SGS THOMSON uc3843 Service Manual

UC2842B/3B/4B/5B
UC3842B/3B/4B/5B

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 TOTEM POLEOUTPUT

.

UNDERVOLTAGE LOCKOUT WITH HYSTER-

.

ESIS
LOWSTART-UPANDOPERATINGCURRENT

.

DESCRIPTION

TheUC384xBfamilyofcontrolICsprovidesthenec­essaryfeaturesto implement off-line or DC to DC
fixedfrequencycurrent modecontrolschemeswith
a minimal external parts count. Internally imple­mentedcircuitsincludea trimmedoscillatorfor pre­cise DUTY CYCLECONTROL undervoltagelock­outfeaturingstart-upcurrentlessthan0.5mA,a pre­cision 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 off­state.

Differencesbetweenmembersof thisfamily arethe
under-voltagelockoutthresholdsandmaximumduty
cycle ranges. The UC3842B and UC3844B have
UVLOthresholds of 16V (on) and 10V (off), ideally
suitedoff-lineapplicationsThecorrespondingthresh­oldsfortheUC3843BandUC3845Bare8.5Vand7.9
V. TheUC3842B andUC3843B canoperateto duty
cyclesapproaching100%.A rangeof the zeroto <
50 % is obtainedby the UC3844Band UC3845Bby
theadditionofaninternaltoggleflipflopwhichblanks
theoutputoffeveryotherclockcycle.

Minidip SO8

BLOCK DIAGRAM (t oggle flip flop use d only in UC3844B and UC3845B)

7

Vi

2.50V

UVLO

S/R

2R

R1V

5V

REF

VREF GOOD

LOGIC

S
R

CURRENT

SENSE

COMPARATOR

INTERNAL

BIAS

T

LATCH

UC3842B

August1996

GROUND

RT/CT

VFB

COMP

CURRENT

SENSE

34V

5

4

2

1

3

OSC

ERROR AMP.

+

- PWM

D95IN331

8

6

VREF

5V 50mA

OUTPUT

1/15

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B

ABSOLUTEMAXIMUM RATINGS

Symbol Parameter Value Unit

V
V
I

O

E

O

P

tot

P

tot

T

stg

T

L

*

Allvoltagesare with respectto pin5, all currents arepositiveintothe specifiedterminal.

PI N CONNE CTIO N (top view)

SupplyVoltage (low impedance source) 30 V

i

SupplyVoltage (Ii < 30mA) Self Limiting

i

Output Current

1

±

Output Energy (capacitive load) 5
Analog Inputs (pins 2, 3) – 0.3to 5.5 V
ErrorAmplifierOutput Sink Current 10 mA
Power Dissipationat T

amb

≤25°

C (Minidip)

Power Dissipationat Tamb≤25°C (SO8)

1.25 W
800 mW

Storage TemperatureRange – 65 to 150
Lead Temperature (soldering 10s) 300

Mini di p/ SO 8

A

J

µ

C

°

C

°

COMP

V

I

SENSE

RT/C

1

FB

T

2
3
4

D95IN332

8
7
6
5

V

REF

Vi
OUTPUT
GROUND

PIN FUNCTIONS

No Function Description

1 COMP This pin is the Error Amplifier output and is made available for loop compensation.
2V

3I

4R

SENSE

T/CT

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

7V

CC

8V

This is the inverting input of the Error Amplifier. It is normally connected to the switching

FB

power supply output through a resistor divider.
A voltage proportional to inductor current is connected to this input. The PWM uses this

information to terminate the output switch conduction.
The oscillator frequency and maximum Output duty cycle are programmed by connecting

resistor R

to Vref and cpacitor CTto ground. Operation to 500kHz is possible.

T

and sunk by this pin.
This pin is the positive supply of the control IC.

