA) and the required outputvoltage (VOH) to reach the comparators 1.0 V clamp level:
Rf(min) 3.0 (1.0 V) + 1.4 V0.5 mA = 8800
Current Sense Comparator and PWM LatchThe UC3842B, UC3843B operate as a current mode
controller, whereby output switch conduction is initiated bythe oscillator and terminated when the peak inductor currentreaches the threshold level established by the Error AmplifierOutput/Compensation (Pin 1). Thus the error signal controlsthe peak inductor current on a cyclebycycle basis. TheCurrent Sense Comparator PWM Latch configuration usedensures that only a single pulse appears at the Output duringany given oscillator cycle. The inductor current is convertedto a voltage by inserting the groundreferenced senseresistor RS in series with the source of output switch Q1. Thisvoltage is monitored by the Current Sense Input (Pin 3) andcompared to a level derived from the Error Amp Output. Thepeak inductor current under normal operating conditions iscontrolled by the voltage at pin 1 where:
Ipk =V(Pin 1) 1.4 V
3 RS
Abnormal operating conditions occur when the powersupply output is overloaded or if output voltage sensing islost. Under these conditions, the Current Sense Comparatorthreshold will be internally clamped to 1.0 V. Therefore themaximum peak switch current is:
Ipk(max) = 1.0 VRSWhen designing a high power switching regulator it
becomes desirable to reduce the internal clamp voltage inorder to keep the power dissipation of RS to a reasonablelevel. A simple method to adjust this voltage is shown inFigure 22. The two external diodes are used to compensatethe internal diodes, yielding a constant clamp voltage overtemperature. Erratic operation due to noise pickup can resultif there is an excessive reduction of the Ipk(max) clampvoltage.
A narrow spike on the leading edge of the currentwaveform can usually be observed and may cause the powersupply to exhibit an instability when the output is lightlyloaded. This spike is due to the power transformerinterwinding capacitance and output rectifier recovery time.The addition of an RC filter on the Current Sense Input with atime constant that approximates the spike duration willusually eliminate the instability (refer to Figure 26).
UC3842B, 43B UC2842B, 43B
9MOTOROLA ANALOG IC DEVICE DATA
+
ReferenceRegulator
VCCUVLO
+
VrefUVLO
3.6V
36V
S
RQ
InternalBias
+ 1.0mA
Oscillator
2.5VR
R
R
2R
ErrorAmplifier
VoltageFeedback
Input
Output/Compensation Current SenseComparator
1.0V
VCC 7(12)
Gnd 5(9)
VC
7(11)
Output
6(10)Power Ground
5(8)Current Sense Input
3(5) RS
Q1
VCC Vin
1(1)
2(3)
4(7)
8(14)
RT
CT
Vref
= Sink Only Positive True LogicPin numbers adjacent to terminals are for the 8pin dualinline package.Pin numbers in parenthesis are for the D suffix SO14 package.
Figure 17. Representative Block Diagram
Figure 18. Timing Diagram
Large RT/Small CT Small RT/Large CT
PWMLatch
(SeeText)
Capacitor CT
LatchSet Input
Output/Compensation
Current SenseInput
LatchReset Input
Output
UC3842B, 43B UC2842B, 43B
10 MOTOROLA ANALOG IC DEVICE DATA
Undervoltage LockoutTwo undervoltage lockout comparators have been
incorporated to guarantee that the IC is fully functional beforethe output stage is enabled. The positive power supplyterminal (VCC) and the reference output (Vref) are eachmonitored by separate comparators. Each has builtinhysteresis to prevent erratic output behavior as theirrespective thresholds are crossed. The VCC comparatorupper and lower thresholds are 16 V/10 V for the UCX842B,and 8.4 V/7.6 V for the UCX843B. The Vref comparator upperand lower thresholds are 3.6 V/3.4 V. The large hysteresisand low startup current of the UCX842B makes it ideallysuited in offline converter applications where efficientbootstrap startup techniques are required (Figure 33). TheUCX843B is intended for lower voltage dctodc converterapplications. A 36 V zener is connected as a shunt regulatorfrom VCC to ground. Its purpose is to protect the IC fromexcessive voltage that can occur during system startup. Theminimum operating voltage (VCC) for the UCX842B is 11 Vand 8.2 V for the UCX843B.
