ST UC2842B User manual

UC2842B/3B/4B/5B
UC3842B/3B/4B/5B
March 1999
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
.TRIMMED OSCILLATOR FOR 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
UNDERVOLTAGE LOCKOUT
.HIGH CURRENT TOTEM POLE OUTPUT
.UNDERVOLTAGE LOCKOUT WITH HYSTER-
ESIS
.LOW START-UP AND OPERATING CURRENT
DESCRIPTION
TheUC384xBfamilyofcontrolICsprovidesthenec-
essary features to implement off-line or DC to DC
fixed frequency current mode control schemeswith
a minimal external parts count. Internally imple-
mented circuits include a trimmed oscillator for pre-
cise DUTY CYCLE CONTROL under voltage lock-
outfeaturingstart-upcurrentlessthan0.5mA,apre-
cision reference trimmed for accuracy at the error
ampinput, logicto insure latched operation, a PWM
comparatorwhichalsoprovidescurrentlimitcontrol,
and a totem pole output stage designed to source
orsinkhighpeakcurrent.Theoutputstage,suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differences between members of this family are the
under-voltage lockout thresholds and maximum duty
cycle ranges. The UC3842B and UC3844B have
UVLO thresholds of 16V (on) and 10V (off), ideally
suited off-line applications Thecorresponding thresh-
oldsfortheUC3843BandUC3845Bare8.5Vand7.9
V. The UC3842B and UC3843B can operate to duty
cycles approaching 100%. A range of the zero to <
50 % is obtained by the UC3844B and UC3845B by
the addition of an internal toggle flip flop which blanks
the output off every other clock cycle.
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 respect to pin 5, all currents are positive into the specified terminal.
PIN CONNECTION (top view)
COMP
V
FB
I
SENSE
R
T
/C
T
GROUND
OUTPUT
Vi
V
REF
1
3
2
4
6
5
7
8
D95IN332
Minidip/SO8
ORDERING NUMBERS
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
ViSupply Voltage (Ii < 30mA) Self Limiting
IOOutput Current ±1A
EOOutput Energy (capacitive load) 5 µJ
Analog Inputs (pins 2, 3) – 0.3 to 5.5 V
Error Amplifier Output Sink Current 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
TJJunction Operating Temperature – 40 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
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ELECTRICAL CHARACTERISTICS ( [note 1] Unless otherwise stated, these specifications apply for
-25 < Tamb < 85°C for UC284XB; 0 < Tamb < 70°C for UC384XB; Vi= 15V (note 5); 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°C Io= 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
TA= Tlow to Thigh
TJ= 25°C (RT= 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= Tlow 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
TA= Tlow 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 Resistance Junction-ambient. max. 100 150 °C/W
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
3/15

Notes : 1. Max package power dissipation limits must be respected; low duty cycle pulse techniques are used during test maintain Tjas
close to Tamb as possible.
2. These parameters, although guaranteed, are not 100% tested in production.
3. Parameter measured at trip point of latch with VPIN2 = 0.
4. Gain defined as :
∆VPIN1
A = ; 0 ≤VPIN3 ≤0.8 V
∆VPIN3
5. Adjust Viabove the start threshold before setting at 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°C CL= 1nF (2) 50 150 50 150 ns
tfFall Time Tj= 25°C CL= 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 474850474850 %
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 = VPIN3 = 0V 12 17 12 17 mA
Viz Zener Voltage Ii= 25mA 30 36 30 36 V
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
4/15

