UC3845AN
SGS Thomson Microelectronics UC3845AN, UC3844AN, UC3843AN, UC3845AD, UC3844AD Datasheet
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UC2842A/3A/4A/5A
UC3842A/3A/4A/5A
March 1999
HIGH PERFORMANCE CURRENTMODE PWM CONTROLLER
.
TRIMMED OSCILLATOR DISCHARGE CUR-
RENT
.
CURRENT MODEOPERATIONTO500kHz
.
AUTOMATIC FEED FORWARD COMPENSA-
TION
.
LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
.
INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGELOCKOUT
.
HIGHCURRENTTOTEMPOLEOUTPUT
.
UNDERVOLTAGE LOCKOUT WITH HYSTER-
ESIS
.
LOWSTART-UPCURRENT (< 0.5mA)
.
DOUBLEPULSESUPPRESSION
DESCRIPTION
TheUC384xAfamilyofcontrolICsprovidesthenec-
essaryfeaturesto implement off-lineor DC to DC
fixedfrequencycurrent modecontrolschemeswith
a minimal external parts count. Internally imple-
mentedcircuitsincludea trimmedoscillatorfor pre-
cise DUTY CYCLECONTROL under voltagelock-
outfeaturingstart-upcurrentlessthan0.5mA,a pre-
cision reference trimmed for accuracyat the error
ampinput,logictoinsurelatchedoperation,a PWM
comparatorwhichalsoprovidescurrentlimit control,
and a totempole output stage designed to source
orsinkhighpeakcurrent.Theoutputstage,suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differencesbetweenmembersof thisfamily arethe
under-voltagelockoutthresholdsandmaximumduty
cycle ranges. The UC3842A and UC3844A have
UVLOthresholds of 16V (on) and 10V (off), ideally
suitedoff-lineapplicationsThecorrespondingthresh-
olds for the UC3843A and UC3845A are 8.5 V and
7.9V.The UC3842A and UC3843A can oper ateto
dutycyclesapproaching100%.Arangeofthezeroto
<50%is obtainedbytheUC3844AandUC3845Aby
theadditionofaninternaltoggleflipflopwhichblanks
theoutputoffeveryotherclockcycle.
BLOCK DIAGRAM (toggle flip flop used only in UC3844A and UC3845A)
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
UC3842A
1/15
*
Allvoltagesare with respect to pin 5, all currents arepositiveinto the specifiedterminal.
PIN CONNE C TI ON
(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
Minid i p/ SO8
ORDERINGNUMBERS
SO8 Minidip
UC2842AD1; UC3842AD1
UC2843AD1; UC3843AD1
UC2844AD1; UC3844AD1
UC2845AD1; UC3845AD1
UC2842AN; UC3842AN
UC2843AN; UC3843AN
UC2844AN; UC3844AN
UC2845AN; UC3845AN
ABSOLUTEMAXIMUMRATINGS
Symbol Parameter Value Unit
V
i
SupplyVoltage (low impedance source) 30 V
V
i
SupplyVoltage (Ii < 30mA) Self Limiting
I
O
Output Current
±
1
A
E
O
Output Energy (capacitive load) 5
µ
J
Analog Inputs(pins 2, 3) – 0.3to 5.5 V
ErrorAmplifier Output SinkCurrent 10 mA
P
tot
Power Dissipation at T
amb
≤
25
°
C (Minidip)
1.25 W
P
tot
Power Dissipation at Tamb
≤
25
°
C (SO8)
800 mW
T
stg
Storage Temperature Range – 65 to 150
°
C
T
J
JunctionOperating Temperature – 40 to 150 °C
T
L
Lead 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
/C
T
The oscillator frequency and maximum Output duty cycle are programmed by connecting
resistor R
T
to Vref and cpacitor C
T
to 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 C
T
through resistor R
T
.
UC2842A/3A/4A/5A - UC3842A/3A/4A/5A
2/15
ELECTRICALCHARACTERISTICS
( [note 1] Unless otherwise stated, these specificationsapply for
-25 < T
amb
<85°C forUC284XA;0 < T
amb
<70°CforUC384XA;V
i
=15V(note 5); R
T
= 10K; C
T
= 3.3nF)
Symbol Parameter Test Conditions
UC284XA UC38 4XA
Unit
Min. Typ. Max. Min. Typ. Max.