These devices contain a single totem pole output stagethat was specifically designed for direct drive of powerMOSFETs. It is capable of up to 1.0 A peak drive current andhas a typical rise and fall time of 50 ns with a 1.0 nF load.Additional internal circuitry has been added to keep theOutput in a sinking mode whenever an undervoltage lockoutis active. This characteristic eliminates the need for anexternal pulldown resistor.
The SO14 surface mount package provides separatepins for VC (output supply) and Power Ground. Properimplementation will significantly reduce the level of switchingtransient noise imposed on the control circuitry. Thisbecomes particularly useful when reducing the Ipk(max) clamplevel. The separate VC supply input allows the designeradded flexibility in tailoring the drive voltage independent ofVCC. A zener clamp is typically connected to this input whendriving power MOSFETs in systems where VCC is greaterthan 20 V. Figure 25 shows proper power and control groundconnections in a currentsensing power MOSFETapplication.
ReferenceThe 5.0 V bandgap reference is trimmed to 1.0%
tolerance at TJ = 25C on the UC284XB, and 2.0% on theUC384XB. Its primary purpose is to supply charging currentto the oscillator timing capacitor. The reference has shortcircuit protection and is capable of providing in excess of20 mA for powering additional control system circuitry.Design Considerations
Do not attempt to construct the converter onwirewrap or plugin prototype boards. High frequencycircuit layout techniques are imperative to preventpulsewidth jitter. This is usually caused by excessive noisepickup imposed on the Current Sense or Voltage Feedbackinputs. Noise immunity can be improved by lowering circuitimpedances at these points. The printed circuit layout shouldcontain a ground plane with lowcurrent signal andhighcurrent switch and output grounds returning onseparate paths back to the input filter capacitor. Ceramicbypass capacitors (0.1 F) connected directly to VCC, VC,and Vref may be required depending upon circuit layout. Thisprovides a low impedance path for filtering the high frequencynoise. All high current loops should be kept as short as
possible using heavy copper runs to minimize radiated EMI.The Error Amp compensation circuitry and the converteroutput voltage divider should be located close to the IC andas far as possible from the power switch and othernoisegenerating components.
Current mode converters can exhibit subharmonicoscillations when operating at a duty cycle greater than 50%with continuous inductor current. This instability isindependent of the regulators closed loop characteristicsand is caused by the simultaneous operating conditions offixed frequency and peak current detecting. Figure 19Ashows the phenomenon graphically. At t0, switch conductionbegins, causing the inductor current to rise at a slope of m1.This slope is a function of the input voltage divided by theinductance. At t1, the Current Sense Input reaches thethreshold established by the control voltage. This causes theswitch to turn off and the current to decay at a slope of m2,until the next oscillator cycle. The unstable condition can beshown if a perturbation is added to the control voltage,resulting in a small I (dashed line). With a fixed oscillatorperiod, the current decay time is reduced, and the minimumcurrent at switch turnon (t2) is increased by I + I m2/m1.The minimum current at the next cycle (t3) decreases to (I +I m2/m1) (m2/m1). This perturbation is multiplied by m2/m1on each succeeding cycle, alternately increasing anddecreasing the inductor current at switch turnon. Severaloscillator cycles may be required before the inductor currentreaches zero causing the process to commence again. Ifm2/m1 is greater than 1, the converter will be unstable. Figure19B shows that by adding an artificial ramp that issynchronized with the PWM clock to the control voltage, theI perturbation will decrease to zero on succeeding cycles.This compensating ramp (m3) must have a slope equal to orslightly greater than m2/2 for stability. With m2/2 slopecompensation, the average inductor current follows thecontrol voltage, yielding true current mode operation. Thecompensating ramp can be derived from the oscillator andadded to either the Voltage Feedback or Current Senseinputs (Figure 32).