Figure 1: Open Loop Test Circuit.
R
T
A2N2222
4.7KΩ
1KΩ
ERROR AMP.
ADJUST
4.7KΩ5KΩ
I
SENSE
ADJUST
100KΩCOMP
V
FB
I
SENSE
R
T
/C
T
1
2
3
4
C
T
7
6
5
8
V
REF
V
i
OUTPUT
GROUND
0.1µF
0.1µF
V
REF
V
i
OUTPUT
GROUND
1W
1KΩ
D95IN343
Highpeakcurrentsassociatedwithcapacitiveloads
necessitate careful grounding techniques. Timing
and bypass capacitors should be connected close
to pin 5 in a single point ground. The transistor and
5KΩpotentiometerareusedtosampletheoscillator
waveform and apply an adjustable ramp to pin 3.
10K 20K 30K 50K 100K 200K 300K 500K f
OSC
(KHz)
1
2
5
10
20
50
D95IN333
C
T
=10nF
C
T
=5nF
C
T
=2nF
C
T
=1nF
C
T
=500pF
C
T
=200pF
C
T
=100pF
V
i
=15V
T
A
=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
%
C
T
=10nF
C
T
=5nF
C
T
=2nF
C
T
=1nF
C
T
=500pF
C
T
=200pF
C
T
=100pF
D95IN334
V
i
=15V
T
A
=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
V
i
=15V
V
OSC
=2V
Figure 4: Oscillator Discharge Current vs. Tem-
perature.
1235
40
50
60
70
80
90
D
max
(%)
0.8 R
T
(KΩ)
D95IN336
V
i
=15V
C
T
=3.3nF
T
A
=25˚C
I
dischg
=7.5mA
I
dischg
=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
V
i
=15V
V
O
=2V to 4V
R
L
=100K
T
A
=25˚C
Gain
Phase
Figure 6: Error Amp Open-Loop Gain 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
V
i
=15V
T
A
=-40˚C
T
A
=125˚C
T
A
=25˚C
Figure 8: Reference Voltage Change vs.
Source Current.
-55 -25 0 25 50 75 100 T
A
(˚C)
D95IN340
50
60
70
80
90
100
I
SC
(mA) V
i
=15V
R
L
≤0.1Ω
Figure 9: Reference Short Circuit Current vs.
Temperature.
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
6/15

0 200 400 600 I
O
(mA)
0
1
2
3
-2
-1
V
sat
(V)
D95IN341
V
i
=15V
80µs Pulsed Load 120Hz Rate
T
A
=-40˚C
T
A
=25˚C
V
i
T
A
=-40˚C T
A
=25˚C
GND
Sink Saturation
(Load to V
i
)
Source Saturation
(Load to Ground)
Figure 10: Output Saturation Voltagevs. Load
Current.
0 102030V
i
(V)
0
5
10
15
20
I
i
(mA)
UCX843/45 UCX842/44
R
T
=10K
C
T
=3.3nF
V
FB
=0V
I
Sense
=0V
T
A
=25˚C
D95IN342
Figure 11: Supply Current vs. Supply Voltage.
Figure 12: Output Waveform. Figure 13: Output Cross Conduction
5V REG
OSCILLATOR
PWM
CLOCK
8
4
5
6
R
T
C
T
GND
OUTPUT
7
V
i
I
D
CT
OUTPUT
LARGE R
T
/SMALL C
T
CT
OUTPUT
SMALL R
T
/LARGE C
T
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
VO
ICC
100ns/DIV
100mA/DIV
20V/DIV
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
7/15

Figure 15 : Error Amp Configuration.
Z
i
Z
f
1mA
2
1
V
FB
COMP
2.5V
D95IN345
+
-
Figure 16 : Under Voltage Lockout.
UC3842B
UC3844B UC3843B UC3845B
16V 8.4V
10V 7.6V
V
ON
V
OFF
V
i
ON/OFF COMMAND
TO REST OF IC
7
<0.5mA
<17mA
I
CC
V
CC
V
OFF
V
ON
D95IN346
Figure 17 : Current Sense Circuit .
ERROR
AMPL. 2R
R1V
CURRENT
SENSE
COMPARATOR
1
CURRENT
SENSE
COMP
CR
S
R3
5
GND
I
S
D95IN347
Peak current (is) is determined by the formula
1.0 V
IS max ≈RS
A small RC filter may be required to suppress switch transients.
During UVLO, the Output is low
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
8/15

Figure 18 : Slope Compensation Techniques.
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
Figure 19 : Isolated MOSFET Drive and Current Transformer Sensing.
7
6
COMP/LATCH
ISOLATION
BOUNDARY
D95IN349
5.0Vref
V
CC
+
-
+
-
Q
S
R
+
-
3R
R
S
N
S
C
V
in
Q1
N
P
V
GS
Waveforms
+
0+ 0
-- 50% DC 25% DC
I
pk
= V
(pin 1)
-1.4
3R
S
N
S
N
P
()
UC3842B UC3842B
UC2842B/3B/4B/5B - UC3842B/3B/4B/5B
9/15

Figure 20 : Latched Shutdown.
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 T
A(min)
.
The simple two transistor circuit can be used in place of the SCR as shown. All resistors are 10K.
5
D95IN351
+
-EA
R
i
+
1mA
R
d
R
2R
5
C
f
R
f
1
2
From V
O
2.5V
+
-EA
R
P
+
1mA
R
d
R
2R
5
C
f
R
f
1
2
From V
O
2.5V
Error Amp compensation circuit for stabilizing any current-mode topology except
for boost and flyback converters operating with continuous inductor current.
C
P
R
i
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