REFERENCE SECTION
V
REF
Output Voltage
T
j
=25°CI
o
= 1mA
4.95 5.00 5.05 4.90 5.00 5.10 V
∆V
REF
Line Regulation
12V ≤ V
i
≤ 25V
220 220mV
∆V
REF
Load Regulation
1 ≤ I
o
≤ 20mA
325 325mV
∆
V
REF
/
∆
T
Temperature Stability (Note 2) 0.2 0.2
mV/
°
C
Total Output Variation Line, Load, Temperature 4.9 5.1 4.82 5.18 V
e
N
Output Noise Voltage
10Hz
≤
f
≤
10KHz T
j
=25
°
C
(note 2)
50 50 µ
V
Long Term Stability
T
amb
= 125
°
C, 1000Hrs
(note 2)
525 525mV
I
SC
Output Short Circuit -30 -100 -180 -30 -100 -180 mA
OSCILLATOR SECTION
f
OSC
Frequency
T
j
=25
°
C
47 52 57 47 52 57 KHz
∆
f
OSC
/
∆
V
Frequency Change with Volt. V
CC
= 12V to 25V – 0.2 1 – 0.2 1 %
∆
f
OSC
/
∆
T
Frequency Change with Temp. T
A
=T
low
to T
high
–5––5–%
V
OSC
Oscillator Voltage Swing (peak to peak) – 1.6 – – 1.6 – V
I
dischg
Discharge Current (V
OSC
=2V) T
J
=25°C 7.8 8.3 8.8 7.8 8.3 8.8 mA
ERROR AMP SECTION
V
2
Input Voltage V
PIN1
= 2.5V 2.45 2.50 2.55 2.42 2.50 2.58 V
I
b
Input Bias Current V
FB
= 5V -0.1 -1 -0.1 -2
µA
A
VOL
2V ≤ V
o
≤ 4V
65 90 65 90 dB
BW Unity Gain Bandwidth T
J
=25°C 0.7 1 0.7 1 MHz
PSRR Power Supply Rejec. Ratio
12V
≤
V
i
≤
25V
60 70 60 70 dB
I
o
Output Sink Current VPIN2 = 2.7V V
PIN1
= 1.1V 2 12 2 12 mA
I
o
Output Source Current VPIN2 = 2.3V V
PIN1
= 5V -0.5 -1 -0.5 -1 mA
V
OUT
High V
PIN2
= 2.3V;
R
L
= 15K
Ω
to Ground
5 6.2 5 6.2 V
V
OUT
Low V
PIN2
= 2.7V;
R
L
= 15K
Ω
to Pin 8
0.8 1.1 0.8 1.1 V
CURRENT SENSE SECTION
G
V
Gain (note 3 & 4) 2.85 3 3.15 2.85 3 3.15 V/V
V
3
Maximum Input Signal V
PIN1
= 5V (note 3) 0.9 1 1.1 0.9 1 1.1 V
SVR Supply Voltage Rejection
12
≤
V
i
≤
25V (note 3)
70 70 dB
I
b
Input Bias Current -2 -10 -2 -10
µ
A
Delay to Output 150 300 150 300 ns
THERMAL DATA
Symbol Description Mi n i dip SO 8 U n it
R
th j-amb
Thermal ResistanceJunction-ambient. max. 100 150
°
C/W
UC2842A/3A/4A/5A - UC3842A/3A/4A/5A
3/15
Notes : 1. Max package power dissipation limitsmust be respected; low duty cycle pulse techniquesare used duringtestmaintainT
j
as
close to T
amb
aspossible.
2. Theseparameters, although guaranteed, are not100%testedin production.
3. Parametermeasured at trippointof latchwithV
PIN2
=0.
4. Gain definedas :
∆ V
PIN1
A= ;0
≤
V
PIN3
≤
0.8V
∆ V
PIN3
5. AdjustV
i
abovethestart threshold before setting at 15 V.
ELECTRICAL CHARACTERISTICS
(continued)
Symbol Pa ram et er Test C o n diti o n s
UC284X A UC384 X A
Unit
Min. Typ. Max. Min. Typ. Max.
OUTPUT SECTION
V
OL
Output Low Level I
SINK
= 20mA 0.1 0.4 0.1 0.4 V
I
SINK
= 200mA 1.6 2.2 1.6 2.2 V
V
OH
Output High Level I
SOURCE
= 20mA 13 13.5 13 13.5 V
I
SOURCE
= 200mA 12 13.5 12 13.5 V
V
OLS
UVLO Saturation V
CC
= 6V; I
SINK
= 1mA 0.7 1.2 0.7 1.2 V
t
r
Rise Time
T
j
=25
°
CC
L
= 1nF (2)
50 150 50 150 ns
t
f
Fall Time
T
j
=25°CC
L
= 1nF (2)
50 150 50 150 ns
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold X842A/4A 15 16 17 14.5 16 17.5 V
X843A/5A 7.8 8.4 9.0 7.8 8.4 9.0 V
Min Operating Voltage
After Turn-on
X842A/4A 9 10 11 8.5 10 11.5 V
X843A/5A 7.0 7.6 8.2 7.0 7.6 8.2 V
PWM SECTION
Maximum Duty Cycle X842A/3A 94 96 100 94 96 100 %
X844A/5A 47 48 50 47 48 50 %
Minimum Duty Cycle 0 0 %
TOTAL STANDBY CURRENT
I
st
Start-up Current V
i
= 6.5V for UCX843A/45A 0.3 0.5 0.3 0.5 mA
V
i
= 14V for UCX842A/44A 0.3 0.5 0.3 0.5 mA
I
i
Operating Supply Current V
PIN2
=V
PIN3
=0V 12 17 12 17 mA
V
iz
Zener Voltage I
i
= 25mA 30 36 30 36 V
UC2842A/3A/4A/5A - UC3842A/3A/4A/5A
4/15
Figure1: OpenLoop Test Circuit.
R
T
A2N2222
4.7KΩ
1KΩ
ERROR AMP.
ADJUST
4.7KΩ
5KΩ
I
SENSE
ADJUST
100KΩ
COMP
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 capacitorsshould be connectedclose
to pin 5 ina singlepointground.The transistorand
5KΩpotentiometerareusedtosampletheoscillator
waveformand applyan adjustablerampto pin 3.
300 1K 3K 10K 30K R
T
(Ω)
1K
10K
100K
1M
f
o
(Hz)
D96IN362
CT=470pF
1nF
2.2nF
4.7nF
Figure 2:
Oscillator Frequency vs Timing Resis-
tance
300 1K 3K 10K 30K R
T
(Ω)
0
20
40
60
f
o
(Hz)
D96IN363
80
Figure 3:
Maximum Duty Cycle vs Timing Resis-
tor
UC3842A
UC2842A/3A/4A/5A - UC3842A/3A/4A/5A
5/15
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