Control Voltage
InductorCurrent
Oscillator Period
Control Voltage
InductorCurrent
Oscillator Period
(A)
(B)
Figure 19. Continuous Current Waveforms
m1 m2
t0 t1 t2 t3
m3
m2
t4 t5 t6
I
m1I
l l m2m1l l m2
m1m2m1
UC3842B, 43B UC2842B, 43B
11MOTOROLA ANALOG IC DEVICE DATA
Figure 20. External Clock Synchronization Figure 21. External Duty Cycle Clamp andMultiUnit Synchronization
Figure 22. Adjustable Reduction of Clamp Level Figure 23. SoftStart Circuit
2(3) EA
Bias
+
Osc
R
R
R
2R
5(9)
1(1)
4(7)
8(14)
RT
CT
Vref
0.01
The diode clamp is required if the Sync amplitude is large enough to cause the bottomside of CT to go more than 300 mV below ground.
ExternalSyncInput
47
+
R
R
R
2R
Bias
Osc
EA
5(9)
1(1)
2(3)
4(7)
8(14)
To AdditionalUCX84XBs
R
S
Q
8 4
6
5
2
1
C
3
7
RA
RB 5.0k
5.0k
5.0kMC1455
f 1.44(RA 2RB)C D(max) RB
RA 2RB
+
5.0V Ref
+
S
RQ
Bias
+
Osc
R
R
R2REA
1.0V
5(9)
7(11)
6(10)
5(8)
3(5) RS
Q1
VCC Vin
1(1)
2(3)
4(7)
8(14)
R1
VClampR2
5.0V Ref
+
S
RQ
Bias
+
1.0mA
Osc
R
R
R2R
EA 1.0V
5(9)
1(1)
2(3)
4(7)
8(14)
C
1.0M
tSoftStart 3600C in F
7(12)
Comp/Latch
1.0 mA
Ipk(max) VClampRS
Where: 0 VClamp 1.0 VVClamp 1.67
R2R1 1
+ 0.33x103 R1R2R1 R2
UC3842B, 43B UC2842B, 43B
12 MOTOROLA ANALOG IC DEVICE DATA
Figure 24. Adjustable Buffered Reduction of Clamp Level with SoftStart
Figure 25. Current Sensing Power MOSFET
Figure 26. Current Waveform Spike Suppression Figure 27. MOSFET Parasitic Oscillations
+
+
S
R
+
R
R
R2R
VClamp 1.67
R2R1 1
Ipk(max) VClampRS
5.0V Ref
Q
Bias
Osc
EA1.0V
5(9)
7(11)
6(10)
5(8)
3(5) RS
Q1
VCC Vin
1(1)
2(3)
4(7)
8(14)
R1
VClamp
R2
Where: 0 VClamp 1.0 V
C MPSA63
tSoft-Start In1 VC3 VClamp C R1 R2R1 R2
+
5.0V Ref
+
S
RQ
(11)
(10)
(8)Comp/Latch
(5) RS1/4 W
VCC Vin
KM
D SENSEFET
GS
Power Ground:To Input Source
Return
Control Circuitry Ground:To Pin (9)
Virtually lossless current sensing can be achieved with the implementation of aSENSEFET power switch. For proper operation during overcurrent conditions, areduction of the Ipk(max) clamp level must be implemented. Refer to Figures 22 and 24.
VPin 5 RS Ipk rDS(on)rDM(on) RS
If: SENSEFET = MTP10N10MRS = 200
Then : VPin 5 0.075 Ipk
7(12)
1.0 mA
Comp/Latch
(12)
+
5.0V Ref
+
S
RQ
7(11)
6(10)
5(8)
3(5)
RS
Q1
VCC Vin
S
R
5.0V Ref
Q
7(11)
6(10)
5(8)
3(5) RS
Q1
VCC Vin
C
R
The addition of the RC filter will eliminate instability caused by the leadingedge spike on the current waveform.
Series gate resistor Rg will damp any high frequency parasitic oscillationscaused by the MOSFET input capacitance and any series wiring inductance inthe gatesource circuit.