This is the reference output. It provides charging current for capacitor CTthrough resistor RT.

ref

ORDERINGNUMBERS

UC2842BD1; UC3842BD1
UC2843BD1; UC3843BD1
UC2844BD1; UC3844BD1
UC2845BD1; UC3845BD1

2/15

SO8 Minidip

UC2842BN; UC3842BN
UC2843BN; UC3843BN
UC2844BN; UC3844BN
UC2845BN; UC3845BN

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B

THERMAL DATA

Symbol Descriptio n Mi n i dip SO 8 U n it

R

th j-amb

Thermal ResistanceJunction-ambient. max. 100 150

°

C/W

ELECTRICALCHARACTERISTICS

-25 < T

<85°C forUC284XB;0 < T

amb

( [note 1] Unless otherwise stated, these specifications apply for

<70°CforUC384XB;Vi=15V (note 5); RT= 10K;CT= 3.3nF)

amb

Symbol Parameter Test Conditions

REFERENCE SECTION

V

∆V
∆V

V

∆

REF

Output Voltage

REF

Line Regulation

REF

Load Regulation

REF

Temperature Stability (Note 2) 0.2 0.2

/∆T

=25°CIo= 1mA

T

j

12V ≤ V
1 ≤ I

i

≤ 20mA

o

≤ 25V

Total Output Variation Line, Load, Temperature 4.9 5.1 4.82 5.18 V

e

Output Noise Voltage

N

10Hz≤f≤10KHz T
(note 2)

Long Term Stability

= 125°C, 1000Hrs

T

amb

(note 2)

I

Output Short Circuit -30 -100 -180 -30 -100 -180 mA

SC

OSCILLATOR SECTION

f

OSC

Frequency

=25°C

T

j

T

A=Tlow

to T

high

TJ=25°C(RT= 6.2k, CT=1nF)

f

∆

OSC

f

∆

OSC

V

I

dischg

Frequency Change with Volt. V

/∆V

Frequency Change with Temp. T

/∆T

Oscillator Voltage Swing (peak to peak) – 1.6 – – 1.6 – V

OSC

Discharge Current (V

=2V) TJ=25°C

OSC

= 12V to 25V – 0.2 1 – 0.2 1 %

CC
A=Tlow

T

A=Tlow

to T

to T

high

high

ERROR AMP SECTION

V

BW Unity Gain Bandwidth T

PSRR Power Supply Rejec. Ratio

Input Voltage V

2

I

Input Bias Current VFB= 5V -0.1 -1 -0.1 -2 µ

b

A

VOL

I

Output Sink Current V

o

I

Output Source Current V

o

V

High V

OUT

V

Low V

OUT

= 2.5V 2.45 2.50 2.55 2.42 2.50 2.58 V

PIN1

2V≤V

J

12V≤V

PIN2
PIN2
PIN2

R

L

PIN2

R

L

4V

≤

o

=25°C 0.7 1 0.7 1 MHz

25V

≤

i

= 2.7V V

= 2.3V V

PIN1

PIN1

= 2.3V;

= 15KΩ to Ground

= 2.7V;

= 15KΩ to Pin 8

CURRENT SENSE SECTION

G

V

SVR Supply Voltage Rejection

Gain (note 3 & 4) 2.85 3 3.15 2.85 3 3.15 V/V

V

Maximum Input Signal V

3

I

Input Bias Current -2 -10 -2 -10

b

= 5V (note 3) 0.9 1 1.1 0.9 1 1.1 V

PIN1

12 ≤ V

≤ 25V (note 3)

i

Delay to Output 150 300 150 300 ns

UC284XB UC38 4XB

Min. Typ. Max. Min. Typ. Max.

