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

OSC

R1V

CURRENT

SENSE

COMPARATOR

-PWM

LATCH

ERROR AMP.

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

SENSE

GROUND

OUTPUT

REF

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.

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

power supply output through a resistor divider.

SENSE A voltage proportional to inductor current is connected to this input. The PWM uses this

information to terminate the output switch conduction.

/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.

CC This pin is the positive supply of the control IC.

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

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

2/15

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

∆

REF Load Regulation 1 ≤Io≤20mA 325 325mV

∆

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

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)

225

–

250

275

225

–

250

275

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–%

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

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.

A2N2222

4.7KΩ

1KΩ

ERROR AMP.

ADJUST

4.7KΩ5KΩ

SENSE

ADJUST

100KΩCOMP

SENSE

REF

OUTPUT

GROUND

0.1µF

REF

OUTPUT

GROUND

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)

D95IN333

=10nF

=5nF

=2nF

=1nF

=500pF

=200pF

=100pF

=15V

=25˚C

(KΩ)

0.8

Figure 2: Timing Resistor vs. Oscillator Fre-

quency

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

=10nF

=5nF

=2nF

=1nF

=500pF

=200pF

=100pF

D95IN334

=15V

=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

=15V

OSC

=2V

Figure 4: Oscillator Discharge Current vs. Tem-

perature.

1235

max

(%)

0.8 R

(KΩ)

D95IN336

=15V

=3.3nF

=25˚C

dischg

=7.5mA

dischg

=8.8mA

Figure 5: Maximum Output Duty Cycle vs. Tim-

ing Resistor.

10 100 1K 10K 100K 1M f(Hz)

-20

(dB)

180

150

120

D95IN337

=15V

=2V to 4V

=100K

=25˚C

Gain

Phase

Figure 6: Error Amp Open-Loop Gain and

Phase vs. Frequency.

0246V

(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

=15V

=-40˚C

=125˚C

=25˚C

Figure 8: Reference Voltage Change vs.

Source Current.

-55 -25 0 25 50 75 100 T

(˚C)

D95IN340

100

(mA) V

=15V

≤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

(mA)

-2

-1

sat

(V)

D95IN341

=15V

80µs Pulsed Load 120Hz Rate

=-40˚C

=25˚C

=-40˚C T

=25˚C

GND

Sink Saturation

(Load to V

)

Source Saturation

(Load to Ground)

Figure 10: Output Saturation Voltagevs. Load

Current.

0 102030V

(V)

(mA)

UCX843/45

UCX842/44

=10K

=3.3nF

=0V

Sense

=0V

=25˚C

D95IN342

Figure 11: Supply Current vs. Supply Voltage.

Figure 12: Output Waveform. Figure 13: Output Cross Conduction

5V REG

OSCILLATOR

PWM

CLOCK

GND

OUTPUT

LARGE R

/SMALL C

OUTPUT

SMALL R

/LARGE C

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

ICC

100ns/DIV

100mA/DIV

20V/DIV

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

7/15

Figure 15 : Error Amp Configuration.

1mA

COMP

2.5V

D95IN345

Figure 16 : Under Voltage Lockout.

UC3842B

UC3844B UC3843B

UC3845B

16V 8.4V

10V 7.6V

OFF

ON/OFF COMMAND

TO REST OF IC

<0.5mA

<17mA

OFF

D95IN346

Figure 17 : Current Sense Circuit .

ERROR

AMPL. 2R

R1V

CURRENT

SENSE

COMPARATOR

CURRENT

SENSE

COMP

GND

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.

ISRSLOPE CT

VREG 8

RT/CT

ISENSE

GND RS

RSLOPE CT

VREG 8

RT/CT

ISENSE

GND

D95IN348

Figure 19 : Isolated MOSFET Drive and Current Transformer Sensing.

COMP/LATCH

ISOLATION

BOUNDARY

D95IN349

5.0Vref

Waveforms

-- 50% DC 25% DC

= V

(pin 1)

-1.4

()

UC3842B UC3842B

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

9/15

Figure 20 : Latched Shutdown.

D95IN350

BIAS

-EA

OSC

3905

3903

1mA

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.

D95IN351

-EA

1mA

From V

2.5V

-EA

1mA

From V

2.5V

Error Amp compensation circuit for stabilizing any current-mode topology except

for boost and flyback converters operating with continuous inductor current.

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

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