7(12) 7(12)
Rg
Comp/Latch Comp/Latch
+
+
UC3842B, 43B UC2842B, 43B
13MOTOROLA ANALOG IC DEVICE DATA
Figure 28. Bipolar Transistor Drive Figure 29. Isolated MOSFET Drive
Figure 30. Latched Shutdown Figure 31. Error Amplifier Compensation
6(10)
5(8)
3(5) RS
Q1
Vin
C1
Base ChargeRemoval
The totem pole output can furnish negative base current for enhanced transistorturnoff, with the addition of capacitor C1.
S
R
5.0V Ref
Q
7(11)
6(10)5(8)
3(5)
RS
Q1
VCCIB
+
0
Vin
IsolationBoundary
VGS Waveforms
+
0
+
0
50% DC 25% DC
IpkV(Pin1) 1.4
3 RS
NSNp
Comp/Latch
7(12)
R
C NS NP
+
+
Bias
+
Osc
R
R
R
2R
EA
5(9)
1(1)
2(3)
4(7)
8(14)
The MCR101 SCR must be selected for a holding of < 0.5 mA @ TA(min). The simple two transistor circuit canbe used in place of the SCR as shown. All resistors are 10 k.
MCR101
2N3905
2N3903
+
R
2R1.0mA
EA
2(3)
5(9)
2.5V
1(1)
RfCfRd
Ri
From VO
Error Amp compensation circuit for stabilizing any current mode topology except for boost and flybackconverters operating with continuous inductor current.
Rf 8.8 k
+
R
2R1.0mA
EA
2(3)
5(9)
2.5V
1(1)
RfCfRd
Rp
From VO
Error Amp compensation circuit for stabilizing current mode boost and flybacktopologies operating with continuous inductor current.
Cp
Ri
1.0 mA
+
+
5.0V Ref36V
S
RQ
Bias
+1.0mA
Osc
R
R
R
2R
EA 1.0V
7(12)
7(11)
6(10)
5(8)
3(5) RS
VCC Vin
1(1)
2(3)
4(7)
8(14)RT
CT
The buffered oscillator ramp can be resistively summed with either the voltagefeedback or current sense inputs to provide slope compensation.
Figure 32. Slope Compensation
m 3.0m
m
RfCf
Ri
Rd
From VO RSlope
MPS3904
Figure 33. 27 W OffLine Flyback Regulator
5(9)
Comp/Latch
MUR110+
+
S
R
+
R
R
5.0V Ref
Q
Bias
EA
5(9)
7(11)
6(10)
5(8)
3(5) 0.5
MTP4N50
1(1)
2(3)
4(7)
8(14)10k
4700pF
470pF
150k100pF
18k
4.7k
0.01
100
+
1.0k
115 Vac
4.7MDA202
250
56k
4.7k 3300pF
1N4935 1N4935
+ +6847
1N4937
1N4937
680pF
2.7k
L3
L2
L1
+ +
+ +
+ +
1000
1000
2200
10
10
10005.0V/4.0A
5.0V RTN
12V/0.3A
12V RTN
12V/0.3A
Primary: 45 Turns #26 AWGSecondary 12 V: 9 Turns #30 AWG (2 Strands) BifiliarWoundSecondary 5.0 V: 4 Turns (six strands) #26 Hexfiliar WoundSecondary Feedback: 10 Turns #30 AWG (2 strands)
Bifiliar WoundCore: Ferroxcube EC353C8Bobbin: Ferroxcube EC35PCB1Gap: 0.10 for a primary inductance of 1.0 mH
MUR110
MBR1635T1
22Osc
T1
7(12)
Comp/Latch
L1L2, L3
15 H at 5.0 A, Coilcraft Z7156 25 H at 5.0 A, Coilcraft Z7157
1N5819
UC3842B, 43B UC2842B, 43B
14 MOTOROLA ANALOG IC DEVICE DATA
Test Conditions ResultsLine Regulation: 5.0 V
12VVin = 95 to 130 Vac = 50 mV or 0.5%
= 24 mV or 0.1%