Unit

4.95 5.00 5.05 4.90 5.00 5.10 V
220 220mV
325 325mV

mV/°C

=25°C

j

50 50 µ

525 525mV

49

52

55

49

52

55

KHz

48

–

56

48

–

56

KHz

225

250

275

225

250

275

KHz

–1––0.5–%

7.8

8.3–8.8

7.5

8.8

7.8

7.6

8.3–8.8

8.8mAmA

65 90 65 90 dB

60 70 60 70 dB

= 1.1V 2 12 2 12 mA

= 5V -0.5 -1 -0.5 -1 mA

5 6.2 5 6.2 V

0.8 1.1 0.8 1.1 V

70 70 dB

µA

V

A

3/15

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B

ELECTRICAL CHARACTERISTICS

(continued)

Symbol P a rameter Test C o n ditions

OUTPUT SECTION

V

V

V

Output Low Level I

OL

Output High Level I

OH

UVLO Saturation VCC = 6V; I

OLS

t

Rise Time

r

t

Fall Time

f

= 20mA 0.1 0.4 0.1 0.4 V

SINK

I

= 200mA 1.6 2.2 1.6 2.2 V

SINK

= 20mA 13 13.5 13 13.5 V

SOURCE

I

T
T

= 200mA 12 13.5 12 13.5 V

SOURCE

= 1mA 0.1 1.1 0.1 1.1 V

SINK

=25°CCL= 1nF (2)

j

=25°CCL= 1nF (2)

j

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

I

Start-up Current Vi= 6.5V for UCX843B/45B 0.3 0.5 0.3 0.5 mA

st

V

= 14V for UCX842B/44B 0.3 0.5 0.3 0.5 mA

i

I

V

Operating Supply Current V

i

Zener Voltage Ii= 25mA 30 36 30 36 V

iz

PIN2=VPIN3

=0V 12 17 12 17 mA

UC284X B UC384 X B

Min. Typ. Max. Min. Typ. Max.

50 150 50 150 ns
50 150 50 150 ns

Unit

Notes : 1. Max package power dissipationlimitsmustbe respected; lowduty cycle pulse techniques are used duringtest maintain Tjas

close to T

2. Theseparameters, although guaranteed, are not100%tested in production.

3. Parametermeasured at trippoint of latch withV

4. Gain definedas :
A= ;0≤V

5. AdjustViabovethe start thresholdbefore settingat 15 V.

∆ V
∆ V

aspossible.

amb

PIN1
PIN3

PIN3

≤

0.8V

PIN2

=0.

4/15

Figure1: OpenLoop Test Circuit.

4.7KΩ

UC2842B/3B/4B/5B - UC3842B/3B/4B/5B

V

REF

R

T

V

REF

ERROR AMP.

ADJUST

4.7KΩ

1KΩ

100KΩ

I

SENSE

ADJUST

D95IN343

5KΩ

COMP

V

I

SENSE

RT/C

FB

T

1
2

UC2842B

3

4

Highpeakcurrentsassociatedwithcapacitiveloads
necessitate careful grounding techniques. Timing
and bypasscapacitors should be connectedclose

Figure 2:

Timing Resistor vs. Oscillator Fre­quency

T

=100pF

D95IN333

OSC

(KHz)

RT

(KΩ)

C

C

T

=1nF

T

=200pF

C

T

=500pF

CT=2nF

C

50

20

10

5

2

Vi=15V
TA=25°C

1

0.8
10K 20K 30K 50K 100K 200K 300K 500K f

CT=5nF

CT=10nF

A2N2222

7

6

5

0.1µF

V

i

OUTPUT

GROUND

0.1µF

1W

1KΩ

8

C

T

V

i

OUTPUT

GROUND

to pin5 ina singlepointground.The transistorand
5KΩpotentiometerareusedtosampletheoscillator

waveformand applyan adjustableramp topin 3.

Figure 3:

Output Dead-Time vs. OscillatorFre­quency

%

50

30

20

10

5

3

2

1

CT=200pF

10K 20K 30K 50K 100K 200K 300K 500K fOSC(KHz)

CT=500pF

CT=1nF

CT=10nF

CT=100pF

CT=5nF

CT=2nF

Vi=15V
TA=25°C

D95IN334

5/15