ST UC2842B, UC2843B, UC2844B, UC3843B, UC3844B User Manual

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

March 1999

HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER

.

TRIMMED OSCILLATOR FOR PRECISE FRE­QUENCY CONTROL

.

OSCILLATOR FREQUENCY GUARANTEED
AT 250kHz

.

CURRENT MODE OPERATION TO 500kHz

.

AUTOMATIC FEED FORWARD COMPENSA­TION

.

LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING

.

INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGE LOCKOUT

.

HIGH CURRENT TOTEM POLE OUTPUT

.

UNDERVOLTAGE LOCKOUT WITH HYSTER­ESIS

.

LOW STAR T-UP AND O PERA TING CURR ENT

DESCRIPTION

The UC38 4xB family of control ICs provi des the nec­essary features to implement off-line or DC to DC
fixed freq uency c urrent mod e cont rol sch emes with
a minimal external parts count. Internally imple­mented cir cuits i nclude a trimmed os cil lator fo r pre­cise DUTY CYCLE CONTROL under voltage lock­out featuring start-up current les s than 0.5mA, a pre­cision reference trimmed for accuracy at the error
amp input , logic to in sure la tched oper ation, a PW M

comparator wh ich also provides current limit control,
and a totem pole output stage designed to source
or sink high pe ak current. The output stage, su itable
for driving N-Channel MOSFETs, is low in the off­state.

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 (o ff), ideally
suited off-line applications The corresponding thresh­olds for the UC3843B and UC3845B are 8.5 V and 7.9
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 respe ct to pi n 5, al l currents are pos itive i nto t he spec i fied terminal.

PIN CONNECTION (top view)

COMP

V

FB

I

SENSE

RT/C

T

GROUND

OUTPUT

Vi

V

REF

1

3

2

4

6
5

7

8

D95IN332

Minidi p/ SO 8

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

V

i

Supply Voltage (low impedance source) 30 V

V

i

Supply Voltage (Ii < 30mA) Self Limiting

I

O

Output Cur re nt

±

1

A

E

O

Output Energy (capacitive load) 5

µ

J
Analog Inputs (pins 2, 3) – 0.3 to 5.5 V
Error Amplifier Output Sink Current 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

Junction Operating Temperature – 40 to 150 °C

T

L

Lead Temperature (soldering 10s) 300

°

C

PIN FUNCT IO NS

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/CT

The oscillator frequency and maximum Output duty cycle are programmed by connecting
resistor R

T

to Vref and cpacitor CT 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 CT through resistor RT.

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

2/15

ELECTRICAL CHARACTERISTIC S ( [note 1] Unless other wise stated, these specifications apply for

-25

< T

amb

< 85°C for UC284XB; 0 < T

amb

< 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

V

REF

Output Voltage

T

j

= 25°C Io = 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

T

A

= T

low

to T

high

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

∆

f

OSC

/∆V

Frequency Cha nge wit h V olt. 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

–1––0.5–%

V

OSC

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

I

dischg

Discharge Current (V

OSC

=2V) TJ = 25°C

T

A

= T

low

to T

high

7.8

7.5

8.3–8.8

8.8

7.8

7.6

8.3–8.8

8.8mAmA

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 VFB = 5V -0.1 -1 -0.1 -2

µ

A

A

VOL

2V ≤ Vo ≤ 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 ≤ Vi ≤ 25V

60 70 60 70 dB

I

o

Output Sink Current V

PIN2

= 2.7V V

PIN1

= 1.1V 2 12 2 12 mA

I

o

Output Source Current V

PIN2

= 2.3V V

PIN1

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

V

OUT

High V

PIN2

= 2.3V;

R

L

= 15KΩ to Ground

5 6.2 5 6.2 V

V

OUT

Low V

PIN2

= 2.7V;

R

L

= 15KΩ 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 Minidip SO8 Unit

R

th 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 l imits must be respected; low duty cycle puls e techniq ues are used during test maintain T

j

as

close to T

amb

as possible.

2. These parameters, alth ough guaranteed, are not 100% tested in produc ti on.

3. Parameter measured at tri p poi nt of la tch wit h V

PIN2

= 0.

4. Gain defined as :

∆

V

PIN1

A = ; 0 ≤ V

PIN3

≤ 0.8 V

∆

V

PIN3

5. Adjust Vi above the start th reshold bef ore set ti ng at 15 V .

ELECT RIC AL CH ARACTERI S TI CS ( co ntinued)

Symbol Parameter Test Conditions

UC284XB UC384XB

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 VCC = 6V; I

SINK

= 1mA 0.1 1.1 0.1 1.1 V

t

r

Rise Time

T

j

= 25°C CL = 1nF (2)

50 150 50 150 ns

t

f

Fall Time

T

j

= 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

I

st

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

V

i

= 14V for UCX842B/44B 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 Ii = 25mA 30 36 30 36 V

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

4/15

Figure 1: Open Loop Te st Ci rc ui t.

R

T

A2N2222

4.7KΩ

1KΩ

ERROR AMP.

ADJUST

4.7KΩ
5KΩ

I

SENSE

ADJUST

100KΩ

COMP

V

FB

I

SENSE

RT/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

High peak curr ents associat ed with capac itive loads
necessitate careful grounding techniques. Timing
and bypass capacitors should be connected close

to pin 5 in a si ngle point ground. The transis tor and
5 KΩ potentiomet er are used to sample the oscillato r
waveform a nd ap ply an adjustable ra m p to pin 3 .

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

OSC

(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
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

%

CT=10nF

CT=5nF

CT=2nF

CT=1nF

CT=500pF

CT=200pF

CT=100pF

D95IN334

Vi=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