Load Regulation: 5.0 V
12V
Vin = 115 Vac, Iout = 1.0 A to 4.0 A
Vin = 115 Vac, Iout = 100 mA to 300 mA
= 300 mV or 3.0%
= 60 mV or 0.25%
Output Ripple: 5.0 V12V
Vin = 115 Vac 40 mVpp80 mVpp
Efficiency Vin = 115 Vac 70%All outputs are at nominal load currents, unless otherwise noted
UC3842B, 43B UC2842B, 43B
15MOTOROLA ANALOG IC DEVICE DATA
OUTLINE DIMENSIONS
N SUFFIXPLASTIC PACKAGE
CASE 62605ISSUE K
D1 SUFFIXPLASTIC PACKAGE
CASE 75106(SO8)
ISSUE T
NOTES:1. DIMENSION L TO CENTER OF LEAD WHEN
FORMED PARALLEL.2. PACKAGE CONTOUR OPTIONAL (ROUND OR
SQUARE CORNERS).3. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
1 4
58
F
NOTE 2 A
B
TSEATINGPLANE
H
J
GD K
N
C
L
M
MAM0.13 (0.005) B MT
DIM MIN MAX MIN MAXINCHESMILLIMETERS
A 9.40 10.16 0.370 0.400B 6.10 6.60 0.240 0.260C 3.94 4.45 0.155 0.175D 0.38 0.51 0.015 0.020F 1.02 1.78 0.040 0.070G 2.54 BSC 0.100 BSCH 0.76 1.27 0.030 0.050J 0.20 0.30 0.008 0.012K 2.92 3.43 0.115 0.135L 7.62 BSC 0.300 BSCM 10 10 N 0.76 1.01 0.030 0.040
SEATINGPLANE
14
58
A0.25 M C B S S
0.25 M B M
h
C
X 45
L
DIM MIN MAXMILLIMETERS
A 1.35 1.75A1 0.10 0.25B 0.35 0.49C 0.19 0.25D 4.80 5.00E
1.27 BSCe3.80 4.00
H 5.80 6.20h
0 7 L 0.40 1.25
0.25 0.50
NOTES:1. DIMENSIONING AND TOLERANCING PER ASME
Y14.5M, 1994.2. DIMENSIONS ARE IN MILLIMETER.3. DIMENSION D AND E DO NOT INCLUDE MOLD
PROTRUSION.4. MAXIMUM MOLD PROTRUSION 0.15 PER SIDE.5. DIMENSION B DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBARPROTRUSION SHALL BE 0.127 TOTAL IN EXCESSOF THE B DIMENSION AT MAXIMUM MATERIALCONDITION.
D
E H
A
B e
BA1
C A
0.10
UC3842B, 43B UC2842B, 43B
16 MOTOROLA ANALOG IC DEVICE DATA
OUTLINE DIMENSIONS
D SUFFIXPLASTIC PACKAGE
CASE 751A03(SO14)ISSUE F
NOTES:1. DIMENSIONING AND TOLERANCING PER
ANSI Y14.5M, 1982.2. CONTROLLING DIMENSION: MILLIMETER.3. DIMENSIONS A AND B DO NOT INCLUDE
MOLD PROTRUSION.4. MAXIMUM MOLD PROTRUSION 0.15 (0.006)
PER SIDE.5. DIMENSION D DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBARPROTRUSION SHALL BE 0.127 (0.005) TOTALIN EXCESS OF THE D DIMENSION ATMAXIMUM MATERIAL CONDITION.
A
B
G
P 7 PL
14 8
71M0.25 (0.010) B M
SBM0.25 (0.010) A ST
T
FR X 45
SEATINGPLANE
D 14 PL K
C
JM
DIM MIN MAX MIN MAXINCHESMILLIMETERS
A 8.55 8.75 0.337 0.344B 3.80 4.00 0.150 0.157C 1.35 1.75 0.054 0.068D 0.35 0.49 0.014 0.019F 0.40 1.25 0.016 0.049G 1.27 BSC 0.050 BSCJ 0.19 0.25 0.008 0.009K 0.10 0.25 0.004 0.009M 0 7 0 7 P 5.80 6.20 0.228 0.244R 0.25 0.50 0.010 0.019
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UC3842B/D
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